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.