Quartzite.—This rock is simply an unusually hard sandstone. Now the hardness of any rock depends upon two things: (1) the hardness of the individual grains or particles; and (2) the firmness with which they are united one to another. Therefore, the hardest sandstones must be those in which grains of quartz are combined with an abundant siliceous cement; and that is precisely what we have in a typical quartzite, such as specimen 33. Quartzite is distinguished, in the hand-specimen, from ordinary quartz by its granular texture (compare specimens 15 and 33); and of course in large masses the stratification is an important distinguishing feature.
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.