Prof. M. A. Usov.

Phases of Effusives.

1. Introduction.

Effusive rocks not infrequently enter into the composition of sedimentary formations. Therefore the geological age of effusives is often established with great precision. And it has long since been observed that ancient effusives differ in many respects from the most recent formations, for which reason these two categories of erupted rocks have received different names. Recently there has been a tendency, especially among American and English petrographers, to unite effusive rocks of different ages into a single group—on the grounds that the composition of lavas in the geological periods known to us has not changed, and that the entire difference between ancient effusives and later rocks consists only in a certain decomposition of them, quite natural for formations which, in the course of time, are subjected to the action of various agents. The latter proposition may, of course, be applied also to intrusive rocks, among which it is permissible to seek ancient and young varieties differing in degree of preservation; however, such differences have hitherto scarcely been distinguished, for in general we know only more or less ancient intrusives, which are freed from the cover of the continent and brought out to the present-day surface over a comparatively long interval of time.

Thus, given the emerging attitude toward the old division of effusives into age groups, one may expect that their dualistic nomenclature will soon disappear. But if the age criterion is losing its significance, there are other grounds for preserving a double nomenclature of effusives, the homologous groups of which do indeed differ from one another so greatly, at least in outward appearance, that giving them the same name does not appear possible. In what follows an attempt is made to show that effusive rocks, by their very origin, represent metastable formations which, with time and under the influence of various other factors, adapt themselves to new conditions by changing the phases of their components and, in this altered form, present forms deserving a special name.

2. History of the question.

In order to approach the proposed subject more closely and to reveal its significance, it is first of all necessary to turn to the history of the question, which can be set forth in fairly brief form.

Werner, who laid the foundation of petrography, divided all rocks according to geological age, proceeding from the proposition that each epoch is characterized by its own special formations. This proposition, derived from the data of the study of the central part of Germany, where geological formations are indeed sharply isolated, was wholly shared by the petrographers of the first half of the nineteenth century, who received their training chiefly in German higher schools. Such a direction was quite natural at a time of exclusively macroscopic determination of rocks—a method which, as is known to every field geologist, is quite irreplaceable—

to them for establishing the general character of those changes to which rocks are subjected and which in part represent a function of time. This tendency found expression also in the first handbook of petrography, which was proposed in 1848 by Naumann, who separated even modern lavas from Tertiary and post-Pliocene effusives (1, 644).

But as acquaintance with the mineralogical composition and structure of igneous rocks became closer, voices began to be raised against too detailed a division of these rocks according to age. In this respect it is necessary to note Zirkel, who, even before the proper application of the microscopic method, came to the conviction that one can only contrast young effusives in general with ancient igneous rocks, as having managed to undergo one or another alteration (2, 446). At approximately the same time belongs the very original proposal of the well-known geologist-traveler Richthofen to connect the different age of igneous rocks with the conditions of their occurrence (3). This proposal, with minor corrections, was supported by Richthofen even at the beginning of the current century and is expressed as follows. Rocks of granitic structure are of ancient age, for they reach the earth’s surface after a long interval of time following their formation; porphyritic rocks are associated with Mesozoic formations, having formed at a comparatively small depth; finally, trachyte-basalt rocks, which are typical effusives, have been preserved only in the upper horizons of the newest formations and therefore are of young age (23, 529).

Of course, such an understanding of the cause of the different external manifestation of igneous rocks could not be shared by petrographers, who had long already believed that effusives may be encountered in all geological systems. Moreover, already in the 1870s it began to be insistently repeated that the composition of effusives in all geological periods is not subject to noticeable changes and that there is no need to apply generic names to effusive rocks of identical composition but of different antiquity. Thus, Fouqué very eloquently describes the difficulty of a student who is asked to distinguish, for example, quartz porphyry from rhyolite (4, 516), and believes that if an effusive somewhat changes its properties over time, this should be indicated only in the specific name of the rock (4, 521). English petrographers especially insisted on the monistic designation of effusives; for example, Allport, according to whose investigations the effusives of the Carboniferous of England are essentially no different from the Tertiary and post-Pliocene traps (6, 566).

Under the influence of this propaganda even Zirkel, who had introduced into the classification of igneous rocks the division of effusives into ancient and new formations, begins to incline to the idea of the necessity of designating effusives of identical composition by one name (5, 292). However, the external difference between ancient and new effusives in general is so significant that the majority of German petrographers, always striving to subdivide concepts, remained of the former opinion on this question. Thus, Rosenbusch, in his well-known work on the description of igneous rocks, acknowledges that the old division of effusives by age fully corresponds to their actual relationship (7, 4), and in a separate article explains the absence of a division of intrusive rocks by age by the fact that only ancient representatives of this group of rocks reach the present-day land surface (8, 16). Incidentally, the main cause of the different appearance of ancient and young effusives nevertheless remains unclear both in Rosenbusch and in Roth, who soon published a course of general and chemical geology (10, 42). Of course, one need not take into account Werner’s idea, revived at that time by F. Frech, that with the passage of time the properties of the earth change and, in connection with this, the composition of igneous rocks changes (9, 63).

Thus, in the 1880s German petrographers, and after them others, stubbornly continued to adhere to a dualistic understanding of effusives, expressing their division according to age—though with a somewhat indefinite distinction of outward appearance. Moreover, Lossen, proceeding from purely stratigraphic relations of the formations of Germany, and also from the external aspect of the effusives connected with these formations (11), divides the latter rocks, which he called rhyotaxites in contradistinction to deep-seated formations, into three series: paleoporphyry-diabase, mesoporphyry-melaphyre, and neotrachyte-basalt (12, 925), with which division, as will be shown below, one may agree, but only on entirely different grounds.

In general, toward the end of the nineteenth century all mineralogical classifications of igneous rocks provided for the division of effusives into paleotypal and neotypal groups. Thus, Michel-Lévy unconditionally carries out such a classification (13); Roth becomes still more firmly convinced of the necessity of taking geological age into account in designating all kinds of rocks (14, 2), and Zirkel, after some hesitation noted above, now stands firmly for this division, indicating that the ancient effusives had more chance to undergo various changes and therefore do in fact differ from the most recent rocks, deserving a special name (16, 840).

But with the beginning of the present century a new revision of this question begins—especially in England and America, where a whole series of authors give effusives only one name, proper—namely—to the neotypal group. Thus, Cross, in his review of the development of petrographic systematics in the nineteenth century, says that, were it not for Zirkel, such a relic of antiquity as the division of effusives according to time would long ago no longer exist (24, 367). Then the no less well-known petrographer Harker notes with regret that this division, which is a survival of Werner’s theories, is still preserved in many manuals (35, 361). Iddings (36, 342) holds the same opinion; however, he did not find it convenient to abandon the former system in his manual on igneous rocks.

The criticism of the former division of effusives according to age finds a corresponding echo also among German petrographers. Even Rosenbusch admits that there are no grounds for such a division, but for the time being decides to leave his classification in its former form (32, 724), thereby delaying for a long time the evolution of the question, for his manual, truly exhaustive, is used by all petrographers as an indispensable handbook. Further, Weinschenk states still more definitely that there is nothing more erroneous than to divide igneous rocks according to age, for in all geological periods identical magmas were poured out (25, 27); at the same time he believes that effusives, in the course of time, are subjected to a certain pathological alteration, and on this basis retains the old dualistic system, giving it only another meaning (26, 19), and—thus—approaching the solution of the question that is proposed in the present essay. Let us also note that the no less well-known petrographer Teichmüller, while retaining the old system, nevertheless considers this to be merely a matter of habit (48, 53). Likewise Milch, in his summary of the systematics of igneous rocks, which reflected all directions of petrographic thought in 1913, believes that the difference between ancient and young effusives consists only in the degree of their preservation and that—consequently—the double designation of these rocks must be removed from their classification (51, 201).

In recent years, judging from the accessible literature, no new ideas have appeared on the question that interests us. In processing their materials, petrographers make use chiefly of the reference books of Zirkel and Rosenbusch and determine effusives according to a template, taking into account their geological age and, as many express it, their freshness (66, 3).

and the degree of preservation (17, 214) of these rocks. It is possible, of course, that in some works on regional geology the question of the true relation between ancient and young effusives was, in passing, subjected to one or another analysis, but such ideas have not yet entered the widely used manuals of petrography, and the present state of general conceptions on this subject may be formulated as follows.

Intrusive rocks that emerge at the earth’s surface, in the overwhelming majority of cases, are of relatively ancient age and are sufficiently altered, so that a division of these rocks by age or by degree of preservation does not appear necessary. Effusive rocks are another matter. They very easily change their outward appearance and in part their mineralogical composition, preserving the primary properties of lava formations for only a rather short interval of time, approximately only since the Tertiary period. Therefore effusives may be divided, according to degree of preservation, into ancient (pre-Tertiary) and young (Tertiary and post-Pliocene), but whether these groups deserve special names is doubtful. Moreover, under the influence of internal geological processes, effusives undergo a particularly sharp alteration, allowing such formations to be set apart as a third group, still insufficiently characterized and established.

3. Statement of the Problem.

The uncertainty, noted in the historical survey, in the solution of the question of the significance of the change in the outward appearance of effusives over time, established by direct observations, is a natural consequence of the fact that, up to now, two basic points controlling the phenomena under consideration have not even been distinguished and formulated:

1) in what consists the essence of the change of effusives over time, and
2) what are the factors of these changes.

To elucidate these points, it must be recalled that every erupted rock, after some time, changes, whatever stratigraphic position it may occupy. This means that the principal cause of the alteration of effusives lies in the nature of the rocks themselves, which at the very moment of their formation acquire unstable properties. And indeed, it has long been established that erupted rocks are aggregates of solid phases, usually not in a state of equilibrium, for each solid phase, successive in the order of separation, came into equilibrium only with the residual liquid phase (52,3). As for effusives, in them the phases themselves are ordinarily metastable, having undergone strong supercooling and quenching. This proposition applies not only to phenocrysts, almost always quenched, but also to the groundmass of erupted rocks, representing often glass or a fine-grained formation distinguished by considerable strain.

Consequently, the essence of the changes to which effusives are inevitably subjected must be expressed in the transformation of metastable phases of supercooling and quenching, with the formation of entirely stable components.

Turning now to the factors of transformation of effusives, we must first of all pay attention to time, the significance of which manifests itself in two directions. First, time plays a great role in all geological processes, whether caused by small forces and proceeding extremely slowly. Therefore, whatever external factors may determine the alteration of effusives—these unstable formations—its intensity must, other conditions being equal, be directly proportional to time. But the interaction and transformation of solid phases may also occur independently of external causes, namely as a result of diffusion, which always manifests itself where equality of vapor pressure occurs, and which has been demonstrated by a whole series

olites (31, 23). Since the diffusion of solids proceeds very slowly at not especially high temperatures and, without some external push, begins only with difficulty, it is quite clear that an effusive rock will prove to be the more equilibrated the longer it has existed.

