carbonic acid gas and crumbles or decays to a mixture of kaolin (clay), carbonate of potash, and silica (quartz). Clay is an important constituent of most good soils, while the carbonate of potash is essential as a food for most plants. Due to yielding of the grains or crystals of feldspar, the granite falls apart (see Plate 1). The grains of quartz remain chemically unchanged, though they may be more or less broken by changes of temperature, and the other minerals, which are mostly iron-bearing, yield more or less to weathering, resulting in a variety of products, among which are oxides of iron. A typical granite, therefore, gives rise to a good heavy soil which is yellow, brown or red according to climate. Such granite soils are common in many parts of the Piedmont Plateau from Maryland to Georgia. Most of the dark-colored igneous rocks, like ordinary basaltic lava, contain much feldspar, various iron-bearing minerals, and little or no quartz. Such rocks yield to the weather like granite but, because of lack of quartz, the soils are more clayey. Rich soils of this kind occur in the great lava fields of the northwestern United States and in the Hawaiian Islands. The importance of the breaking down of feldspar under the influence of the weather, as above described, not only from the standpoint of soil development, but also as regards the wearing down of the lands of the earth, is difficult to overemphasize because that mineral is by far the most abundant constituent of the earth's crust. The term "erosion" is one of the most important in geologic science. It comprises all the processes whereby the lands of the earth are worn down. It involves the breaking up of earth material, and its transportation through the agency of water, ice, or wind. Weathering, including the various subprocesses as above described, is a very important process of erosion. By this process much rock material is got into condition for transportation. Another process of erosion, called "corrasion," consists in the rubbing or bumping of rocks fragments of all sizes carried by water, ice, or wind against the general country rock, thus causing the latter to be gradually worn away. A fine illustration of exceedingly rapid corrasion of very hard rock was that of the Sill tunnel in Austria, which was paved with granite blocks several feet thick. Water carrying large quantities of rock fragments over the pavement at high velocity caused the granite blocks to be worn through in only one year. Ordinarily in nature, however, the rate of wear is much slower than this. Pressure exerted upon the country rock by any agency of transportation may cause relatively loose joint blocks, into which most rock formations are separated, to be pushed away. This process, called "plucking," is especially effective in the case of flowing ice. CHAPTER III STREAM WORK