CLAY AND ITS MODE OF OCCURRENCE CONTENTS. Definition.-Clay is the term applied to those earthy materials but others in all stages of decay, and representing chemically many different compounds, such as oxides, carbonates, silicates, hydroxides, etc. (See Chap. III.) The discussion of the chemical and physical properties of the clay will be left until later, but it may simply be stated here that two prominent minerals in clay are quartz (silica) and kaolinite (a mixture of water, silica and alumina, and known chemically as a hydrated silicate of alumina). ORIGIN OF CLAY.1 Clay results primarily from the decomposition of other rocks, and very frequently from rocks containing feldspar, so that for this reason most writers have stated that it was derived from feldspathic rocks. There are some rock species, however, that contain no feldspar (such as serpentine), and others with very little (as some gabbros), which, on weathering, produce some of the most plastic clays known. In all of these clays there is found a variable amount of the mineral kaolinite, which is of secondary origin, i. e., it is derived from other minerals by decomposition. This is termed the clay base. In order to trace the process of clay formation, let us take the case of granite, a rock which is commonly composed of three minerals, viz., quartz, feldspar, and mica. When such a mass of rock is exposed to the weather, minute cracks are formed in it, due to the rock expanding when heated by the sun and contracting when cooled at night, or they may be joint planes formed by the contraction of the rock as it cooled from a molten condition. Into these cracks the rain water percolates and, when it freezes in cold weather, it expands, thereby exerting a prying action, which further opens the fissures, or may even wedge off fragments of the rock. Plant roots force their way into these cracks and, as they expand, in growth, supplement the action of the frost, thus further aiding in the breaking up of the mass. In this chapter the different kinds of clay are discussed according to their origin. A classification according to uses is given in the Introduction to Part III. This process alone, if kept up, may reduce the rock to a mass of small angular fragments. The rain water, however, acts in another way. It not only carries oxygen into the pores of the rock, but also acids in solution, the latter having been gathered in part from the decaying vegetable matter in the soil and in part from the air. The result of this is that the oxygen and the acids attack many of the mineral grains of the rock and change them into other compounds. Some of these are soluble and can be carried off by the water circulating through the mass, but others are insoluble and are left behind. It will thus be seen that one effect of this action. is to withdraw certain elements from the rock, and, the structure of the minerals as well as the rock being destroyed, it crumbles down to a clayey mass. The three minerals mentioned as being commonly present in granite are not equally affected, however, by the weathering agents. Thus the quartz grains are but slightly attacked by the soil waters, while the feldspar loses its lustre and changes slowly to a white, powdery mass, which is usually composed entirely of grains of kaolinite. The mica, if whitish in color, remains unattacked for a long time, and the glistening scales of it are often visible in many clays. If the mica is dark colored, due to iron in its composition, it rusts rapidly and the iron oxide, thus set free, may permeate the entire mass of clay and color it brilliantly. If now a granite, which is composed chiefly of feldspar, decays under weathering action, the rock will be converted into a clayey mass, with quartz and mica scattered through it. Remembering that the weathering began at the surface and has been going on there for a longer period than in deeper portions of the rock, we should expect to find on digging downward from the surface, a) a layer of fully formed clay, b) below this a poorly defined zone containing clay and some partially decomposed rock fragments, c) a third zone, with some clay and many rock fragments, and d) below this the nearly solid rock. (Fig. 1.) In other words, there is a gradual transition from the fully formed clay at the surface into the parent rock beneath. The only exception to this is found in clays formed from limestone, where the passage from clay to rock is sudden. The reason for |