Thus, effusives by themselves may, in the end, come into a state of equilibrium. However, this internal process is always accelerated by external factors corresponding to the natural geological setting into which the erupted rocks fall. Indeed, an effusive can be preserved from denudation only in the event that it is covered by some rocks and—consequently—passes to a certain depth, where it is subjected to the action of elevated temperature and pressure. There is no need to dwell on the significance of the change of temperature in physicochemical processes: it is well known that an increase in temperature accelerates the motion of particles of matter and—thereby—intensifies diffusion and ordinary chemical reactions, especially those taking place in a heterogeneous system. As for pressure, its role in the phenomena under consideration must be acknowledged as comparatively modest, since what is involved is static pressure occurring at a relatively small depth. But if an effusive is subjected to dynamic pressure connected with mountain-building movements, then this factor can exert a great influence on the alteration of the rock, acting like the push that brings particles into stable equilibrium, and promoting the formation of at least submicroscopic cracks along which liquid or gaseous solutions will circulate, while also controlling the very character of the new formations.

It must be said that, in studying the natural transformation of effusives into stable formations, one cannot disregard the liquid and gaseous solutions that are usually recognized as agents of true metamorphism. The point is that every erupted rock contains a certain amount of liquid and gaseous substances, either filling the tiniest cavities and pores or occluded throughout the entire mass of the rock (33, 35). There is no doubt that, during the movement of the molecules of the components of effusives, the occluded substances also move, accelerating the natural transformation of the rock and, at the same time, possibly leading to new formations as a result of exchange reactions. Of course, these new formations must be comparatively very small, since the rock has not been subjected to mountain-building pressure, which promotes the development of cracks along which the movement of solutions proceeds more freely, nor to sufficiently strong heating, which in general intensifies reactions.

Thus, effusives, which after their formation are in a state of unstable equilibrium, undergo gradual transformation into a stable phase; this process proceeds slowly, not without the assistance of elevated temperature and pressure, and also of liquid and gaseous substances connected with the rock itself, which lead to the appearance of new formations. In general, this alteration of effusives may be paralleled with the diagenesis of sediments, which occurs under the influence of the same factors and conditions the formation of normal sedimentary rocks. Therefore the process under consideration, the transformation of an effusive rock into an aggregate of stable phases, may permissibly be called diagenetic. Extending this analogy further, it is easy to arrive at the conclusion that, since different names exist for sediments and for sedimentary rocks consolidated by diagenesis, with still greater justification special names may be given to primary and diagenetized effusives, which differ in the different state of their components; for example, the names liparite and quartz porphyry have the same right to exist as clay and argillite or sand and sandstone. Of course, it is as difficult to draw a perfectly sharp boundary between homologous effusives as—

for example, between clays and argillites: rocks may always be encountered in which only individual areas have managed to undergo diagenetic transformation. But the existence of transitional formations does not exclude the possibility and necessity of making a classification based on accepted criteria; at the same time, such transitional members have a special interest and a special historical significance.

Diagenesis is a process of metamorphic character, although it is not considered to belong to metamorphism in the strict sense of that concept, for it represents a necessary stage of normal lithogenesis. But if one proceeds from the practice of sedimentary rocks, especially fine-grained ones, then one may distinguish one more type of transformation, which is characteristic of rocks subjected to strong tangential pressure and which also is usually not taken to be a form of normal metamorphism. Such is the formation of clayey schists, which are often produced directly at the expense of clays undergoing a kind of diagenesis with the manifestation of schistosity and with the appearance of special, extremely fine mineral products, distinct from the new formations of simple diagenesis. Of course, clayey schists are close to phyllites of the upper zone of regional metamorphism (27,57), so that drawing a sharp boundary between them is very difficult, if at all possible; but the geological separation of the formations being compared encounters no special difficulties, and—moreover—it is scarcely the case that definite boundaries exist at all between geological and petrographic groups, which often have to be separated not qualitatively but quantitatively.

Taking such an analogy into account, one may distinguish a special phase of transformation of primary unstable effusives in the presence of sharp tangential pressure, which brings to the fore the action of gaseous and liquid substances found in the erupted rock itself, intensifying this action by the formation of a mass of cracks of various kinds and giving it a special direction. In any case, it is well known to everyone how sharply, at least in external appearance, the effusives of epicontinental formations, which had been only under static pressure or had experienced reflected folding, differ from the erupted rocks that entered into geosynclinal masses and were subjected directly to tangential pressure: suffice it to say that epicontinental effusives possess a predominantly brown coloration, whereas geosynclinal effusives are usually characterized by a green color. Unfortunately, this kind of alteration of effusives is not marked by a definite name, unless one counts Lossen’s proposal to separate the Paleozoic effusives, which in Central Germany usually have a geosynclinal character, into the keratophyre-diabase group (12,924). If one proceeds from sedimentary rocks, then geosynclinal effusives might be called the corresponding schists, but this would not be entirely correct, for usually such rocks retain a massive appearance. Therefore, for the time being one should stop at the definition of the process under consideration as greenstone alteration, in the terminology of the macroscopists, who correctly noted the external feature of such rocks.

Thus, effusives unstable by their very origin are transformed into stable phases of two kinds, depending on the environment into which they fall after their formation: effusives of epicontinental formations undergo simple diagenesis, while geosynclinal effusives undergo greenstone alteration. The remaining kinds of alteration of rocks, connected with their more or less complete recrystallization, must be assigned to true metamorphism, or the fundamental transformation of mineral aggregates, and will not be considered here.

Having thus outlined the essence and the principal factors of the natural alterations of effusives, let us now turn to a more detailed characterization

…of these transformations, without dwelling on a separate description of the primary phase of effusives as the original and sufficiently well-known one.

4. Diagenesis of effusives.

In order to reveal the nature of the diagenesis of effusives, occurring under normal conditions of occurrence of epicontinental formations, it is necessary to study the natural alteration of the primary phases of their individual components.

Free silica, according to experimental investigations (53), separates from the molten phase in the form of cristobalite, which, with a lowering of temperature, may pass into tridymite, and this latter into $\beta$-quartz, which, finally, passes into normal $\alpha$-quartz. But in intrusive rocks free silica always appears in the form of ordinary quartz, in which traces of a former $\beta$-variety can sometimes still be observed (37, 438). This is explained by the fact that acid magma, being rich in liquid and gaseous components, crystallizes at a comparatively low temperature, while under the great pressure to which an intrusive body is subjected the melting point of individual minerals is raised. Therefore, during abyssal and hypabyssal solidification of magma, free silica separates from it beginning with $\beta$-quartz, which then, with the slow decrease in temperature characteristic of intrusive bodies, is transformed into the normal variety.

It is natural that the free silica of effusive phenocrysts, as intratelluric formations, always appears in the form of quartz. But it is possible that in primary effusives, the segregations of which solidified in the first phase near the temperature of crystallization, the quartz of the phenocrysts belongs to the $\beta$-variety, which passes into the ordinary form only as a result of diagenesis. To resolve this question, particularly high-precision optical investigations are needed, for the two polymorphous varieties of quartz are with difficulty distinguished from one another. In any case it is characteristic that the quartz of liparites seems considerably purer and more transparent than the quartz of porphyries.

As for free silica crystallizing in the surface stage of lava eruption, it can manifest itself, and indeed does manifest itself, in higher-temperature modifications, for when the pressure is reduced to atmospheric pressure the melting temperature of the mineral components is considerably lowered, as is excellently demonstrated by the constantly observed resorption of quartz phenocrysts in effusives. Of course, the study of the mineralogical composition of the groundmass of acid effusives is extremely difficult, but a number of investigators have already established in the most recently extruded rocks the presence of cristobalite and—especially—tridymite, some holding that the greater part of the free silica of the groundmass of young effusives is represented precisely by tridymite (28, 124). One may even think that there is no quartz at all in the groundmass of primary effusives, since the simultaneous formation of quartz and tridymite would be strange. But if we turn to descriptions of quartz porphyries and other diagenetically altered acid effusives, then here we do not find indications of the presence of tridymite and cristobalite; apparently, the latter minerals, being metastable at low temperatures, have had time to change into stable quartz, and not only a molecular rearrangement has taken place, but also a change in the external form of the aggregates, with the destruction of the penetration twins and the scale-like texture characteristic of tridymite.

Passing now to the lime-soda feldspars, it must first of all be noted that the study of these generally widespread and important rock-forming minerals is far from complete, despite the large number of investigations that have been carried out. This is explained both by the complexity of the structure of these minerals and by the difficulty of their artificial [[unclear: continuation cut off at bottom of page]]

case. On the basis, for example, of the recent summary work of the American O. Bowen (70 and 76), the scheme of the relations among the different representatives of the feldspar group under consideration may be represented as follows. Potassic and sodic feldspars, on crystallization from molten solutions, yield isomorphous crystals with a still not quite precisely established limit of their miscibility; moreover, potassic feldspars with an admixture of sodium separate in the form of monoclinic sanidine and orthoclase, while varieties of these minerals with predominant sodium give triclinic anorthoclases. At lowered temperature the formations that have separated from the magma are metastable and gradually change in two directions: first of all, exsolution of the component in solid solution occurs, with the production of perthite or antiperthite, and the potassic feldspar often changes into triclinic microcline.

Dwelling on the phases sanidine—microcline, we must establish the position of orthoclase and adularia, which are, as it were, superfluous in this system. And indeed, these minerals cannot be independent species: orthoclase, by its paragenesis and by its frequent microperthitization, must be regarded as submicroscopically twinned microcline, a view expressed by a whole series of investigators and to which, although not quite decisively, Rosenbusch (29, 322) also adhered; while adularia is a hydrogen orthoclase, displaying a microcline lattice even under slight mechanical action on the mineral (76, pl. } C–D). Consequently, the primary form of potassic feldspar is sanidine, often containing a large amount of sodium.

Sanidine also forms the chilled phenocrysts of young effusives, as well as feldspar individuals that take part in the composition of the recrystallized groundmass of these rocks, with the exception, perhaps, of the components of its micro-poikilitic and spherulitic portions, which in general are difficult to resolve because of the insignificant size of the grains. This primary mineral, when entering into the composition of the segregations, is generally colorless and glassy, and microscopically appears entirely homogeneous, forming rare simple twins, so that it often resembles quartz; but in a rapidly solidifying groundmass sanidine may rather be taken for albite, forming elongated first-axis and polysynthetic-twinned laths. It is interesting that sanidine, like the orthoclase and microcline derived from it, generally has no zonal structure and cannot be zoned, for in the temperature–concentration diagram of the potassium–sodium feldspars both lines separating the liquid and solid phases pass almost side by side, at an insignificant distance from one another. Therefore the statements found in various handbooks and manuals concerning zoning in potassium–sodium feldspars are based on misunderstandings; in particular, the example of strongly zoned orthoclase illustrated by Rosenbusch (29, table IX, fig. 6) undoubtedly pertains to a thin-platy oligoclase cut along the twinning plane.

