Part 6
3. _Argillaceous group._—Just as the conglomerate group shades off gradually into the arenaceous group, so we find it difficult to draw any sharp line of division between the arenaceous group and the argillaceous, but we pass from the largest pebble to the most minute clay-particle by an insensible gradation. For the sake of convenience, however, we draw the line at the limit of visibility, and say that in the true clay and slate the individual particles are invisible to the naked eye; in other words, these rocks have a perfectly compact texture, while the two preceding groups are characterized by a granular texture. Although clay, like sand and gravel, may be of almost any composition, yet it usually consists chiefly, often entirely, of the mineral kaolin. The reason for this is easily found. Quartz resists both mechanical and chemical forces, and is rarely reduced to an impalpable fineness; but all the other common minerals, such as feldspar, hornblende, mica, and calcite, on account of their cleavage and inferior hardness, are easily pulverized; but it is practically impossible that this should happen without their being broken up chemically at the same time. Decomposition follows disintegration; and, while calcite is completely dissolved and carried away, the other minerals are reduced, as we have seen, to impalpable hydrous silicates of aluminum, _i.e._, to kaolin. Hence, we find that the fragmental rocks are composed principally of two minerals, quartz and kaolin,—the former predominating in the conglomerate and arenaceous groups, and the latter in the argillaceous group.
Clay.—That kaolin is the basis of common clay is proved by the argillaceous odor, which is so characteristic of moist clay. Pure kaolin clay is white and impalpable, like China clay; but pure clays are the exception. They often become coarse and gritty by admixture with sand, forming _loam_; and they also usually contain more or less carbonaceous matter, which makes black clays; or more or less _ferrous_ oxide, which makes blue clays; or more or less _ferric_ oxide, which makes red, brown, and yellow clays. By mixing these coloring materials in various proportions, almost any tint may be explained. Clays are sometimes calcareous, from the presence of shells and shell-fragments or of pulverized limestone. These usually effervesce with acid, and are commonly known as _marl_. It is the calcareous material in a pulverulent and easily soluble condition that makes the marls valuable as soils.
Slate.—Consolidated clay. The compact texture and argillaceous odor are usually sufficient to prove this. To get the odor we need simply to breathe upon the specimen, and then smell of it. We find all degrees of induration in clay. It sometimes, as every one knows, becomes very hard by simple drying; but this is not slate, and no amount of mere drying will change clay into slate; for, when moistened with water, the dried clay is easily brought back to the plastic state. To make a good slate, the induration must be the result of pressure, aided probably to some extent by heat. True slate, then, is a permanently indurated clay which will not soak up and become soft when wet.
Slate is usually easily scratched with a knife, and it is distinguished from limestone by its non-effervescence with acid. As we should expect, it shows precisely the same varieties in color and composition as clay. A good assortment of colors is afforded by the roofing-slates. Specimen 34 is a typical slate, for it not only has a compact texture and argillaceous odor, but it is very distinctly stratified. The stratification is marked by alternating bands of slightly different colors, and is much finer and more regular than we usually observe in sandstone, and of course entirely unlike the stratification of conglomerate. These differences are characteristic. Some slates, however, are so homogeneous that the stratification is scarcely visible in small pieces. Thus the roofing-slates (specimen 35) rarely show the stratification; for it is an important fact that the thin layers into which this variety splits are entirely independent of the stratification. This is the structure known as slaty cleavage; it is not due to the stratification, but is developed in the slate subsequently to its deposition, by pressure. Some roofing-slates, known as ribbon-slates, show bands of color across the flat surfaces. These bands are the true bedding, and indicate the absolute want of conformity between this structure and the cleavage. Few rocks are richer in fossils than slate, and these prove that it is a stratified rock. Slate which splits easily into thin layers _parallel with the bedding_ is known as _shale_.
Porcelainite.—This is clay or slate which has been baked or partially vitrified by heat so as to have the hardness and texture of porcelain.
2. CHEMICALLY AND ORGANICALLY FORMED ROCKS.—We have already learned that from a geological point of view the differences between chemical and organic deposition are not great, the process being essentially chemical in each case; and since the limestones and some other important rocks are deposited in both ways, it is evidently not only unnatural, but frequently impossible, to separate the chemically from the organically formed rocks. Unlike the fragmental rocks, the rocks of this division not only admit, but require, a chemical classification. This is natural because they are of chemical origin; and it is practicable because, with few exceptions, only one class of minerals is deposited at the same time in the same place,—a very convenient and important fact. Therefore our arrangement will be mineralogical, thus:—
(1) Coal Group. (2) Iron-ore Group. (3) Siliceous Group. (4) Calcareous Group. (5) Metamorphic Group (Silicates).
Most of the silicate rocks are mixed, _i.e._, are each composed of several minerals; but some silicate rocks and all the rocks of the other divisions are simple, each species consisting of a single mineral only.
