F. I. Shnell.

Petrographic Study of Diabase Outcrops in the Vicinity of the City of Tomsk.

The collection of diabases described in the present article was gathered (together with sandstones, clay shales, and conglomerates) in the summer of 1912 by me and by the student B. L. Stepanov in the valleys of the small rivers Ushaika and Basandaika and—between the mouths of these streams—along the right bank of the Tom River.

We carried out our geological investigations within the limits of the one-verst map of the Military General Staff that was at our disposal. I arrange the present description according to the following plan:

1) The stratigraphy of the region studied, with the description following the order in which the diabase outcrops are actually observed in the field, beginning from the outermost points (the village of Rodionovka and the village of Protopopovo on the Ushaika River, and the village of Nekrasovka on the Basandaika River) and proceeding downstream along the streams, as well as along the Tom River between the mouths of these same streams, likewise in the direction of the current.

2) Contact metamorphism of the clay shales and sandstones, among which the diabase veins chiefly occur, and endocontact metamorphism observed in the veins themselves.

3) Macroscopic and microscopic description of the diabases.

4) Chemical characterization of the types of diabase.

5) General conclusions.

Stratigraphic Data.

Proceeding downstream along the Ushaika River, the first diabase outcrop was found by us on the right bank of the Ushaika, opposite the mouth of the Kamenka stream, which flows in from the left into the Protopopovskaya Ushaika. This outcrop here served, as it were, as an obstacle to the widening of the river channel, and therefore the post-Pliocene deposits of the river occupy here a narrow strip of 80–100 sazhens. The diabase vein (with a thickness of up to 0.6 sazhen) appears at the top of the scarp of the first terrace in the form of a mass,

wedged between beds of clay slate—on the eastern side—and sandstone—on the western.

The strike of these rocks is 314° N., with a dip to NO ∠ 82°.

In its upper part the vein has spheroidal jointing, the dimensions of the spheroids varying from the size of a fist to that of a human head. The spheroids are easily extracted with the aid of a pick, and, when struck with a hammer, several concentric weathering crusts separate off; inside they contain a fresh, very hard spheroid, which is already difficult to split. The sandstone and clay slate near the vein are baked.

In the direction toward the foot of the same cliff, the vein just described, as is evident from the shafts present here, becomes much thicker and, together with spheroidal jointing, acquires platy jointing, passing into parallelepipedal jointing.

The next diabase outcrop is Rodionovskii, on the right bank of the Rodionovka Ushaika. Here the diabase vein likewise exhibits spheroidal jointing and, in the character and conditions of its occurrence, is analogous to that observed at the Protopopovskii outcrop; but, unfortunately, it was impossible to determine the elements of its occurrence, since the working face had long been inactive and was heavily covered by talus.

Farther on, following downstream, 200 sazhen from the confluence of the two Ushaikas into one common stream, we observed an outcrop of a vein, the thickness of which is difficult to establish, but, judging by the mass and size of the spheroids (1–1½ feet in diameter), it must be supposed to be no less than 2–3 sazhen. After 40 sazhen, following along the foot of the hill, there is revealed a cliff about 6 sazhen high, consisting entirely of separate parallelepipeds of diabase, and in the upper marginal parts of the cliff—of spheroids with weathering crusts. Study of this outcrop makes it possible to judge that, during cooling, the diabase mass separated into individual parallelepipeds, and then, under the influence of weathering penetrating along the joint cracks, these parallelepipeds were transformed into spheroids. The strike of the vein here is 308° N., with a dip to NO / 80°.

At 250 sazhen downstream along the Ushaika from the cliff just described, on the opposite left bank, its steep slope gives a splendid section, in which, among clay slates and sandstones having a strike of 8° and a dip to NW / 78°, there is an outcrop of two veins: the first with a thickness of 4.50 sazhen, the second—about 0.92 sazhen. Both are separated by a bed of clay slate 0.9 sazhen thick.

