Since the several stratified rocks, such as sandstones, shales, claystones, clays, limestones, etc., are themselves but the outcome of the same disintegrating and decomposing influences upon the crystalline rocks by which soils are now formed, we must study the action of these influences upon the minerals composing the latter rocks in order to gain a comprehensive understanding of the subject. While the number of different minerals known to science is very large, such study need not go beyond a small number of the chiefly important, rock-forming species which are so generally distributed as to require consideration in this connection. These minerals are the following: Quartz and its varieties; the several feldspars; hornblende and augite; the micas; talc and serpentine. Calcite, gypsum and dolomite, though not contained in the older rocks, must be considered because of their forming large rock deposits by themselves; and zeolites require mention because, though rarely forming a large proportion of rocks, they are of special importance as soil ingredients. _Quartz_ and the minerals allied to it consist essentially of dioxid of silicon, usually without (quartz proper) but partly also with water in combination (opal and its varieties). _Silicon_ is next to oxygen the most abundant element found on the earth’s surface. It occurs largely in the various forms of quartz, alone, or as one ingredient of compound rock-masses; the rest, in combination (as silica) with various metallic oxids, forms the important group of silicate minerals, constituting the bulk of most rocks. Quartz occurs frequently in crystals (rock crystal; six-sided prisms terminated by six-sided pyramids), clear or variously colored; but more abundantly as quartz rock or quartzite, readily known by its hardness, so as to strike fire with steel, and by its glass-like, irregular fracture. Besides the crystalline quartz rock we find close-grained and at least partly non-crystalline varieties, such as hornstone and flint. Sandstones most commonly consist of grains of quartz cemented by some other mineral, or by silica itself; in the latter case the siliceous sandstone frequently passes insensibly into true quartzite. The loose sand so well known to common life is prevalently composed of quartz grains, whose hardness and resistance to weathering enables them to survive longest the soil-forming agencies.