(1) _Coal Group._—These are entirely of organic origin, and include two allied series, which are always merely the more or less extensively transformed tissues of plants or animals; viz.:—
Coals and Bitumens.—At the first lesson we examined a sample of peat (specimen 8), and considered the general conditions of its formation, peat being in every instance simply partially decayed marsh vegetation. It was also stated that, as during the lapse of time the transformation becomes more complete, the peat is changed in succession to _lignite_, _bituminous coal_, _anthracite_, and _graphite_. The coals, indeed, make a very beautiful and perfect series, whether we consider the composition—there being a gradual, progressive change from the composition of ordinary woody fibre in the newest peat to the pure carbon in graphite,—or the degree of consolidation and mineralization—since there is a gradual passage from the light, porous peat, showing distinctly the vegetable forms, to the heavy crystalline graphite, bearing no trace of its vegetable origin. This relation is easily appreciated by a child, if a proper series of specimens is presented. The coals also make a chronological series, graphite and anthracite occurring only in the older formations, and lignite and peat in the newer, while bituminous coal is found in formations of intermediate age.
Bituminous coal is the typical, the representative coal; and from a good specimen of this variety we may learn two important facts:—
(1) That true coals, no less than peat, are of vegetable origin. To see this we must look at the flat or charcoal surfaces of the coal. These soil the fingers like charcoal, and usually show the vegetable forms distinctly.
(2) That coals are stratified rocks. These dirty charcoal surfaces always coincide with the stratification, being merely the successive layers of vegetation deposited and pressed together to build up the coal; and when we look at the edge of the specimen the stratification shows plainly enough.
The bitumens form a similar though less perfect series, beginning with the organic tissues, and ending, in the opinion of some of the best chemists and mineralogists, with diamond. In fact the coals and bitumens form two distinct but parallel series. The coals are exclusively of vegetable origin, while the bitumens are largely of animal origin. The organic tissues in which the two series originate are chemically similar,—the animal tissues, which produce the lighter forms of bitumen, however, containing more hydrogen and less carbon and oxygen than vegetable tissues; while the final terms, as just shown, are probably chemically identical, being pure carbon,—graphite for the coals and diamond for the bitumens; so that the entire process of change in each series is essentially carbonization, a gradual elimination of the gaseous elements, oxygen and hydrogen, until pure solid carbon alone remains.
The principal differences between the coals and bitumens are the following:—
Coals are rich in carbon, with some oxygen and little hydrogen.
Bitumens are rich in hydrogen, with some carbon and little or no oxygen.
Coals are entirely insoluble.
Bitumens are soluble in ether, benzole, turpentine, etc., and the solid forms are soluble in the more fluid, naphtha-like varieties.
Coals are never liquid, and cannot be melted or, with trifling exceptions, even softened by heat.
Many bitumens are naturally liquid, and all become so on the application of heat.
The coals partake of the characteristics of their chief constituent element, carbon, the most thoroughly solid substance known; while the bitumens similarly show the influence of hydrogen, the most perfectly fluid substance known.
The two bitumens of the greatest geological importance are asphaltum or mineral pitch and petroleum; but these substances are too familiar to require any farther description here.
(2) _Iron-ore Group._—These interesting and important stratified rocks include the three principal oxides of iron,—limonite, hematite, and magnetite,—as well as the carbonate of iron, siderite; and the rocks have essentially the same characteristics as the minerals. In economical importance they are second only to the coals; and the history of their formation through the agency of organic matter is one of the most interesting chapters in chemical geology (see page 26). The three oxides are easily distinguished from each other by the colors of their powders or streaks, and the magnetism of magnetite, and from all other common rocks by their high specific gravity. Magnetite is the richest in iron, and limonite the poorest. As regards the degree of crystallization and order of occurrence in the formations, they form a series parallel with the coal series, thus:—
Limonite, never crystalline, and found in recent formations.
Hematite, often crystalline, and found in older formations.
Magnetite, always crystalline, and found in oldest formations.
Siderite effervesces with strong acid; and this separates it from all other rocks, except limestone and dolomite; and from these it is distinguished by its high specific gravity. As a mineral, siderite is often light colored; but as a rock it is always dark, and usually black, from admixture chiefly of carbonaceous matter. In studying dynamical geology, we have learned (page 28) the reason for the intimate association of siderite with beds of coal, and this accounts equally for the carbon contained in the rock itself. The connection of this rock with the coal-formations adds much to its value as an ore of iron.
Finally, the iron-ores, at least where of much economical importance, are truly stratified. This can often be seen in hand-specimens; and is well shown by their relations to other rocks, in quarries and mines; and in many cases, for limonite and hematite, by the fossils which they contain.