Blocks of diabase are also visible on the bed of the river, and from here, precisely, the extraction of this rock for paving the streets of the city of Tomsk was first begun.

Heading north along a forest path running approximately parallel to the first ravine, which flows from the right into the Ushaika River, at a distance of 150 sazhen from it we found a cliff, 1½ sazhen high and also about 1½ sazhen wide, consisting of diabase and sandstone.

Workings here have long since been abandoned, and therefore, both in this and in the next, more westerly quarry, the bottom of which is flooded with water and the edges covered with fallen soil deposits, the character and elements of the vein’s occurrence have remained undetermined.

Continuing farther along all the stone quarries, which are confined to the valleys of the Ushaika River, we encounter no outcrops of igneous rocks as far as Stepanovskaya Luka, where, on the contrary, there is a series of diabase outcrops; owing to their contact action, the enclosing clay shales have become compacted, made resistant to agents of weathering, and, at the same time, have forced the river to bend around them and to lay its channel in the more untouched, softer shales. Leaving the hill on its right side, the Ushaika emerges onto a fairly broad plain, which, however, narrows somewhat over the stretch beginning from the Uprava quarry at the 75th verst, near the railway track, and extending to the “Goltsa” cliff. In the first place (i.e., near Stepanovskaya Luka) there remain traces of former diabase-mining works here: enormous pits filled with water, the building of a water pump, and other facilities, which have not yet been demolished since the closing of operations at this quarry. The faces of these workings have collapsed and are flooded with water, but from the spoil heaps and the stacks still partly preserved one may judge the rock formerly extracted—a diabase porphyrite.

After the “Goltsa” cliff, consisting entirely of sandstone cut by an innumerable number of quartz veinlets, the Ushaika enters the lower part of its valley. Flowing at first northward, near Khromov’s zaimka, one verst below the railway bridge, the river turns to the northwest. At this bend, in the riverbed, there are outcrops of diabase, which is mined here by private owners for their own needs, but the quality of the rock is not high: it is coarse-grained and not resistant to blows from a hammer.

About 200 sazhen still farther downstream, on the right bank of the Ushaika River, lies the so-called 2nd Uprava quarry, with an excavation area of diabase porphyrite approximately equal to 99 square sazhen. The jointing of the rock here is polyhedral; the occurrence is in sheet-like veins with a strike of 270° and a dip to the S ∠ 80°.

Our second route was along the Basandayka River, beginning at the village of Nekrasovka, downstream to the mouth of the Basandayka and then farther down along the Tom.

The valley of the Basandayka River is, comparatively, broad; its right slope descends steeply into the valley, while the left one is gentle, and only below the village of Pisarevka does it become steeper, narrowing the riverbed and not giving it the same expanse that it had throughout its course from Nekrasovka to Pisarevka. Here there are calm, soft outlines of the banks, whose sections almost everywhere expose dark-gray loam, or are even turfed over; there is a mass of oxbow lakes.

The first outcrop of diabase is encountered only 30 sazhens before reaching the first house of the village of Pisarevka, on the steep left bank at the level of the water horizon in the river. The rock forms a vein (1½ sazhens thick) in shales, having a strike of $3^\circ$ and a dip to $NW \angle 75^\circ$.

Proceeding farther toward the mouth of the Basandayka, we then encountered outcrops of a vein of diabase porphyrite on the left bank, 40 sazhens below the bridge, at a bend made by the river. The rock here contains large segregations of augite and is rather rich in pyrite. The thickness of the vein reaches 3 sazhens.

After going another 20 sazhens down the valley, we found the next outcrop of diabase porphyrite, in the upper horizons—with spherical separateness. The porphyrite is strongly destroyed and indistinctly bounded, and therefore it was impossible to determine precisely the elements of its occurrence. The clayey shale, however, enclosing the vein, has a strike of $NO30^\circ$ and a direction of dip of $SO120^\circ$.

