The Sedimentary Rocks
To this class belong all those rocks which have been laid down by water or wind, or are the results of organic depositions. They include loose material like sand or day, and also the same materials, when cemented into more or less solid rocks, like sandstone or shale. So long as the material has not been altered from what it was when laid down, the rock is termed sedimentary.
In general the material of which these rocks are composed comes from the weathering and disintegration of other rocks. This does not apply to the organic deposits, for each type of which there is a peculiar mode of formation. To illustrate the typical formation of sedimentary rocks, we may look at the fate of a granite when exposed. At once the surface is attacked by changes of temperature, frost and rain. The various minerals of the granite expand and contract with every change of temperature, but each component mineral has a different coefficient of expansion under heat, so that minute cracks are quickly formed between the minerals. Water gets into these cracks and begins to dissolve the minerals. Feldspar is the most easily attacked, part of it being dissolved and carried away, a small part changing to quartz, and by far the largest part changing to kaolin. The dark mineral is also attacked and partly dissolved, and partly changed to kaolin and iron oxides. The quartz resists solution almost completely. Of these products the kaolin and iron oxides are carried far away and deposited in still water. The quartz and perhaps some of the dark mineral are heavier and carried more slowly, being deposited as sand. This happens to granite everywhere, but in the regions where there is frost the action is greatly hastened; for water gets into the cracks and expands every time it freezes and thus widens the cracks rapidly, which greatly facilitates the entrance and movement of water in the rock. In a similar way any original rock will be disintegrated, and the residue, after the soluble part has been carried away, becomes sand or clay or mud.
Particles of quartz, kaolin, and lime, separately, or mixed, loose or more or less cemented, with accompanying impurities, make up the great bulk of the sedimentary rocks. They are usually arranged in layers, of varying thickness, as they were laid down by water or the wind. In the same way layered accumulations which are either products of plants or animals, or parts of the plants or animals, are considered sedimentary, as for instance, coal, chalk, petroleum, etc.