Sedimentary rocks are secondary in their origin, the materials of which they are composed having been derived from the decay and disintegration of some previously existing rock masses and which has been deposited from some fluid by which it has been moved from its former position. The shifted material may have been moved and deposited by the action of water, the atmosphere, or glacial ice. The first case is by far the most prominent and important, especially with respect to the volume of the masses involved, and the frequency of their occurrence, and thus when stratified rocks are mentioned such water-formed rocks are always understood, unless it is otherwise stated. In contradistinction to them, the material which has been moved and deposited by the action of the atmosphere, forms the class known as Aeolian rocks, one of far less importance.
Those which have been formed by a deposition of sediments in a body of water may be divided into two classes, depending upon whether their origin has been mechanical or chemical in its nature. In the case of the sedimentary rocks of a mechanical origin, their constituent particles have been derived from the disintegration of some rock mass, and have been transported by streams into a large body of quiet water, where they have been deposited in practically horizontal layers. Sedimentary rocks of chemical origin have had the materials of which they are composed dissolved by waters circulating through the rocks and brought ultimately by these waters into a sea, where through some chemical change they are precipitated upon its floor, also in horizontal layers. These horizontal beds of sediments are ultimately consolidated into the masses known as sedimentary rocks.
From what has been said, it is clear that the stratified rocks are secondary ones in the respect that their material in some form or other has been derived from already existent ones. An exception to this would be found in beds of coal, which are truly stratified rocks derived from plant life, or in beds of volcanic ashes which have been deposited from the atmosphere, and which have been described by preference under the igneous rocks. But in general the statement, that the material of the stratified rocks is secondary, holds true, and it has been derived from former rocks of all classes igneous, metamorphic and stratified.
When rocks are exposed to the effects of the atmosphere they gradually weather and decay. This is brought about by a variety of agencies. All rock masses are penetrated in various directions by cracks and fissures called joints; these are both great and small, and in addition the individual mineral grains contain cleavage and other cracks. Thus water is able to thoroughly permeate the rock masses, and in cold regions where alternate thawing and freezing goes on, the expansion of the water in turning to ice keeps on splitting and crumbling the rocks until on the surface they are reduced to a mass of debris. The expansion and contraction of rocks in hot countries and in arid regions, under great daily and yearly changes of temperature, accomplishes the same thing more slowly. The expansion of the growing roots of trees and plants tends to the same end.
By such processes there is a constant tendency for the rock masses to be broken up, mechanically, into smaller and smaller fragments. In the meantime the substances dissolved in the water, such as air, acids from decaying vegetation, and especially carbonic acid gas, are acting chemically upon the rock minerals, converting the silicates, oxides, and sulphides into other forms, into carbonates, hydrated silicates, hydroxides, sulphates, etc. Much material goes into solution, is leached out, and by running water is carried into lakes and the ocean, where it concentrates, and where we must again consider it under the formation of the chemically precipitated sediments. Some minerals, such as quartz, are not attacked to any appreciable extent, or but very slowly, under ordinary circumstances, and these remain to form the chief part of the rock debris. It is for this reason that silicates, and especially quartz, play the chief mineral role in the sedimentary rocks formed by mechanical processes.
The sedimentary rocks, as geological masses, differ greatly from the igneous ones in that they form widely extended, relatively thin bodies, making part of a coating or mantle upon the earth's outer surface; they never prolong themselves by extension into the depths, as the latter always do. It is thus their horizontal, as contrasted with their vertical extension, which gives them importance as geological masses. The most characteristic feature about their structure is that they are stratified. This means that they consist of layers, beds, or strata, varying in material, texture and color, and in thickness, which, if undisturbed by geological events more recent than their formation, are in general horizontally disposed, the younger ones upon the older.
Except where they have been disturbed, sedimentary rocks are therefore characterized by a parallel arrangement of their constituent particles into layers and beds which are to be distinguished from each other by differences in thickness, size of grain and often in color. It is to be noted, further, that sedimentary rocks in general are composed of an aggregate of individual mineral particles, each of which stands out in a way by itself and does not have that intimate interlocking relation with the surrounding particles which is to be seen in the minerals of an igneous rock. In all the coarser grained sedimentary rocks there is some material which, acting as a cement, surrounds the individual mineral particles and binds them together. This cement is usually either silica, calcium carbonate or iron oxide. The chief minerals to be found in sedimentary rocks are quartz and a carbonate, calcite or dolomite. These give rise to the two chief types of sedimentary rocks, the sandstones and the limestones. A brief description of these rocks follows.
Sandstone and Related Rocks
Shale and Related Rocks
Limestone and Other Carbonate Rocks
Iron Ore Sedimentary Rocks
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