Farther on there follows a whole series of outcrops of diabase porphyrite along the same left bank of the Basandayka River; the outcrops alternate with layers of clayey shales enclosing them and, owing to their hardness, protrude from the thickness of these shales, but at most by ½ arshin; the level of occurrence of all the outcrops above the water horizon in the river almost nowhere exceeds 1 sazhen. Five outcrops of veins on the right bank of the Tom River are concentrated over the stretch between the town water-pumping station and the powder magazines. Here all the veins, with the exception of one, barely reach a thickness of 0.5 sazhen, and moreover most of them are so destroyed that it is difficult to determine the elements of occurrence.

The principal veins are situated: one 14 sazhens east of the underwater rock called “Boets,” and the other immediately beyond this rock on the western side. The western vein has a thickness of more…

2 sazhens and gives off several apophyses; two of the latter, nearly parallel, cut across the sandstones and have a strike of \(301^\circ\) and a dip close to vertical. The second—the eastern—vein, 0.5 sazhen thick, has the same strike of \(301^\circ\) and likewise wedges in between the sandstones, bakes them, and forms a steep cliff lying near the municipal water-pumping station. The three remaining diabase outcrops are situated at the level of the bank terrace on which the water-pumping-station building has been constructed; these outcrops are not thick and have been destroyed.

Exo- and endocontact metamorphism.

The contact changes in the sandstones and clay shales enclosing the diabase veins, as is already evident from the preceding description, are very slight. They are manifested chiefly by compaction and baking of the sedimentary rocks, which gives the altered rocks a greater resistance, in comparison with the unaltered ones, to the processes of weathering and erosion. In addition, in the immediate vicinity of the diabase veins the sedimentary rocks show changes in color and in the character of their jointing; namely, the sandstones in contact with the veins have not a gray color, as usual, but a brownish coloration and acquire fine columnar jointing, while the clay shales change their usual gray color to bluish-gray, more rarely yellowish, and split with a thin platy jointing.

Endocontact changes in the diabase itself are observed only in some outcrops and are expressed in the appearance, near the selvages of the vein, of a hornfels-like structure.

Macro- and microscopic description of the diabases.

On the basis of the totality of the morphological characters of the rocks composing the described veins, they may be divided into two groups:
1) augite, quartz-bearing porphyrites, which have the predominant distribution in the area studied; and 2) quartz diabase aphanites. We shall describe them separately.

I. Augite diabase, quartz-bearing, porphyrites.

Macroscopically this type is characterized by a holocrystalline, distinctly granular groundmass and by the presence of segregations of augite, more rarely of augite and plagioclase.

The former, i.e., the phenocrysts of augite, reach a size of 6–7 mm along the vertical axis and, possessing idiomorphic outlines, have

in most cases they have a prismatic habit and exhibit very distinct cleavage. Their color is brownish-black. The plagioclase segregations sometimes exceed the augite segregations in size and appear as flat bluish-gray tablets, the length of which exceeds the width by approximately \(1 \frac{1}{2}\) times.

In the groundmass, which has a general greenish-gray coloration, the following can be distinguished macroscopically: indistinct whitish-greenish prisms of plagioclase, biotite—xenomorphic plates, and pyrite—also xenomorphic and readily situated near the augite phenocrysts, sometimes clothing them on two sides and helping them to stand out most distinctly against the general greenish-gray background of the groundmass.

Microscopic investigation.

Phenocrysts: Augite—automorphic; sections perpendicular to the vertical axis are octagonal, those along \((101)\) are hexagonal; in the former sections a system of cracks of prismatic cleavage is visible, intersecting at an angle of \(87^\circ\); in longitudinal sections the same cracks are parallel to the vertical axis \(c\); in addition, irregular cracks are often observed. The color of the augite ranges from colorless to pinkish-yellow and pale yellow.