(3) _Siliceous Group._—These rocks are composed of pure silica in the forms of quartz and opal. When first deposited, whether organically, like tripolite, or chemically, like siliceous tufa, the siliceous rocks are soft and light, and the silica is in the form of opal. Subsequently it changes to quartz, and the rocks assume the much harder and denser forms of chert and novaculite, respectively.
Tripolite or Diatomaceous Earth.—This interesting rock is soft, light, and looks like clay; but it is lighter, and the argillaceous odor is faint or wanting. It does not effervesce with acid. Hence, it is neither clay nor chalk. Notwithstanding its softness, it is really composed of a hard substance, viz., silica, in the form known as opal. By rubbing off a little of the dust, and examining it under the microscope, we easily prove that the silica is mainly or entirely of organic origin; for the dust is seen to be simply a mass of more or less fragmentary organic remains, occurring in great variety, and of wonderful beauty and minuteness. There are few rocks so unpromising on the exterior, and yet so beautiful within. We have already learned that these organic bodies are principally Diatom cases, Radiolaria shells, and Sponge spicules. We can form some idea of their minuteness from Ehrenberg’s estimate that a single cubic inch of pure tripolite contained no less than 41,000,000,000 organisms.
The lightness of tripolite (sp. gr., 1-1.5) is due to the facts that opal is a light mineral (sp. gr., 1.9-2.2), and that many of the shells are hollow. Tripolite is a good example of a soft rock composed of a hard mineral; and it owes its value as a polishing material to the fact that it consists of a hard mineral in an exceedingly fine state of division. Tripolite, when pure, is snow-white; but it is rarely pure, being commonly either argillaceous or calcareous. This rock is now forming in thousands of places, in both fresh water and the ocean.
Flint and Chert.—During the course of geological time, beds of tripolite are gradually consolidated, chiefly by percolating waters, which are constantly dissolving and re-depositing the silica; and, finally, in the place of a soft, earthy rock, we get a hard, flinty one, which we call _flint_ if it occurs in the newer, or _chert_ if it occurs in the older, geological formations. Besides forming beds of nearly pure silica, which we call tripolite, the microscopic siliceous organisms are diffused more or less abundantly through other rocks, especially chalk and limestone. In such cases the consolidation of the silica implies its segregation also; _i.e._, the silica dissolved by percolating water is deposited only about certain points in the rock, building up rounded concretions or nodules. Thus, a siliceous limestone becomes, by the segregation of the silica, a pure limestone containing nodules of chert, which are usually arranged in lines parallel with the stratification. Specimen 16 is a fragment of a flint-nodule from the chalk-formation of England.
Siliceous Tufa.—Hot water, and especially hot alkaline water, circulating through the earth’s crust, is always charged with silica dissolved out of the rocks; and when such water issues on the surface in a hot spring or geyser, it is cooled by contact with the air, its solvent power is diminished thereby, and a large part of the silica is deposited around the outlet as a snowy-white porous material called _siliceous tufa_. Silica deposited in this way is, like organic silica, always in the form of opal. Siliceous tufa is distinguished from clay, slate, chalk, and limestone by the same tests as tripolite, and from tripolite itself by the absence of microscopic organisms.
Novaculite.—Through the action of percolating water and pressure, siliceous tufa, like tripolite, becomes harder and denser and is thus changed to _novaculite_, which holds the same relation to chemically deposited silica that chert and flint do to organically deposited silica. The white novaculite obtained at the Hot Springs of Arkansas, and commonly known as Arkansas stone, is a typical example of this rock. The rock which, on account of the use to which it is put, is known as buhr-stone, is also an excellent example of chemically deposited silica. It is usually somewhat porous and fossiliferous.
(4) _Calcareous Group._—These are the lime-rocks, including the carbonate of lime, in limestone and dolomite, the sulphate of lime, in gypsum, and the phosphate of lime, in phosphate rock. These rocks are even more closely connected in origin than in composition; and it is for this reason that rock-salt, which of course contains no lime, is also included in this group. Limestones are formed abundantly in the open sea, through the accumulation of shells and corals; but when portions of the sea become detached from the main body and gradually dry up, like the Dead Sea and Great Salt Lake, dolomite, gypsum, and rock-salt are deposited in succession as chemical precipitates. Phosphate rock may be regarded as a variety of limestone, resulting from the accumulation of the skeletons and excrement of the higher animals.
Limestone.—This is the lithologic or rock form of carbonate of lime or calcite, and one of the most important, interesting, and useful of all rocks. Although so simple in composition,—calcite being the only essential constituent,—limestone embraces many distinct varieties, and is really equivalent to a whole family of rocks. A highly fossiliferous limestone, such as specimen 38, is, perhaps, the best variety with which to begin the study of the species. The softness of the fossil shells of which the rock is so largely composed, and the fact that they effervesce readily with dilute acid, proves that they are still carbonate of lime; and by applying the acid more carefully, it can be seen that the compact matrix of the rock also effervesces, consisting of shells more finely broken or comminuted and mixed with more or less clay and other impurity, almost the entire rock being of organic origin.