Thus sanidine, as the primary potassium–sodium feldspar of young effusives, with time changes into the modification stable at lower temperatures, which in diagenetized effusives is orthoclase, as submicroscopically twinned microcline; in the case of an excess of the sodium component, the latter separates in the form of microperthitic intergrowths, always of small dimensions. This character of the regenerated mineral is explained by the following causes. The constant association of a sharply expressed microcline lattice and of broad microperthitic and even perthitic bands with those intrusive rocks that cooled very slowly, or the magma of which contained a large amount of liquid and gaseous components, as well as with crystalline schists (52, 98), compels one to think that for the formation of such coarse-platy forms a considerable external stimulus is necessary and that, in ...

in a calm and very slow process of crystallization and recrystallization a tendency toward a denser structure of the mineral is manifested.

In connection with the regrouping of the molecules there is the clouding of the potash-soda feldspar, which loses the homogeneity of its internal structure. In addition, the feldspar not infrequently acquires a reddish or yellow coloration, caused by extremely finely distributed iron oxides, which are attested by almost all analyses of these minerals and, apparently, are admixtures to the solid solution of sanidine and anorthoclase; at least, Andersen experimentally proved that the thin hematite plates of aventurine, which impart a red color to these feldspars, disappear when heated to 1235°, returning into the composition of the solid solution (59, 398); the absorption of a small quantity of iron oxides has also been proved for plagioclases (30, 93). But besides iron oxides, upon the decomposition of sanidine, alumina is also split off (50, 6), which is why orthoclase, having a fine structure, appears somewhat cloudy under the microscope, whereas microcline plates are perfectly transparent, for in microcline this clayey substance, being located between the plates, does not catch the eye. However, both the browning and the clayey clouding of a diagenetized feldspar are by no means obligatory, for the composition of the admixtures of the solid solution of feldspar may be rather varied (19, 1406—1417). There is no doubt that an analogous change may also occur as a result of the action of atmospheric agents. As for adularia, then it, as a hydrothermal mineral, received its structure at the very moment of formation and is therefore transparent.

The question of the state of the potash-soda feldspars forming the isomorphous field near the sodium component is considerably less clear. First of all it must be noted that such minerals occur comparatively rarely, being usually characteristic of alkaline effusives rich in sodium, in which case, according to the law of mass action, the acid plagioclase dissolves the potash component in itself, not allowing it to individualize as independent sanidine. Thus, observations show that in the most recent effusives the primary sodium feldspar with an admixture of potassium, analogous to sanidine and possessing transparency and a light-gray coloration, belongs to the triclinic system, revealing the finest albite striation, to which pericline twins are sometimes also joined, giving the impression of the finest microcline lattice. True, sometimes this mineral, called anorthoclase, appears homogeneous and monoclinic, for which reason it has even been proposed to distinguish a monoclinic natronorthoclase (34, 89), but one cannot agree with such a proposal, since not infrequently “natronorthoclase” and anorthoclase, having one and the same chemical composition, are found in one and the same rock (17, 400). Evidently “natronorthoclase” stands in the same relation to anorthoclase as orthoclase to microcline, having a submicroscopically twinned structure. In any case, Marx is right in supposing that the transition point of natronorthoclases into the monoclinic phase lies above the temperature of their melting, even under the hypabyssal conditions of formation of the phenocrysts of effusives (44, 102).

The theory of the submicroscopic—or microscopically twinned—structure of anorthoclases, as acid plagioclases with an admixture of the potassic component in solid solution, is the most natural and admirably explains the strange properties of these minerals. Indeed, anorthoclases usually contain up to 5 and more percent of anorthite (19, 1425); consequently, anorthoclases usually are a solid solution of potash feldspar in oligoclase, and not in pure albite. But everyday experience in the study of oligoclases, for example in granites, shows that oligoclases possess the ability, depending on the almost symmetrical arrangement within them of the axes of elasticity, to give extraordinarily fine polysynthetic twins, somewhat resembling the striation of anorthoclases, often visible under the microscope. This ability

is further intensified by the presence of potassium, whose aluminosilicates in general are finely lamellar—for example microcline, leucite, and phillipsite. It is quite possible that pure albite as well, which always crystallizes in coarsely lamellar twins, when the potash feldspar is dissolved obeys this tendency. As for the solubility of the potassium component in plagioclases more basic than albite-oligoclase, it is comparatively insignificant (40, 330); moreover, in igneous rocks, together with basic plagioclase, independent microcline-orthoclase occurs. In general, anorthoclase is albite-oligoclase in solid solution with potash feldspar.

This point of view on the relation of anorthoclase to the acid plagioclases is not contradicted by the crystallographic and optical properties of the feldspars under consideration. Thus, it is quite natural that, with a fine twin structure, a triclinic mineral mimics monoclinic symmetry—more or less completely, depending on the fineness of the twinning intergrowth; and in anorthoclases the angle between (010), which represents the twin plane of these minerals, and the summational pinacoid (001) is almost equal to 90°. Likewise, owing to the superposition of the ellipsoids of elasticity of finely intergrown twin individuals, in anorthoclases the apparent extinction angle on (001) is close to 0°, while the angle between the optical axes assumes a value of about \((-)\,40^\circ\), with variations that probably depend both on the composition of the mineral and on its structure. In a word, anorthoclase is related to “natronorthoclase” as microcline is to orthoclase, with the essential difference that the former minerals have a primary character, whereas the latter represent a diagenetized modification, stable at lower temperatures.

It is further necessary to note the following. If anorthoclases are, in the majority of cases, potassic oligoclases, then it is quite natural that they are not infrequently zonal. And it is possible that the zonality of sodium-potassium feldspars mentioned in the literature refers—precisely—to anorthoclases. It is also not without interest to observe that, in crossed nicols, these minerals often appear irregular, depending on the position of the section of the thin section relative to the fine twin individuals, the superposition of which causes changes in the position of the optical ellipsoid, as is especially well known for microclines, which possess undulatory extinction and a change of optical properties even in different parts of one and the same grain (41, 66).

Anorthoclases belong to formations metastable at low temperature, since the solubility of the potassium component in this limited group of solid solutions decreases with lowering temperature. Upon the decomposition of anorthoclase, an acid plagioclase with antiperthitic intergrowths of potash feldspar should be obtained. Observations on phenocrysts of the corresponding diagenetized effusive rocks fully confirm this, indicating at the same time that, with the limited magnitude of the displacement of the molecules of the solid solution decomposing as a result of simple diagenesis, the complex mineral is characterized by a fine structure—chiefly of the “chessboard albite” type, with veinlets and cement of orthoclase between the scattered twinned areas of plagioclase (43, 127). It must be said that such decomposition is sometimes manifested also in basic plagioclases, with the formation, of course, of a few antiperthitic intergrowths of potash feldspar (76, table II, fig. L). It goes without saying that the decomposed anorthoclase becomes cloudy and usually acquires the reddish coloration characteristic of orthoclase-microcline.

A less abrupt change during diagenesis is shown by the plagioclases, although this change proceeds in two directions. First of all, according to the temperature-concentration diagram of the plagioclases, whose curves diverge strongly from one another (54, 583), these minerals have a greater tendency

to the formation of zonal crystals—especially in the acidic half of the series, for the corresponding acid magmas are distinguished by a comparatively high viscosity, which hinders diffusion and the equalization of the composition of the separated solid phases of plagioclase. Since the phenocrysts of effusives are usually formed under hypabyssal conditions of rather rapid cooling, the plagioclases of the phenocrysts are always zonal; moreover, oligoclase-andesines represent, as it were, the geometric locus of zonal surfaces, for which reason they cannot be subjected to exact optical investigation. Such are the primary plagioclases of effusives, distinguished both by transparency and by a glassy luster. But with the passage of time diffusion takes place between the zones of the mineral, which becomes increasingly homogeneous, although even in very ancient formations the zonality does not disappear, since diffusion in solid solutions extends over only a small distance. It is clear that, with such a rearrangement of molecules, and in connection with the indefiniteness of the boundaries between the zones of the diagenetized plagioclase, the latter becomes turbid. Of course, this direction in the alteration of the minerals under consideration does not apply to albites, as primarily homogeneous formations, nor also in part to very basic plagioclases, which have time to equalize their composition while still in the intratelluric stage of the life of the liquid magma.

But besides the molecular rearrangement connected with diffusion between the different zones of the mineral, in plagioclases there not infrequently occurs the exsolution from the solid solution of certain admixtures, such as Fe₂O₃, FeO and MgO, found in all analyses of these minerals, as may readily be seen, for example, in Hinze’s handbook (19). But it is interesting that only the albite of effusives undergoes the same reddening as the potash feldspars, acquiring under the microscope a turbid appearance from a fine impregnation with an alumino-ferruginous substance. As for the remaining plagioclases, beginning with oligoclase, they become opaque, retaining a dirty-white color, sometimes with a dirty-greenish tint, distributed usually in zones and spots. Such a difference in the coloration of diagenetized albite and of the other plagioclases is a consequence both of the composition and of the structure of these minerals, not to mention also the composition of the groundmass of the effusives and of the neighboring rocks, which exert no small influence on the character of alteration of the phenocrysts as well. First of all, one must note the difference in the structure of the minerals considered: whereas more or less pure albite appears in rare and broad twinned individuals, oligoclase and the other plagioclases become finely polysynthetic and, in addition, more or less finely zonal. Thus albite possesses a greater structural viscosity, characteristic of potash feldspars, and, like the latter, is little accessible to external solutions; on decomposition it releases only a clayey-ferruginous pigment, finely distributed and usually imparting a reddish coloration to the mineral. It is possible, however, that such a character of the pigmentary admixture belongs only to the alkali feldspars—just as alkaline rocks are relatively rich in alumina and iron oxide. As for the true plagioclases, in them there soon appears a mass of hairlike cracks along the planes of intergrowth of twins and along the zonal surfaces—cracks through which thin solutions, occluded in the effusive and migrating chiefly toward its groundmass, can easily penetrate. These solutions already produce a sufficient alteration of the mineral in a metasomatic manner—all the more because the anorthite molecule is comparatively readily susceptible to chemical action. As a result there are obtained predominant sericite and clay of indefinite composition, then often calcite, epidote, zoisite, and chloritoserpentine with the precipitation of free quartz. The inner zones, as those rich in anorthite, and also the basichromatic periphery of phenocrysts, corroded by the groundmass of the effusive, are especially strongly altered. Thus to the natural diagenesis of plagio-

of effusives, expressed in the destruction or weakening of zoning and in the exsolution from the solid solution of certain impurities, is almost always accompanied by a chemical change in the minerals, proceeding in the manner of their partial replacement, as may be judged from the fact that the optical composition of the plagioclases of analogous primary and diagenetized eruptive rocks remains the same. Of course, such fine new formations are very varied, depending on the composition and structure of the groundmass, which constitutes the principal part of liquid solutions; then on the presence of additional cracks and on the depth to which the effusive descended during its life, and also on the character of the adjacent rocks, for the latter may supply material for the process described, which, at the suggestion of Academician Fersman, may be called katagenesis (72, 30), and which therefore has significance also with respect to the erupted rocks; unfortunately, the results of such recrystallization and alteration of the plagioclases of effusives may be to a considerable degree masked by analogous products of hypergenesis, or surface weathering.