Twins on \((100)\) are very widespread, more often simple ones composed of only two equally developed individuals, or in the form of more or less fine polysynthetic lamellation; twins of intergrowth on \((101)\) occur more rarely. The extinction angle \(c : ng\) varies within the limits from \(33^\circ\) to \(42^\circ\), and \(2V = 54^\circ\) to \(60^\circ\). As inclusions in the augite segregations, only flakes of biotite are observed.

The indicated properties of augite are equally characteristic of its segregations in the various representatives of the type under consideration; only the dimensions of the segregations and the tendency to form twins are not constant. The outcrops along the Basandaika and Tom rivers are distinguished by the greatest size and number of augite segregations.

Plagioclase—automorphic, more often regularly bounded by two parallel sides and elongated along the brachydiagonal; more rarely tabular owing to the predominance of the basopinacoid. Colorless, in rare cases faintly grayish or yellowish. Of the twins, those subordinate to the Manebach, Carlsbad, or pericline laws are most widespread, and twins according to the law of La Roc Tourné and others are comparatively rare. Investigations on the Fedorov stage yielded close limits within which the numbers of the plagioclases vary, hav—

but from No. 52 to No. 58; Nos. 35 and 37 occur very rarely. As inclusions there are occasionally found augite, apatite, and iron ores. Sometimes the plagioclase is filled with chloritic products of decomposition. The outcrop of the 2nd uprava quarry near the Khromovskaya zaimka is distinguished by especially large segregations of plagioclase.

The groundmass is characterized by the content of the following minerals: plagioclase, which has an idiomorphic lath-shaped form (lath length 2–3 mm, more rarely 5 mm); augite, filling the greater part of the interstices between the plagioclase laths; biotite, distinctly pleochroic (from pale yellow to dark brown), likewise xenomorphically wedged between the plagioclase laths; hornblende, forming short columns, often without terminal faces, showing pleochroism with \(n_g\)—brown, \(n_p\)—pale yellow (with the absorption scheme \(n_g \ge n_m > n_p\)) and possessing an extinction angle \(c : n_g = 20^\circ\); quartz, occurring in rare and small interspaces between the plagioclase laths and here and there found in micropegmatitic intergrowth with orthoclase.

Among accessory constituents there are present: magnetite—usually irregular grains, very rarely square sections; it sometimes fills irregular cracks by which the feldspar crystals are broken; apatite—small idiomorphic prisms, usually enclosed in biotite, more rarely in other minerals; titaniferous iron ore occurs in separate, ragged forms of insignificant size; pyrite—irregular, sometimes rather large grains.

Finally, as secondary components there are observed: calcite—as a product of the destruction of plagioclase, in the form of a coating on its laths; epidote and kaolin—also as products of decomposition of the feldspars; and chlorite—as a product of decomposition of augite.

Variations in the composition of the groundmass of the rocks under consideration are expressed chiefly in the differing relative content of augite, biotite, and quartz. Namely: in the rock exposed at Stepanovskaya Luka, the groundmass is poorer in augite and richer in biotite and quartz; whereas in the rocks from the other outcrops, inverse quantitative relations are observed among the indicated components of the groundmass.

II. Quartz diabase aphanites.

Blackish-gray, even rather close to black, very dense rocks, of whose constituent parts only plagioclase laths and irregular grains of pyrite are macroscopically distinguishable.

Under the microscope, the plagioclase laths, scarcely exceeding 3 mm/m, appear very fresh, colorless, and transparent; they give the same twins as the plagioclases in the preceding type, and resemble them in their numbers, i.e., in the percentage content of anorthite. Individual plagioclase laths are sometimes broken by cracks into separate fragments, the intervals between which are filled partly with magnetite, partly with chlorite; the augites occur as small, irregular grains, comparatively rarely approaching the plagioclases in size; their color ranges from colorless to greenish-yellow and even dirty green. Biotite, platy, brown, distinctly pleochroic, occurs together with accumulations of augite grains. Quartz—a xenomorphic constituent—fills the intervals between the plagioclase laths and, as in the preceding type, is sometimes found in micropegmatitic intergrowth with orthoclase. Magnetite and pyrite are scattered over the entire surface as separate crystals and grains. Magnetite occasionally also forms accumulations. The relative amount of ore minerals is especially abundant in specimens taken from the marginal parts of the veins.

