Most of the soils of the earth are the direct result of weathering. Important exceptions are soils which have been transported by the action of water, ice, or wind. Although the process of weathering is very slow and relatively superficial, it is, nevertheless, true that in many places, the products of weathering form faster than they can be carried away. Such weathered materials accumulate in their place of origin to form soils. The upper few hundred feet of the earth's crust is everywhere more or less fractured and porous and the rocks are there affected in varying degrees by most of the ordinary agents of weathering. In such cases, outside the areas which were recently covered by ice during the great Ice Age, it is common to find the loose soil grading downward into rotten rock, and this in turn into the fresh practically unaltered bedrock. Soils of this kind are generally not more than ten or twenty feet deep, though under exceptional conditions, as in parts of Brazil, they attain depths of several hundred feet. In order to make still clearer some of the above principles of weathering and also to give the reader some understanding of the most common types of residual soils, we shall consider what happens to a few rather definite types of ordinary rocks when they are subjected to weathering. A very simple case is that of a sandstone, the mineral grains (mostly quartz) of which are held together by carbonate of lime. The lime simply dissolves and is carried away, while many of the mineral grains may remain to form a soil of nearly pure sand. Where oxide of iron forms the cementing material, the rock yields less readily to weathering, and the sandy soil will be yellowish brown or red according to the climate. Another simple case is that of limestone which when perfectly pure yields no soil because it is all soluble. Pure limestone is, however, rare, and the various mineral impurities in it, being to a considerable degree insoluble, tend to remain to form a residual soil which may vary from sandy to clayey, and usually brown or red due to the setting free of oxides of iron. According to one estimate a thickness of about 100 feet of a certain fairly impure limestone formation in Virginia must weather to yield a layer of soil one foot thick. Soils of this kind, which are usually rich, are common in many limestone valleys of the Appalachian Mountains. In the case of shale rock, which is hardened mud, the cementing materials are removed, some chemical changes in the minerals may take place, and the rock crumbles to a claylike soil. What happens to a very hard, resistant igneous rock like granite when attacked by the weather? Such a rock always consists mainly of the two very common minerals feldspar and quartz, usually with smaller amounts of other minerals such as mica, hornblende, augite, or magnetite. The feldspar, which when fresh is harder than steel, slowly yields when attacked by water containing