Comparatively little help in establishing the character of an effusive is provided by the dark-colored components, which in general are prone to rapid decomposition. Nevertheless, some observations may be made concerning the primary and diagenetized state of these minerals. Thus, among the pyroxenes, which are the most important dark-colored components of effusives, there are cases of zonal structure serving as an expression of their hypabyssal origin; these include titaniferous augite, magnesian diopside, and aegirine-augite. It goes without saying that such an inhomogeneous structure cannot long be preserved by pyroxenes, in which diffusion processes proceed comparatively rapidly; therefore the presence of zones and sectors in these minerals is a reliable indication of the primary character of the corresponding rock. In addition, various impurities are often dissolved in pyroxenes, represented chiefly by oxides of heavy metals and liable to precipitate at low temperatures. But almost always, parallel with the internal regrouping of molecules during the diagenesis of pyroxenes, their chemical alteration also takes place under the influence of those thin solutions of endo- and exogenic origin which could exert such great influence even on the more stable plagioclases. The character of these changes may be extremely varied, depending on the factors indicated above; the most widespread secondary products are chlorite-serpentine, calcite, and epidote with accompanying oxides of iron and titanium and with quartz. But it must be noted that not all individuals of pyroxenes of a given rock undergo the same alteration, and some remain altogether without alteration: everything depends on whether the given mineral was subjected to resorption or not, and on how much it is broken by hairline cracks, as well as on what the composition of the liquid solution proved to be in the given part of the rock.

As for biotite and amphibole, these minerals, occurring almost exclusively among the phenocrysts of effusive rocks, are in themselves formations foreign to the effusive, undergoing resorption and being in a state of extreme internal stress from their abrupt quenching. It is therefore natural that they alter very rapidly, usually becoming covered with precipitating oxides of heavy metals. The alkaline pyroxenes are especially unstable—these delicate minerals, ephemeral in the setting of an effusive, which is so porous and fissured and so convenient for the wandering of various solutions: alkaline pyroxenes are readily leached, usually leaving a smeared oxide of iron, present in excess in the composition of these minerals.

Curious is the relation of olivine to dia- and katagenesis. Ordinarily this mineral is not in a state of internal stress, representing a normal solid solution. And it must be said that it is sometimes preserved in

in very ancient epicontinental effusives. But for the most part olivine quickly falls victim to thin solutions, turning into various kinds of serpentine aggregates with the separation of veinlets or ore disseminations. The point is that this orthosilicate reacts very readily with chemical reagents, and the presence of a few tiny cracks, by which the grains of olivine—as a brittle mineral—are so often traversed, is sufficient for the thin solutions of the effusive to penetrate along them and do their destructive work. This process finds especially favorable ground in the case when, before the lava solidified, the olivine had already begun to dissolve, so as to begin a complete transformation into metasilicate; for, according to Andersen’s investigations, olivine not infrequently represents only an intermediate phase which, during normal crystallization of the magma, passes wholly or partly into pyroxene (58, 287). It is possible that iddingsite, encountered according to Rosenbusch also in basalts (32, 1189), represents precisely the result of superficial burning of dissolved olivine, the alteration of which will not, therefore, be a feature characteristic only of diagenetized effusives.

Among the principal components of igneous rocks, the feldspathoids are also of known importance; to these, as primary minerals of effusives, belong nepheline, sodalite, hauyn, nosean, analcime (79, 530), and leucite. Some of these minerals are in general metastable at low temperatures; thus, leucite is prone to decompose into orthoclase and nepheline, while nepheline contains a limited amount of plagioclase in solid solution (49, 551). And all of them are very unstable with respect to ordinary solutions, especially those containing chlorine and sulfur. This is also evident from the fact that feldspathoid effusives are described only from the most recent formations. True, Rosenbusch says that by phonolites and other feldspathoid effusives are meant both neo- and paleo-volcanic rocks, and that from the example of these formations one may be convinced of the absence of the need for a dualistic classification of the erupted rocks (32, 956); but, turning to the bibliographic lists, we find descriptions almost exclusively of very recent feldspathoid effusives, while the very rare cases recognized as Paleozoic representatives of this group in reality prove to be either already diagenetized or to be of a younger age. Thus, the leucite rock from France cited by Rosenbusch as Carboniferous (32, 1370), according to the description of M. Lévy and Lacroix, is completely diagenetized, with replacement of rounded crystals of leucite or analcime by albite (18, 25). But especially interesting is Rosenbusch’s reference to the Paleozoic leucite phonolite from the Podkamennaya Tunguska River, described by Khrushchov (32, 956). Indeed, Khrushchov says (according to Lopatin’s diary) that this rock, which has an entirely fresh appearance and contains such minerals as sanidine, anorthoclase, nepheline, and leucite, is interbedded with Silurian limestones (15, 224). But it is now well known that the traps, to which this rock also belongs, formed in the Tunguska basin not earlier than the Permian period and continued to be erupted, perhaps, even in the Tertiary period, occurring in the Cambro-Silurian sequence exclusively in the form of dikes and sill-like deposits (65, 27). In any case it is perfectly clear that the primary rock under consideration belongs to the youngest effusives of the region. Thus, for us there can be no doubt that the presence in an effusive rock of such ephemeral minerals as feldspathoids is a sure sign of its primary phase.

As regards the nature of the alteration of feldspathoids during the diagenesis of rocks, it is difficult to say anything definite. These minerals partly decompose into their constituent components, but much more often they undergo alteration under the influence of penetrating thin solutions and, depending on the composition of the latter and on other physico-chemical factors, yield a whole series of products,

such as: zeolites, usually appearing in the form of small spherulites, thin-fibrous aggregates, sometimes calcite, and some infiltrations. We are familiar with these feldspathid alterations properly from the results of the chemical weathering of the most recent effusives; as for the present type of dia-genetized feldspathid effusive rocks, it is little known to us, for the products of alteration of feldspathids are usually recognized in porous rock only very rarely—for example, in a libenerite vein porphyry—giving regular pseudomorphs by which such an altered rock could be assigned to alkaline formations.

As for the ore admixtures, they are generally stable minerals, if they occur in grains of more or less noticeable size. But small grains, as having a comparatively large surface, experience a clear influence from the thin solutions of the effusive, especially such complex formations as titaniferous magnetites, which easily decompose and give a leucoxene coating. In general, however, opaque minerals in petrographic objects have still been studied so little that they cannot be developed from the point of view advanced here.

Up to now we have considered the alteration of the solid phases of the components of effusives chiefly in phenocrysts, which by their size are the most convenient for study. But now one must turn to the groundmass, which often constitutes the whole effusive. First of all let us consider volcanic glass, which is widely distributed among acid lavas, as having great viscosity and a tendency to supercooling, and is relatively rarely encountered among basic effusives, whose lava is characterized by fluidity and crystallizes fairly well even with rapid cooling. It is interesting to note that macroscopically volcanic glasses usually have a black color whatever their composition, since, according to experiments, even such a content of heavy metals as is present, for example, in granites is sufficient to impart a dark color to glass; and only in wholly porous pumice, consisting as it were of thin films of glass, does the latter appear dirty-white. With such a uniform external appearance, volcanic glasses are almost indistinguishable by their composition; it can only be established that acid varieties are rather transparent in thin splinters and under the microscope appear light, whereas basic glasses, in which crystallites of ores and dark-colored components always appear, are dark even in thin section, and in hand specimen appear cloudy. Of course, often an effusive, or its principal mass, does not consist entirely of one glass, which cements the crystals of the stronger components that had managed to separate during the effusive stage. However, we find such a picture almost exclusively in basic rocks, during the rapid crystallization of which, preventing the equalization of phases, the residue of the liquid phase becomes so enriched in silica that, under the given conditions of cooling, it solidifies in the form of glass, with the production of a hyalopilitic and intersertal structure, whereas an acid lava either crystallizes entirely or solidifies in the form of glass. At the same time it must be borne in mind that vitrophyric effusives, as the most superficial formations, have a tendency to acquire breccia-like and porous textures, the latter being characteristic chiefly of basic lavas, which retain fluidity until rapid solidification, fixing the cavities left by gaseous exhalations.

Glass represents a supercooled liquid phase and—therefore—is very unstable, readily recrystallizing, which occurs rather quickly, as is indicated at least by the example of old window glass. Therefore the presence in an effusive of still clear glass may serve as evidence of the primary character of the rock. True, in descriptions of sufficiently ancient, for example Paleozoic, effusives we very often encounter mentions of

on a glassy base; but in the vast majority of cases this is a misunderstanding, as applied to basic rocks, whose glass, upon recrystallization, becomes clogged with a dense dust of opaque oxides of heavy metals, producing the impression of an isotropic mass. Only rarely do such indications correspond to reality. For example, weiselbergite from the Saar–Nahe region (32, 1093), containing clear glass and possessing phenocrysts of the primary phase, is associated with the red bed of the Permian system; but this formation lies here undisturbed, and the effusive rock could preserve its primary properties—just as the Cambrian clay of the environs of Leningrad, forming part of deposits of small thickness, still possesses excellent plasticity.