From the uneven extinction of some individuals of the feldspars and the granulation of the quartz grains, as well as from their undulatory extinction, it may be supposed that the rocks were subjected to considerable pressure.

Some specimens (Nos. 17, 129, and 135) of aphanitic diabases, taken near quartz veins and veinlets, are strongly enriched in quartz and have a microdioritic structure rather than an ophitic one, as in the majority of the other specimens.

Chemical Characterization of the Types Described.

Subjected to chemical analysis were: a specimen of augite diabase porphyrite from the 2nd Upravsky quarry, ½ verst from the Chistyakovskaya zaimka (No. 145/60), and a specimen of diabase aphanite from an outcrop on the hill opposite the bridge at B.-Protopopovka, on the left bank of the Ushayka River (No. 17к/27).

The results of the chemical analysis and their recalculation according to the scheme of the American petrographers are given in the following table.

Sample No. 145/60.

\[ F = 54,98;\qquad Q = 12,54; \]

\[ Sal = 67,52;\qquad Fem = 32\ 27; \]

\[ \frac{Sal}{Fem}=\frac{67,52}{32,27}=2,09<\frac{7}{1}>\frac{5}{3} \text{ — class II — Dosalane} \]

\[ \frac{Q}{F}=\frac{12,54}{54,98}=0,22<\frac{3}{5}>\frac{1}{7} \text{ — order 4 — Austrare} \]

\[ \frac{K_2O' + Na_2O'}{CaO'}=\frac{18+37}{92}=0,6=\frac{3}{5} \text{ — rank 3 (4) — Tonalase (Bandase)} \]

\[ \frac{K_2O'}{Na_2O'}=\frac{18}{37}=0,48<\frac{3}{5}>\frac{1}{7} \text{ — subrank 4 — Tonalose (Harzose)} \]

Sample No. 17k/27

*) Correspond to the percentages of oxides recalculated to 100 after subtracting loss on ignition.

\[ F = 46{,}29; \qquad Q = 18{,}96; \]

\[ Sal = 65{,}25; \qquad Fem = 34{,}75. \]

\[ \frac{Sal}{Fem} = \frac{65{,}25}{34{,}75} = 1{,}9 < \frac{7}{1} > \frac{5}{3} \text{ — class II — Dosalane} \]

\[ \frac{Q}{F} = \frac{18{,}96}{46{,}29} = 0{,}4 < \frac{3}{5} > \frac{1}{7} \text{ — order 4 — Austrare} \]

\[ \frac{K_2O' + Na_2O'}{CaO'} = \frac{14 + 39}{65} = 0{,}81 < \frac{5}{3} > \frac{3}{5} \text{ — rank 3 — Tonaiase;} \]

\[ \frac{K_2O'}{O'} = \frac{14}{39} = 0{,}36 < \frac{3}{5} > \frac{1}{7} \text{ — subrank 4 — Tonalose.} \]

Comparing the results obtained, we see that the two analyzed rocks are very close to one another: the more or less noticeable difference consists only in the fact that the Norm of the second rock is somewhat richer in standard quartz and poorer in standard anorthite.

The position of the specimens of the Tomsk diabases in the quantitative chemical-mineralogical system of the American petrographers, as clarified by the calculation made, may be said to correspond fully to the diabase character of these rocks. Indeed, acquaintance with the chemical character of diabases from the collection “Chemical analyses of igneous rocks, published from 1884 to 1900” by H. S. Washington (Washington, 1903) convinces one that approximately half of the diabases cited in this collection belong to class II, a considerable part belong, at the same time, to order 4, and, undoubtedly, the great majority are characterized by such values of the ratios \(K_2O' + Na_2O' : CaO'\) and \(K_2O' : Na_2O'\), which determine the 3rd rank and the 4th subrank. Comparison with the Norms of various diabases cited in the same collection also shows that a quartz diabase from Richmond, Cape Colony (see p. 245 of the cited collection, analysis No. 119), possesses a significant similarity with the diabases of Tomsk. This similarity is all the more interesting because the Richmond diabase represents one of the varieties of diabases which, like the Tomsk diabases, form an entire suite of sheet-like and vein rocks in the so-called Karroo formation in the Cape Colony in South Africa (see Neues Jahrbuch, Beilage-Band V, 1887, S. 195).

