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.:—