The disintegration of volcanic glass occurs partly in the solid state; but to a considerable degree it is assisted by those liquid and gaseous substances which are dissolved and occluded in this glass and which readily begin to ferment when the molecules are set in motion, the impulse for such processes being an increase in pressure and temperature. The results of the transition of glass into the crystalline state are quite diverse, depending on its composition; but for a general characterization of these results it is sufficient to dwell on the fate of acidic and basic volcanic glasses. Acidic glass contains the elements of alkali feldspars and free silica, with a small amount of oxides of magnesium, iron, and manganese. It is clear that in the very fine and unevenly granular mass obtained during the recrystallization of acidic glass, and which bears the name microfelsite, quartz is quite definitely outlined as a mineral stable at the rather low temperature of the process and in general not susceptible to chemical changes; as for the feldspars, they likewise must appear in stable modifications, although the possibility is not excluded of their partial decomposition under the influence of the thin solutions of diagenesis. In any case, microfelsite gives the impression of a mass with individualized components, usually only slightly darkened by the finest dust of oxides of heavy metals and, in part, chloritoserpentine, which separate from the glass without binding into the pyrogenic composition the dark-colored minerals, as very unstable in the presence of thin solutions. It must be thought that the oxides of this dust, no longer susceptible to more precise determination, contain to a considerable degree hydroxide, the presence of the elements of which has been proved by analyses of fine impurities in all primary effusives; moreover, pechstein contains up to 10% water. Be that as it may, the recrystallized acidic glass of normal effusives of epigenetic formations is always colored in various shades of red, caused above all by finely distributed in the rock, diagenetized alkali feldspars, partly, perhaps, decomposed, and then by the fine dust of partly hydrous oxides of heavy metals, the various combinations of which also contribute to the diversity of color shades in felsites.

As for the alteration of basic volcanic glasses, it is more complex and less definite. The point is that here, parallel with the recrystallization of the glass, there also occurs decomposition of the normal minerals possible for the composition of the rock, which do not have time to appear and enter into reaction with the fine solutions of diagenesis, forming such products as acidic plagioclases, chloritoserpentine, nosite, epidote, calcite, and the very conspicuous oxides of heavy metals, which clog the whole mass of the rock, as well as secondary quartz. As a result we have aphanite, in which the green coloration of the chloritoserpentines mixes with the red shades of the metallic oxides, so that a complex lilac coloration is obtained (20, 359), with different shades depending on the absolute and relative quantity of the oxides of iron, manganese, and partly titanium. It is quite possible that the shades of coloration fully correspond to the composition of the rock, but the corresponding-

...no investigations were carried out, and in general no attention was paid to the coloration of the rocks.

Thus, vitrophyric rocks, during diagenesis, are transformed into felsites and aphanites, which have an argillaceous appearance and are characterized by shades of red, ever darker as the basicity of the rock increases. The character of diagenesis does not change if the glass constitutes only part of the groundmass, for it finely impregnates the rock, imparting to it externally its own properties; moreover, when the microlites of the groundmass are small, the latter undergo an analogous alteration. This is the case chiefly with more or less basic effusives, in which both microlites and a glassy mesostasis may form in the effusive phase. As for acid lavas, they either solidify as glass, or crystallize completely, or, finally, yield glassy masses with original aggregates of forms of rapid growth of crystallites of quartz and alkali feldspar, often forming continuous areas as well and deserving special consideration below.

Passing now to holocrystalline effusives, we shall note first of all that such rocks are characteristic chiefly of basic formations, and that in acid effusives, derived from very viscous and readily supercooled lava, predominantly glassy and crystallitic masses develop. The latter have micropoikilitic, micropegmatitic (granophyric), microfelsitic, and lithophysoidal structures, representing fine intergrowth or overgrowth of tridymite-quartz and alkali feldspars. They are formed in part with the very close participation of an excess of mineralizers, which manifest themselves especially in separate areas, also promoting the occurrence of banded texture in rapidly cooling acid lava. It must be said that rocks of such structures rarely possess the normal light-gray coloration, sometimes acquiring greenish tints in the case of the development of alkali pyroxenes in the groundmass, for under the influence of gaseous components lowering the melting temperature of the minerals, the alkali feldspars undergo diagenetic alteration already in the post-volcanic phase, and the free silica—especially in flow banding (43, 135)—crystallizes directly in the form of quartz. But primary-normal acid effusives with the indicated structures of the groundmass also alter diagenetically rather quickly, since their components have very fine grain and a fairly loose arrangement, under which conditions the fermentation of fine solutions must proceed at an accelerated rate. As for the normally crystallized acid groundmass, usually called microgranitic, it is in fact not encountered in true effusives, being characteristic of vein and hypabyssal formations.

On the other hand, more or less basic lavas, being under some cover—if only a layer of water—or in the central parts of covers and flows, where cooling is somewhat slowed, crystallize rather readily, yielding, depending on chemical composition, various masses of trachytic, microlitic, pilotaxitic, or ophitic structure. The rocks formed in this way consist of normal primary minerals, sometimes characterized only by crystallographic forms, and acquire a simple primary coloration: when feldspars predominate, especially alkali feldspars, the rock is light gray, sometimes with a greenish tint caused by alkali pyroxenes; when a sufficient quantity of femic components appears, the rock darkens and, beginning with andesites, becomes almost black, which is natural with sufficiently fine intermixture of light and dark substances and with the transparency of the salic minerals.

Of course, holocrystalline effusives also undergo a diagenetic alteration analogous to that which is known to us from phenocrysts. Thus, alkali feldspars give reddish tones to rocks that consist essentially of these minerals. As for plagioclase effusives, when they contain a large quantity of dark-colored components and ores, a tendency appears toward the acquisition of a dark-lilac and dark-greenish-gray coloration—depending chiefly on the structure of the groundmass of the rock. Namely, lilac shades are obtained in rocks with a very fine-grained and at the same time usually somewhat porous mass, since in this case there are many adsorbed fine solutions, which, moreover, more readily alter mineral grains with a relatively large surface; in addition, it is possible that in such masses there was also glass, the insignificant quantities of which are difficult to establish, and whose decomposition provides material for the lilac coloration of the rock. With a coarser grain of the rock, characteristic among effusives exclusively of basic formations—for example, dolerites, which compose the central parts of thick sheets—the effusive tends to preserve its primary color and primary composition, undergoing diagenetic alteration only with difficulty; this is due to a number of reasons. First of all, such rocks contain few adsorbed fine solutions, which moreover have restricted paths for their movement. Therefore the circulation of gaseous and liquid components is limited here, and each mineral component is altered more or less independently of neighboring grains. Further, if the ore components do undergo some transformation, its products do not impregnate the mass of the rock and do not impart to it that peculiar lilac coloration which represents a derivative of the dust (streak) of the oxides of heavy metals. Finally, the ophitic or poikilo-ophitic structure characteristic of dolerites gives them such a dense texture that the possibility of catagenetic alteration of the rocks is excluded. Therefore the diagenesis of the basic effusives under consideration is reduced to the simple transition of the solid phases of minerals into more stable polymorphous modifications, and is generally expressed very weakly, since with sufficiently complete crystallization of the basic lava, plagioclase and augite are obtained at once in homogeneous or very weakly zoned individuals. It is true that, in the course of time, under the influence of fine solutions circulating between the grains of the rock and along accidental cracks, pyroxene and olivine may tend to yield chlorite-serpentine products, but the latter not infrequently also appear in quite young formations, being in part of postvolcanic origin. In a word, it is very difficult to divide dolerites, as well as the hypabyssal and vein rocks of corresponding composition that are extremely similar to them, into primary and diagenetized phases. Hence arose confusion in the designation of such rocks, called in some cases dolerites or traps, and in other cases diabases—all depending on their age and degree of alteration.

It is worth dwelling a little on the nomenclature of basic igneous rocks. At present there is probably no particular disagreement that diagenetized basalts should be called melaphyres, and anamesites—differing from basalts by an almost complete absence of porphyritic structure and a sufficiently phanerocrystalline texture with a certain amount of glassy mesostasis—should be called toleiites. During the diagenesis of these rocks, both fine-grained and hypocrystalline, their external appearance and color change so sharply that there is no difficulty in separating primary from diagenetized phases of effusives. But dolerites are almost not altered diagenetically, or, owing to the formation of chlorite-serpentine, acquire a slightly greenish coloration, which compels one to bring these rocks closer to vein or greenstone formations and to call them diabases. It is clear that such a designation is incorrect from the genetic point of view, since diabases have come to be understood as...

take vein and greenstone basic rocks (26, 104). From this point of view it is entirely unacceptable, for example, that Swedish petrographers propose calling “gune—,” “konga—,” and other diabasic recrystallized effusives connected with the Cambro-Silurian epicontinental, very quietly deposited formation of Scandinavia, and macroscopically almost indistinguishable from the most recent rocks of the dolerite type, diabases. Thus, if—indeed—there exist holocrystalline effusive dolerites, then their ancient phases merit a different designation; it would be most convenient to call such rocks, comparatively rarely encountered among true effusive formations, palatinites (32, 1161). As for the term “trapps,” it is applied to all primary basic effusives and has a collective character.

Thus, the diagenesis of effusives consists not only in the alteration of solid phases passing into more stable modifications, but also in the appearance of mineral neoformations that are the result of the action of subtle solutions of both endo- and exogenous origin. That some quantity of substances, liquid and gaseous at ordinary temperature, is present in effusives and can enter them from neighboring rocks, especially sedimentary ones, cannot be doubted. Doubt may rather arise as to whether, under the quiet conditions of epicontinental formations, reactions sufficient for mineral neoformations could occur. However, comparison with sedimentary rocks, which during diagenesis sometimes undergo very considerable changes, the sharp manifestations of diffusion in the solid mineral phases of the effusives themselves, and, finally, the direct comparison of young and ancient effusive rocks—all indicate that the various mineral neoformations are by no means a product of weathering or accidental alteration, but represent the result of the inevitable transformation, or diagenesis, of effusives, accompanying the transition of solid phases into stable forms. If these neoformations are often distributed in the rock very unevenly and have different expression, this depends both on the composition of the given and neighboring rocks and on the structural and textural features of the effusive, both primary and secondary; it is possible, however, that with the closest study of the diagenesis of effusives, more definite regularities will be revealed in the manifestation and in the order of appearance of the corresponding secondary products.

5. The greenstone phase of effusives.

If effusive rocks undergo the rather substantial changes just described under very quiet conditions, chiefly under the influence of diffusion of solid, liquid, and gaseous solutions, then they are subjected to still more considerable transformation under the action of great directed pressure, especially that developing during tectonic processes in geosynclinal masses. Indeed, in connection with intensified tangential pressure, in rocks, along the mass of forming cracks, there occurs a displacement of matter that disrupts the internal structure of the components, brings heterogeneous formations into contact, assists the movement of subtle solutions, and—consequently—causes diffusion and intensified chemical reactions, which at the same time proceed in a special direction.

There is no doubt that the transformation of metastable phases in this case occurs with great speed while producing approximately the same forms as during simple diagenesis. There are, however, some differences as well. Thus, microperthitic intergrowths may develop into larger and even independent individuals, and potassium feldspar may manifest itself in the form of microcline, for the pressure associated with displacement allows the molecules to pass

a greater distance, in order to produce formations more significant in size.—Furthermore, when potash feldspars are altered, a red coloration never appears; this depends on the different composition of the newly formed minerals, which are characteristic of oriented pressure and are evidently characteristic of rocks that have undergone such pressure. The manifestation of such new formations may be explained both by the law of smaller volume and by those small movements that affect the rock under strong lateral pressure.

