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.