Common Minerals and Rocks

by William O. Crosby

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.

The principal accessory minerals occurring in limestone are: (1) kaolin, forming argillaceous or slaty limestone, which may be recognized by the argillaceous odor and dark color; (2) quartz, forming siliceous or cherty limestone, known by its hardness or by the nodules of flint or chert; (3) dolomite, forming dolomitic or magnesian limestone, which effervesces less freely with acid; and (4) serpentine, forming serpentinic limestone, which is sharply distinguished by the green grains of serpentine mingled with the white calcite. A concretionary texture is common with limestone. If the concretions are small, like mustard-seed, we call the rock oölite; if larger, like peas, pisolite.

Dolomite.—If for calcite, which is the sole essential constituent of all limestone, we substitute the allied mineral dolomite, we have the rock dolomite. As might be inferred from its composition, dolomite is very closely related to limestone, although there are some important differences. Physically, the two rocks differ about as the two minerals do. Dolomite is harder than limestone, and being also less soluble, it resists the action of the weather more. Dolomite, if pure, effervesces feebly, or not at all, with cold dilute acid. Here, however, we have to recognize the fact that dolomite is rarely pure; but there exists, in consequence of the admixture of calcite, a perfectly gradual passage from pure dolomite to pure limestone, and parallel with this every degree of vigor in the reaction with acid. Hence, it is entirely an arbitrary matter as to where we shall draw the line between dolomitic limestone and calcareous dolomite. Dolomite is a very much less abundant rock than limestone, and, unlike limestone, it rarely contains many fossils, and is never of organic origin; i.e., there are no organisms which secrete the mineral dolomite to form their hard parts or skeletons. Like gypsum and rock-salt, dolomite is probably never deposited in the open ocean, but only in closed basins. Like limestone, it occurs with both the compact and the crystalline textures.

Gypsum.—When pure, this rock (specimen 36) is identical with the mineral gypsum (specimen 17), except that it is rarely crystalline. It is usually, however, not only perfectly compact, but more or less dark-colored from the admixture of clay and other impurities. Its most notable characteristics are its softness, the absence of the argillaceous odor, except where it contains much clayey impurity, and its non-effervescence with acids. The first two usually serve to distinguish it from slate, while the acid test separates it readily from limestone and all other carbonate rocks. The deposition of gypsum is purely chemical, and it occurs under about the same physical conditions as the deposition of salt; i.e., in drying-up portions of the sea. Hence we usually find gypsum associated with beds of rock-salt; and, since drying-up seas are few in number, and small compared with the whole extent of the ocean, we can easily understand why neither rock-salt nor gypsum are abundant rocks, except in a few localities.