The theorists of crystalline schists, to the upper zone of which the formations under consideration may partly be assigned, have shown that, under sharp tangential pressure, minerals with a small specific volume or with a high specific gravity readily develop. Therefore, instead of kaolin, which contaminates the altered feldspars, mica is preferentially obtained in the form of fine sericite, and from the dark-colored components chlorite is readily formed (39, 53). And since these secondary minerals incorporate all the hydroxyl and a considerable part of the iron oxides, the rock cannot acquire a red or brown coloration, but is colored in green tones, caused by the fine distribution of chlorite and partly of sericite.

The formation of sericite and chlorite, as the most characteristic derivatives in the alteration of unstable effusives under the influence of tangential pressure, may also be explained by those internal displacements that occur in the compressed rock. Sander very vividly brought out the latter circumstance (46, 301), pointing out that such fine-lamellar substances as sericite and chlorite appear as a natural lubricant, necessary for facilitating internal displacements (77, 207), and therefore form readily even at a low temperature, causing, together with the rubbing of displacement surfaces, the phyllitic character in strongly schistose rocks. True, effusives undergo comparatively weak schistosity, but they contain much material for the construction of the indicated minerals, which readily arise because of the stressed state of these rocks, changing considerably, as we have seen, even during simple diagenesis.

But if effusives that have undergone lateral pressure are subjected to greenstone alteration, which is generally characteristic of metamorphic rocks, for example, of the upper and partly the middle zones of regional metamorphism, then doubt may arise as to the necessity of distinguishing a special greenstone phase of effusives not belonging to metamorphic derivatives. Of course, greenstone effusives are very close to the corresponding members of the upper zone of regional metamorphism, or—better to say—they may pass into formations of this zone; yet there are differences between the forms compared, both of a fundamental and of a mineralogical character. First of all, the greenstone phase of effusives appears whenever these rocks come under the influence of appreciable lateral pressure, and consequently it may be paralleled with argillaceous shales, which pass into true metamorphic schists only with a special degree of pressure and temperature, when the entire mass of the rock recrystallizes. Thus, greenstone effusives do indeed represent a phase of special alteration of primary effusive rocks, analogous to the diagenetic phase. One must even think that between the primary and the greenstone phases the diagenetic phase of effusives, as an intermediate formation, does not have time to manifest itself. In fact, geosynclinal strata do not undergo diagenetic hardening, retaining their plasticity up to diastrophism itself, which otherwise could not have manifested itself; then, during diagenesis, there already occurs so thorough a fixation of the hydroxyl and other characteristic oxides among the components that, for redistribu—

…of these radicals, already having entered into sufficiently stable compounds, requires—specifically—a metamorphic process.

To illustrate the relationship between the diagenetic and greenstone phases of effusives one may cite a very interesting example from the section along the Tom’ River between the Belsu River and the city of Tomsk. Here, at the base of the Famennian stage of the Upper Devonian, there is a horizon of effusives represented by albitophyres and metaphyres with their tuffs and breccias. On the foundation of the Kuznetsk Alatau, in connection with the epicontinental Paleozoic of the Kuznetsk Basin, these pyrogenic rocks underwent “normal” diagenesis, giving rise to red tones that have been preserved even in those cases where, under the influence of later pressure from the rising block of the Alatau, the effusives were subjected to very strong schistosity. But to the NW of the basin, in geosynclinal argillaceous sediments of the same age that took part in the structure of the cover of the Tomsk charriage, the effusives already possess a greenstone character, being associated here with argillaceous and marly shales, as is clearly seen, for example, near the village of Mitrofanova.

Thus there exists a clear, fundamental distinction between the greenstone phase of effusives and those greenstone rocks that are produced as the result of true metamorphism. But an objective boundary between these formations must also be found according to their petrographic composition. The problem posed is difficult and can be solved only in approximately the following way. First of all, the structure and texture of effusives in their greenstone phase are in general preserved, although they may be established properly chiefly under the microscope. This does not contradict the proposition that greenstone effusives were subjected to considerable lateral pressure, for these rocks, as massive formations, resist perceptible schistosity rather well; this is often absent where the adjacent sediments have been transformed into true shales. Evidently, tangential stress is resolved in submicroscopic displacements and—especially—in the formation of the indicated finely foliated minerals with low specific volume. If, however, an effusive is compressed to the state of a schist, with complete destruction of structure and texture, then a new rock is obtained, one that merits a special name and belongs to the group of Sander’s tectonites (47, 257), even though no new minerals at all may have appeared in it. As regards the mineralogical diagnosis of the greenstone phase and of metamorphic effusives, it appears less definite. Apparently, for the greenstone phase there are no other more characteristic minerals than sericite and chlorite, which can also form under sufficiently cold conditions (77,248). True, sometimes in descriptions of phyllites, as purely dynamometamorphic formations, amphibole is mentioned (78,540), but this applies to other primary rocks and, probably, already to actual shales of the upper zone of true metamorphism. Of course, besides sericite and chlorite there may also be other secondary products, encountered also in diagenetized effusives, for example epidote and calcite, but there can hardly occur any noticeable decomposition of plagioclase with separation of albite, so characteristic of metamorphic shales.

Consequently, by the greenstone phase of effusives one must understand the result of their transformation into a stable state in the presence of considerable pressure, directing the diagenesis of these unstable rocks along a special path, toward the formation of sericite and chlorite, which finely impregnate the effusive and impart to it a more or less distinct coloration, replacing the reddish tone of true diagenetized effusives. Of course, in this phase the structure and texture of the rocks and the composition of their principal components remain without noticeable change; but when the rocks come under the action of considerably elevated temperature and pressure, then they corres—

are recrystallized by metasomatism or lose their structure and must already be assigned to metamorphic formations; moreover, the effusives of the upper zone of these crystalline schists sometimes differ little from their greenstone phase.

The effusives of the greenstone phase, like simply diagenetized effusives, must have special names, which are absolutely necessary if rocks are to be regarded as geological bodies. Unfortunately, almost no such names have been proposed—or, rather, they cover too large groups of forms, which is explained by the uniformity of outward appearance and, in general, by the considerably altered state of greenstone effusives: even acidic rocks, poor in dark-colored components, are comparatively well recognizable, possessing light tones of green coloration, which often do not affect the phenocrysts; but basic effusives, by contrast, become so clogged with readily shifting chlorite that their exact determination indeed meets with enormous, sometimes insurmountable difficulties. To this must be added that, owing to some crushing of greenstone rocks, their components in general are more readily altered under the action of both internal and external solutions. Thus, for the designation of greenstone effusives only two names have been proposed: keratophyres for acidic rocks and diabases for basic rocks. The first name later came to be applied to designate diagenetized forms of alkaline effusives; but since the latter also have other names, the term “keratophyre” should be left with its original meaning. As for the name “diabase,” or “bedded greenstone rock” (32, 1227), it continues to be used by most petrographers to designate the greenstone phase of more or less basic effusives, although, for example, Rosenbusch (32, 1228) is inclined to restrict the scope of this concept to the greenstone group of the trap family and even only to its phanerocrystalline varieties, which more often have a vein or hypabyssal character. Such a restriction could be recognized as entirely correct if, in parallel, names had been advanced for other families of basic effusives. But since no such proposal has been made, and it is difficult to make one in view of the almost complete impossibility of distinguishing the greenstone analogues, for example, of augite andesite and basalt, for the time being the name “diabase” must be extended to all greenstone rocks of the basic series. In this case such terms as “spilite” and “variolite” acquire an entirely independent particular meaning, and the rocks denoted by them must correspond to structural varieties of primary and diagenetized effusives. It is very possible that if the division of effusives into phases proposed here is accepted and carried through to the end, then by means of literature searches and special proposals it will be possible considerably to expand the inventory of petrographic terms and, at the same time, to make the greenstone phase of effusives more or less equivalent to the phase of diagenetized forms of these rocks.

6. Secondary alterations of effusives.

Thus, unstable effusives of the primary phase, usually having a light-gray or black coloration, under normal diagenesis pass into red-lilac formations, and under the influence of pressure are transformed into forms of the greenstone phase. Consequently, the phases of effusives are excellently distinguished by their coloration, which serves as an expression of their mineralogical composition. But this is so only in the scheme. There is a whole series of exceptions, some of which deserve special consideration.

Thus, the upper parts of lava flows and sheets, owing to contact with the oxygen of the air, are as if burned—oxidized—acquiring respectively a red and lilac coloration and resembling diagenetized forms, although

mineral phases remain almost entirely in their primary state. This mimetism, which can be foreseen and which has been noted by a number of investigators—for example, Lacroix on the Caucasian lavas (56, 218 and 234)—is capable of misleading one at a first acquaintance with the corresponding part of an effusive body; but the truth is quickly restored as soon as observation is extended to the whole thickness of the effusive rock, or as soon as the latter is subjected to microscopic analysis.

Next, no less interesting are the changes in effusive rocks under the influence of post-volcanic processes. It may now be considered fully established that the gaseous components liberated at the end of the crystallization of magma produce, in the same erupted rock, rather considerable changes in the form of chloritization, actinolitization, serpentinization, albitization, and silicification. True, in the main mass of the rock these changes are little noticeable, being confined to separate areas or even grains (75, 170), but they are encountered, in any case, in the freshest rock itself, and they are usually attributed—of course, incorrectly—to the agents of weathering or to later metamorphism. Thus, if an effusive rock, generally fresh and primary in its main mass, contains here and there grains of components that have undergone decomposition or transformation into a stable modification, this still does not mean that the given rock has already begun to undergo diagenesis or greenschist alteration, processes which must embrace the whole mass of the rock; no, such sporadic changes must be ascribed to the action of gaseous substances that accumulated in small quantities at separate points and moved along accidental hairline cracks. But sometimes volcanic emanations, whether of endo- or exogenous origin and having gathered in large masses, permeate the enormous thickness of the effusive rock, converting it into a false-greenschist formation by means of propylitization, pyrophyllitization, onkosinization, etc.; with such processes the deposition of ores is usually associated, predominantly sulfide ores, forming veins or disseminations. These false-greenschist forms of effusive rocks are not difficult to distinguish by their association with mineralization, by the presence of peculiar very fine-scaly minerals that sometimes clog the whole rock, and by the absence in the latter of cataclasis and a schistose texture, which is a necessary condition for complete greenschist alteration and metamorphism of rocks. In addition, it should be noted that, with weak treatment by volcanic gases, acidic rocks undergo coloration, although they do not pass entirely into stable phases.

