The materials of which the sedimentary rocks are composed were, in the first instance at least, derived from the chemical decay or mechanical abrasion of the igneous rocks, and hence they are often called derivative or secondary. They have been laid down under water, or on land, and are therefore almost always stratified and, for the most part, are composed of rounded fragments, seldom crystalline.
Almost all the minerals which we have found in the igneous rocks also occur, in a more or less worn and comminuted condition, in the sedimentary class. However, with the exception of quartz, the great bulk of the sedimentary materials consists of simpler and more stable compounds than the igneous minerals, from the decomposition of which they have been derived. The principal minerals which compose the sedimentary rocks are quartz (Si02), kaolinite (A1203, 2 Si02, 2 H20), and calcite (CaC03).
Quartz is a very simple and stable chemical compound, and hence, in the ordinary process of rock decay, it remains unchanged further than being broken up into smaller pieces and rounded by the action of wind or running water. Kaolinite is derived principally from the decay of the felspars, and the lime of calcite from the complex silicates containing lime, which are so frequent in the igneous rocks. These rocks also yield the iron oxides which are so widely diffused in the sedimentary class, though comparatively seldom in any very great quantity. Very many varieties of rocks are produced by the mixture of the siliceous (quartz), argillaceous (clay), and calcareous (lime) materials in varying proportions. The sorting out of material by water, according to its chemical nature, is usually imperfect (although siliceous and calcareous concentrations are often remarkably pure), and changes from point to point, so that the sedimentary rocks have an even less definite chemical composition than have the igneous.
It is, unfortunately, not yet practicable to apply to the sedimentary rocks the arrangement employed for modern continental and marine deposits, and the most useful classification at present of the sedimentary rocks is, primarily, according to the mode of their formation, and secondarily, according to their composition. This gives two principal divisions: I, the Aqueous Rocks, or those laid down under water; II, the Aeolian Rocks, those which were accumulated on land, which are of more limited extent and importance.