On the coast of Florida, and in many other places, we find beautiful examples of shell-limestone now in process of formation. These are at first very open and porous, because the interstices between the nearly entire shells are not yet filled up with smaller fragments and sand. But when that is done, we shall have a rock similar to the old fossiliferous limestone. Specimen 37.
The shells and fragments, and the grains of calcareous sand, are, as a rule, quickly cemented together by the deposition of carbonate of lime between them; so that limestone is nowhere observed occurring abundantly in an unconsolidated form.
Limestone, as a rule, is not distinctly stratified in hand-specimens, but of course that it is a true sedimentary rock is abundantly proved by the fossils; and it goes almost without saying that limestone, being necessarily mainly composed of organic remains, must be to a greater extent than any other rock the great store-house of fossils; and in no other rock are the fossils so well preserved and perfect as in limestone.
Nevertheless, there are extensive formations of limestone containing no discernible traces of fossils. And some of these non-fossiliferous limestones, too, are of very recent formation. Some of the modern coral-reefs, for example, are composed of limestone which was formed only yesterday, as it were, and which, from its mode of formation, must consist entirely of corals; and yet it shows no trace of its organic origin, but is perfectly compact, or, possibly, crystalline. This frequent obliteration of the organic remains, as well as the perfect consolidation of the rock, is attributed to its solubility. The calcium carbonate is gradually dissolved by the water, and then deposited in the interstices in other parts of the rock.
Specimen 39 is that variety of limestone known as _chalk_. It is soft and earthy, resembling both clay and tripolite, but differing from the former in lacking the distinct argillaceous odor, and from both by its lively effervescence with acids. It appears to be entirely destitute of organic remains, but this is a defect of our vision and not of the rock; for, like the tripolite, it often appears under the microscope to be little else than a mass of shells. Tripolite is a deposit built up of the siliceous shells of Diatoms and Radiolaria, while chalk is chiefly composed of the similar but calcareous shells of Foraminifera. Our specimen is from the Cretaceous formation of England; but we have good reason to believe that chalk is _now forming_ on a very extensive scale. There are millions of square miles in the deeper parts of the ocean where the dredge brings up little else but a perfectly impalpable, gray, calcareous slime or ooze. When examined microscopically, this is seen to be composed chiefly of Foraminifera shells, and among these the genus Globigerina predominates; so that the deposit is frequently called Globigerina ooze. Now this gray, calcareous ooze, when dried and compacted by pressure, makes a soft, _white_ rock which can scarcely be distinguished from chalk; in fact, it is a modern chalk. And there seems no good reason to doubt that the deposition of chalk has gone on continuously since Cretaceous time—for several millions of years at least.
Specimen 40 is also a white rock, easily scratched with the knife, and effervescing freely with acid, and therefore a variety of limestone. But its texture is very different from the other varieties we have studied. It has a sparkling surface, which we explain by saying that the rock is crystalline. It is, in fact, a mass of minute crystals of calcite. The crystalline limestones have not always been crystalline, but it is safe to assume that they were originally entirely uncrystalline, and in many cases rich in fossils; but the fossils have been mainly obliterated by the crystallization.
Crystallization generally in rocks is an indication of great age, so that we usually say crystalline rocks must be older than uncrystalline rocks of the same composition; and this is mainly true with the limestones. When the crystallization is rather fine, as in our specimen, resembling granulated sugar, we have what is commonly called saccharoidal limestone. This is the typical marble. Marble is not a scientific name, and the term is usually applied to any calcareous rock which will take a polish, and sometimes even to rocks which are not calcareous at all.
In the section on dynamical geology, we learned that the carbonate of calcium or calcite is deposited from the sea-water, and limestones formed, in two ways: first, in a purely chemical way, where the water becomes saturated with calcite; and, second, organically, where the calcium carbonate is taken from the water by marine organisms to form their shells and skeletons, and the gradual accumulation of these on the ocean-floor builds up a limestone. As before stated, the difference between these two methods of deposition is not so great as it often seems, because we know that the animals never make the carbonate of calcium which they secrete, but it comes into the sea ready made with the drainage from the land.
The limestones forming at the present time are almost wholly organic; but the rock known as _calcareous tufa_ is an exception. This is formed under the same general conditions as siliceous tufa, but much more abundantly, and in cold water as well as warm; because calcite is far more soluble (especially in water containing carbon dioxide) than opal or quartz. It is deposited, not only around the mouths of springs, but also along the beds of the streams which they form, enveloping stones, roots, grasses, etc., and building up usually a loose, spongy mass having a very characteristic turfaceous texture.