Among this suite there are not infrequently members which, like the rocks described by us, contain biotite.

In conclusion to the morphological characterization of the rocks described, I shall add that, in their qualitative mineralogical composition and in the features of their microstructure, these rocks may be regarded as close to the so-called conga-diabases or mangerites (see Rosenbusch, Elemente d. Gesteinslehre, 3rd edition, pp. 187 and 417).

General Conclusions.

Thus, the igneous rocks described from the vicinity of Tomsk form rather numerous bedded veins or intrusive deposits in Paleozoic clay slates and sandstones; they possess parallelepipedal or platy jointing in the deeper horizons and in freshly exposed parts, and spheroidal jointing—in the heads of the veins and in parts that have undergone more considerable weathering; they show a weak exocontact effect (of a purely physical character) on the enclosing rocks and weak endocontact alterations near the selvages; occurring in two types, differing chiefly in structure—namely, augite diabase quartz-bearing porphyrites and diabase quartz-bearing aphanites—they display variations in mineralogical composition determined by the presence or absence of brown hornblende, while in chemical composition all are very close to one another.

Tomsk. Typo-lithography of the Siberian Printing Association.

TOMSK

Conventional signs

Numbers beside the samples correspond to the sample journal.

Scale: 1 verst = 1 inch.

Visible map labels

• Tomsk
• r. Ushayka
• r. Basandaika
• r. Bolshaya Kirgizka
• r. Malaya Kirgizka
• r. Kislovka
• r. Cheremoshka
• r. Peshchanaya
• r. Stepanovka
• r. Serebryanka
• Zavarzino
• Basandaika
• Petukhovo
• Stepanovka
• Kislovka
• Voronino
• Pravoberezhnaya
• D. Zavarsina
• D. Serebryanka
• D. Stepanovka
• D. Petukhova
• D. Basandaika
• D. Kislovka
• Station Tomsk
• Tomsk I
• Tomsk II
• Northern town
• D. Yurtochnaya
• D. Sadovaya
• D. Berezkina
• D. Trudovaya
• D. Voronina
• D. Baturina
• D. Timiryazeva

[Map of the vicinity of Tomsk, showing rivers, roads, settlements, and numbered diabase sampling points.]

Visible labels on the map:

• River Tom.
• River Basandaika.
• River Malaya Kirgizka.
• River Bolshaya Kirgizka.
• River Ushaika.
• River Zavarzina.
• River Bolshaya Sennaya [[unclear: river name]].
• River Zelenka.
• River [[unclear: small river name]].

Settlements and localities:

• Tomsk.
• Stepanovka.
• Berezkina.
• Zavarzina.
• Bogashevo.
• Basandaika.
• Mezheninovka.
• Protopopovo.
• Petukhovo.
• Borovtsovo.
• [[unclear: settlement name]].
• [[unclear: settlement name]].
• Levashova.
• Berezovka [[unclear: settlement name]].
• Kireevka [[unclear: settlement name]].
• Kolpasheva [[unclear: settlement name]].

Road and direction labels:

• Toward the city of Tomsk.
• Toward the city of Mariinsk.
• Toward Petrograd.
• Toward Kuznetsk.

Numbered points visible on the map:

• 1, 2, 3, 4, 5, 6, 7, 8.
• 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162.
• 163, 164, 165, 166, 167, 168, 169, 170, 171.
• 172, 173, 174, 175, 176, 177.