To this it must be added that propylitized effusive rocks readily undergo subsequent kaolinization and alunitization caused by the activity of vadose solutions. In general, external agents alter effusive rocks, as they do other rocks, into clayey-ferruginous formations, sometimes impregnated with calcite and usually appearing as dense earthy masses which are either colored brownish-red tones or are light greenish-white—depending on the degree of oxidation—and are irresolvable under the microscope. Yes, weathering and the action of surface waters do produce their peculiar products, expressed by clay and various oxide zones, and it is in vain that, in descriptions of rocks, especially effusive rocks, the appearance of secondary minerals is so often explained precisely by the influence of weathering!

In conclusion, it must be noted that the regularity of the change in color of effusive rocks, depending on the phase of these rocks, is sometimes rather grossly violated—at least if one judges by descriptions of collections of effusive rocks. Thus, one may read here and there (43, 142) that keratophyre or even diabase has a black color. In many cases the conditions of occurrence of such rocks turn out not to have been clarified, and it is possible that they form dikes, while veined, as also hypabyssal, rocks differ significantly in their textural and morphological properties.

markedly from effusives, even with identical composition and structure. Further, the deviation of an effusive’s color from the norm may be due to metamorphism that was not noticed in describing the rock; for example, effusives darken and become black under the influence of contact metamorphism. But sometimes, indeed, we find an effusive of a special color, which is always the outward expression of the special character of the products of diagenesis or of zeolitic alteration of the rock. In most cases it can be shown that such anomalies, generally rare, are caused by the influence of some adjacent rock that imparted to the solutions a distinctive composition. Thus, some black diabases prove under the microscope to consist almost exclusively of calcite with a fine dissemination of magnetite, while retaining the structure by which alone the original character of this rock can be recognized. In general, the study of rocks that deviate in their color—and consequently in their mineralogical composition—from the properties of the corresponding effusive phase can yield very interesting results for elucidating the genesis of formations; and attention must be paid to such cases both during field mapping and in the processing of materials.

Finally, cases are possible in which, under the influence of abundant and weakly acting chemically solutions of the country rocks, effusives retain their primary mineralogical composition while completely changing their color. Gerasimov described a flow of Caucasian andesite that had acquired a white color as a result of being permeated by gypsum leached from the enclosing sedimentary rocks (80, 399).

7. Classification of effusives.

The proposed strict distinction of the phases of effusives must considerably complicate the classification of these rocks and at the same time make it more natural. A complete revision of the classification of effusives is an extremely difficult task, since, for the precise determination of these rocks, the combined application of both mineralogical and chemical methods is necessary, and since, owing to the ease of fine differentiation in magmatic chambers and in feeder conduits, a large number of slight lava varieties is obtained. This task must be the object of a separate special investigation; here it is important only to show what form the classification of the principal groups of effusives assumes if their different phases are taken into account.

However, even in such a simplified form the problem posed is not especially easy. The point is that the usual qualitative mineralogical classifications of igneous rocks, for example that of Rosenbusch, are very schematic and do not include a whole series of rocks which have now gained right of citizenship and a high standing among other igneous formations. On the other hand, the new classifications are still poorly developed, and one cannot use them without some criticism and without introducing certain corrections. These new classifications are either wholly or partly quantitative mineralogical ones. Of these, Johannsen’s classification (63 and 67), being very orthodox and representing an analogue of the quantitative chemical classification of the four American petrographers, cannot be used for our purpose—if only because it is too detailed; and it is very difficult, if at all possible, to fit into it those forms of effusives with which we are ordinarily familiar. Among other schemes proposed up to now, two English classifications, those of Holmes (61) and Shand (62), deserve special attention; they are partly qualitative and do not depart so sharply from the old classifications. It is therefore necessary to take into account certain elements of these schemes in order to construct the most acceptable schematic classification of effusives.

Thus, the latest classifications are based on the principle of the degree of saturation of rocks with silica—a principle that must be recognized as entirely natural for igneous rocks, as silicate formations, and which, in a latent form, was carried through all the known mineralogical and partly chemical classifications. This principle is also adopted in the classification of effusives proposed here (see the separate table), with one amendment introduced into it. Namely, it seems necessary to introduce an intermediate group between rocks supersaturated and saturated with silica, with a small amount of free silica—in order to provide a place for such forms as, e.g., trachyte or andesitodacite, which are now considered to have independent significance.

In parallel with this, the idea may arise of the advisability of distinguishing another intermediate group of rocks with a slight deficiency of silica and, consequently, with traces of olivine or feldspathoids. The necessity for such a group seems to be prompted by the fact that there are transitional forms, such as arcotrachytes or trachyandesites, which do not fit entirely into the groups outlined above. However, there is no urgent need to introduce a second intermediate group of effusives. Indeed, turning to the intrusive rocks, we do not find among them corresponding intermediate forms—or, in any case, they are developed to a very limited extent. Consequently, the occurrence in effusives of a small amount of the indicated silica-poor minerals must represent a temporary formation. And it must be said that this proposition has now been demonstrated experimentally through the application of physico-chemical relations. Thus, Andersen and Bowen showed that very often at the beginning of crystallization even of a not particularly basic magma, olivine appears, which then becomes unstable with the remainder of the liquid phase and is wholly transformed into a metasilicate (58, 287); likewise Bowen found that leucite is not infrequently a temporary formation, which, upon further crystallization of the melt, passes into potassium feldspar (73, 10). And so, depending on how far crystallization of the magma had proceeded during the formation of phenocrysts, the latter may be represented by olivine or by a feldspathoid, although normally these minerals are not characteristic of the given rock. Thus the presence of a small amount of olivine and feldspathoids with alkali metasilicates among the phenocrysts is still not grounds for separating the corresponding rocks into a special group.

Turning now to the further subdivision of effusives, it is necessary to agree with Shand that the composition of the plagioclase is the second very important factor in the classification of igneous rocks, including effusives. But it is interesting that none of the above-mentioned authors of the new classifications has given correct boundaries between the most widespread forms of this continuous series of solid solutions. Thus, Holmes’ distinction of only three kinds of plagioclase, accepted also by older classifiers, is wholly insufficient; next, Johannsen’s method—although apparently verified statistically—of dividing the plagioclase series, like the other factors of the classification he proposed, at the limits 0—5—50—95—100, is entirely artificial; finally, Shand’s proposal to divide the isomorphous plagioclase series into four symmetrical parts also does not satisfy us, for it is perfectly clear that the acidic half of the series must be subdivided much more, in accordance with the greater differentiation of acidic magma. In general, it must be acknowledged that the old system of dividing the plagioclases by Tschermak corresponds much better to the actual distribution of these minerals than all later proposals, including Kozu’s decimal grouping (64, 157). And in the classification developed here, the following kinds of plagioclases are taken into account: albite, whose independent significance need not be discussed; oligoclase, which according to the investigations of P. Chirvinsky is so highly

a characteristic component of granitic magma (42, 524); oligoclase-andesine (on average No. 30 according to Fedorov’s terminology), which enters into the composition of such important rocks as granodiorite (21, 277) and monzonite (68, 14); andesine, so characteristic of andesites or diorites; and the basic plagioclases, which, in the present state of our knowledge of basic rocks, and perhaps in general, do not deserve subdivision.

But in subdividing effusives according to the composition of the plagioclases, we must bear in mind that the plagioclases of phenocrysts are always somewhat more basic than the average plagioclase of the rock, for the first solid phases separated from a plagioclase melt are richer in anorthite (54, 597), and only toward the end of slow crystallization does the composition of the mineral become equalized. This circumstance was indicated especially clearly by Levinson-Lessing, who also noted that, when the quantity of porphyritic inclusions is not especially large, the plagioclase of the groundmass of an effusive is fairly close to the average theoretical composition of the mineral of the rock (56, 255). For more or less acid rocks, however, the difference indicated is not so significant. In other cases the composition of the plagioclase of the phenocrysts, determined microscopically, must be lowered by 5–10 numbers in order to approach the actual composition of the mineral adopted in the present classification.

As for the ratio of the potassic and sodic feldspathic components, used by the above-mentioned authors of the most recent classifications for the further subdivision of igneous rocks, this factor may also be disregarded, especially in subdividing effusives, for the following reasons. First, the potassic component is often in a concealed state, entering, for example, into a solid solution with plagioclase; secondly, when present in small quantity this component is the last to separate and participates only in the composition of the groundmass of the effusive, where it is difficult to detect; finally—and this is especially important—the amount of potassic feldspar is, as a rule, inversely proportional to the basicity of the plagioclase of the rock, so that there is no need to introduce this element among the factors of classification. True, sometimes a large amount of free potassic feldspar appears in basic rocks, but such a case, essentially anomalous and occurring as a consequence of delayed differentiation of the synthetic magma, is not provided for in the proposed schematic classification.

Let us now turn to a brief characterization of the groups of effusives distinguished in the proposed classification. Rocks supersaturated with silica almost always contain quartz among the phenocrysts; in any case there is so much free silica that the groundmass is either glassy or microfelsitic. The most basic member of this group can only be dacite. As for the transitional group in acidity, with a small amount of free silica, phenocrysts of quartz can no longer occur in it, and the groundmass is less inclined to form glass, more often acquiring a microlitic character with patches of micropegmatite and microspherulites, with fairly widespread exsolution of excess silica among the predominant microfeldspathic base. In this group there already appears a basic plagioclase, always—however—associated, as in all the more acid representatives of the first two groups, with potassic-sodic feldspar; at least, there must exist an effusive analogue of quartz gabbro that has not yet received a special name. In general, the representatives of the acid primary effusives are well studied and have a rather rich synonymy, from which the names adopted by the Russian petrographer Levinson-Lessing (56, 492) have been chosen here—with the only difference that the form pantellerito-dacite has been omitted, since it can hardly claim existence, being composed of forms too remote from one another, and that comendite and pantellerite have been given the interpretation proposed by Rosenbusch (38, 330 and 333).

On the other hand, the nomenclature of the diagenetized forms of the first two groups is very poor, with the exception only of the alkaline rocks, among which albitophyres stand out distinctly, though for some reason they are so poorly represented among the more recent effusives; in particular, the more basic representatives of these groups are not separated, bearing the names of quartz and felsite porphyries. Of course, the forms of the greenstone facies of the effusives under consideration are not at all separated; in most cases they have a compact structure and are called here keratophyres, in accordance with the original meaning of this concept, while the name “keratophyres” has in recent times been less appropriately applied to alkaline diagenetized rocks, and respectively “lenne- and lan-porphyries.” It should also be noted that the diagenetized felsites correspond to the glasses so highly developed among the rocks of these two groups.

The group of rocks saturated with silica is considerably more extensive. First of all, alkaline formations are significantly developed here. If in the preceding two groups, with free silica, alkaline pyroboles are in fact characteristic only of families containing exclusively alkaline feldspars, then in the group under consideration each family may be represented both by predominantly alkaline-earth and by alkaline forms. Further, it also includes effusives containing basic plagioclase without independent potassic feldspar, the amount of which generally decreases rapidly with increasing basicity of the rocks and which, in the andesitic series of the group, is always concealed in the groundmass, contributing to the manifestation in it of a hypocrystalline, hyalopilitic structure. As for the purely plagioclase basic effusives, usually united under the general name of traps, they very rarely yield glass, being divided, according to the increasing perfection of crystallization, into basalt, anamesite, and dolerite. Therefore, since glassy forms are characteristic only of those representatives of the group which contain potassic feldspars, occurring comparatively rarely, they, under diagenesis, are transformed into aphanites, sometimes difficult to distinguish from felsites corresponding to felsite porphyries. Turning now to the nomenclature of the forms of the group, we must note that it is still poorly developed—partly because the alkaline part of this group had not been so sharply distinguished. Only with respect to the primary representatives of the alkaline-earth series of the group is the position of the rock nomenclature sufficiently favorable, especially thanks to the analysis of Levinson-Lessing, to which one must fully adhere by introducing the names “andesitotrachytes” and “andesitobasalts” (45, 237). But the subdivision of the diagenetized forms of this series has not been carried out in the literature with sufficient clarity. Thus, simple porphyrites are usually opposed to andesitotrachytes and andesites; and it must be supposed that weisselbergites, since this name should be attached to diagenetized effusives, correspond precisely to andesitotrachytes, having a rather acid composition of the plagioclases (38, 382). Without dwelling on the fact that labrador porphyrites are most conveniently accepted as analogues of andesitobasalts, let us consider the diagenetized forms of the traps. Here great disagreement is caused by the definition of the concept “melaphyre.” Many petrographers—especially Levinson-Lessing (57, 759)—consider it impossible to parallelize melaphyre with basalt, noting a noticeable content of alkalies in it and comparing it with orthoclase gabbro. But one need only agree that until now too broad a meaning has been assigned to melaphyre, and that a large number of rocks which have been called melaphyres must in reality be assigned to labrador and essexite porphyries. And since there is no other name for diagenetized basalt, its usual meaning should be retained for melaphyre—all the more because the derivation of this term (mélas—black) corresponds to the comparatively dark coloration of diagenetized basalts, as the basic ...

… and more or less fully recrystallized formations. Of course, toleite and palatinite must have the same limited meaning; they are most conveniently parallelized with anamesite and dolerite, the diagenetized forms of which are somewhat distinctive, as was explained above, and are usually called diabases.

The alkaline series of the group under consideration appears to be little ordered. First of all, trachytes containing alkaline pyroxenes ought to be regarded as alkaline trachytes; but it has already become sufficiently firmly accepted to understand by alkaline trachytes rocks without plagioclase, and to give to trachytes with alkaline pyroxenes the names Ponza, Drachenfels, or Arso trachytes. Further, trachyandesites quite precisely fall into this series of silica-saturated effusives, having only a somewhat broad meaning and possessing a name that does not quite correspond to the internal content of these rocks. It is still less expedient to understand by trachydolerites purely feldspathoid effusives, as was emphasized by Levinson-Lessing, who showed that trachydolerites should be understood as the effusive equivalents of gabbro-syenites (22, 303). Therefore there will be no mistake in calling the alkaline andesitobasalts precisely trachydolerites, thus limiting the meaning of this concept, which has always referred to feldspathic forms. As for alkaline trapps, they usually contain feldspathoids; therefore the corresponding family in the silica-saturated group remains vacant for the time being. There is no need to speak of a weak differentiation of the diagenetized representatives of this group. Here one may only note lamprophyres, as analogues of alkaline trachytes, and essexite porphyrites, which do not quite fully correspond to trachyandesites and trachydolerites.

Let us now turn to the greenstone phase of the group. The acidic half of this phase could be called keratophyres, as, for example, Rosenbusch did. But even according to the latter, even the analogues of trachytes are so enriched in chlorite, developing because of the comparatively large content in these rocks of dark-colored components, that they “look like diabases” (32, 940); moreover, a hornfels-like fracture (horn-por) is characteristic only of such silicate rocks as contain a noticeable amount of free silica. Therefore all the greenstone forms of this group are here called diabases, while the name “keratophyres” I assign to the representatives of the first two groups, with the exception of the form with basic plagioclase, which is rich in dark-colored components and has long since received the designation, attached to its more or less recrystallized manifestations, of quartz diabase.

Silica-undersaturated eruptive rocks are divided into three subgroups, characterized by the presence of olivine or feldspathoids, or of both minerals. The olivine subgroup is closely connected with the alkaline-earth series of the preceding group, being very simple if one accepts that a small amount of olivine phenocrysts belongs to accidental formations, as was clarified above. Essentially olivine trapps belong here, to which correspond the diagenetized representatives of the family of olivine metaphyres. In addition, one may distinguish, under the name hawaiite, an intermediate form between andesite and basalt in the non-diagenetized phase, the primary phase of which is very difficult to mark out among the mass of forms of the newest effusives that have received various names—often on the basis of an insufficiently precise determination of the feldspars. As for the more acidic representatives of effusives, normally olivine cannot occur in them, as is seen among fully recrystallized intrusives, with the exception of feldspathoid forms; in particular, one can hardly agree with Washington concerning an independent olivine andesite, or hawaiite (74, 412). Finally, here belong the forms that have not separated into phases and are rather…

well crystallized, as basic formations, picrite-porphyrites. It is interesting that, up to now, in the alkaline-earth end of the third group there have not been recorded augite porphyrites, unless one counts the few sanukites that approach them.

The next subgroup of feldspathoid effusives represents the development of the alkaline series of the third group, being extremely complex in view of the extreme instability of these rocks, which in essence are not yet fully formed magmatic formations. In this subgroup one might distinguish both a sodium and a potassium series, as well as leucite and melano-kratov forms, but in view of the limited distribution of feldspathoid rocks we shall not complicate the classification scheme of effusives proposed here. The supporting families of the subgroup are the families of phonolites and tephrites. The scope of the first family, however, is not yet quite established; thus it is not clear whether it includes kenite, which contains a small number of phenocrysts of olivine, which may also represent a temporary formation. It is interesting that a diagenetized analogue of phonolite has still not been established; true, a liebenerite porphyry or a rhombic porphyry is sometimes cited as such, but these rocks are vein rocks. In general, very few ancient representatives of feldspathoid rocks are known, and—in particular—their greenstone phases have not been identified at all—evidently because feldspathoids not only alter easily but in many cases are leached out. As for the families intermediate between phonolites and tephrites, there is great confusion in their delimitation and nomenclature, caused in part by the insufficiency of the microscopic methods used for investigating the rock-forming minerals and by excessive confidence in the chemical method, which should have only a purely ancillary significance in petrology. In the proposed scheme, from the whole series of names of corresponding rocks cited in the literature, those are indicated which better than others reflect the mineralogical composition of the rocks and are constructed according to the same principle as in the rest of the classification. Nephelinites and leucitites, which, like all feldspathoid rocks, contain at least an admixture of alkali feldspars, need not be discussed.

The question of the scope of the last subgroup of silica-undersaturated rocks, containing both olivine and feldspathoids, is somewhat unclear, for a small number of phenocrysts of olivine may also be a fortuitous formation preserved from the transformation of olivine into a metasilicate. If we turn to the analogy with intrusive rocks, we shall see that in their more acidic counterpart olivine is characteristic only of certain syenites and monzonites. But in laurdalite and laurvikite olivine is only an admixture, perhaps likewise accidentally preserved in these hypabyssal rocks, the effusive equivalents of which have already been assigned by us to the subgroup of olivine-free rocks, while the olivine monzonites are essentially orthoclase gabbros. Therefore one may think that the subgroup under consideration, like the first subgroup, is comparatively poor, being represented by the families of banakite, basanite, and various feldspar-free forms. In conclusion it should be noted that limburgites and augitites do indeed belong to derivatives of feldspathoid alkaline magma, since they always contain a sufficient quantity of alkalis, which are—among other things—the cause of the appearance of glass in these basic rocks.

The classification of effusives presented has only a schematic character and does not claim to illuminate the question with sufficient completeness; this question requires special treatment with an analysis of the synonymy of rocks. Nevertheless it is considerably broader than the usual classification tables, not only those given in handbooks, but also those recently published, for example the mineralogical classification of Pirsson (69, 58). In any case

the proposed scheme shows that our classifications of eruptive rocks are still very little developed, and that in synonymy petrographers are immeasurably weaker than paleontologists, paying too little attention to this method of studying natural-historical objects.

8. Conclusion.

Thus, in the present study the basic idea has been put forward that effusives, being by the very origin of their formation highly unstable, change with the passage of time. This change is expressed in the gradual transformation of the components of the rock into stable phases, and also, at times, in the chemical transformation of the components with the aid of subtle solutions, always present both in the changing rock and in neighboring rocks and readily set in motion even by a slight change in external conditions. Such new forms of effusives, which have not undergone fundamental recrystallization and have preserved their structural and textural features, have been called phases—just as polymorphic modifications of minerals are called phases of the state of the corresponding substance.

There may be three normal phases of effusives. First of all—the primary phase, preserved for a longer or shorter time only when the effusive is in a calm state and is not subjected to significant static pressure. Effusives of the primary phase are characterized by coloration from light gray to black or light greenish, and, in the absence of glass, by a rough-porous fracture. In the transformation of effusives in epicontinental formations, characterized by weak dislocation, the rocks undergo simple rebirth, or diagenesis, and this diagenetized phase acquires a reddish-brown coloration as a result of the precipitation or formation of oxides of heavy metals that finely impregnate the effusive. Finally, when an effusive rock enters into the composition of geosynclinal masses subjected to intense dislocations, then, in parallel with the transformation of the components into stable phases, abundant formation of sericite and chlorite takes place in it; and this phase of effusives has received the name greenstone, being characterized by the green coloration of the rocks and lying on the boundary with greenstone schists, which already belong to metamorphic rocks.

Of course, in some cases, caused chiefly by the influence of the country rocks, effusives produce forms differing from the phases described above; likewise, post-volcanic processes and weathering of various ages may considerably alter the external appearance of an effusive. But all such cases concern small masses of rocks, are comparatively easily recognized, and do not undermine the correctness of the scheme of relations among the phases of effusives.

Thus the phases of effusives, since the latter have not undergone any metamorphism, are perfectly recognizable even by their coloration. This circumstance is of great practical interest, making it possible to recognize stratigraphic relations and shades of tectonic processes in the field with the aid of effusive rocks, which are usually regarded by field geologists as very random and little indicative formations.

Finally, the necessity, emphasized in the present study, of distinguishing phases of effusives compels us to give these phases different names and makes it possible to improve and to make truly natural the classification of effusive rocks.

Tomsk.

1.5.24.

Scheme of classification of effusive rocks

Silica and plagioclase

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