Fig. 349.—Section of limestone showing abundant fossils imbedded in a matrix made up of comminuted shell matter. About two-thirds natural size. (Photo. by Church.)
Fig. 350.—Limestone composed chiefly of shells. About three-fourths natural size. (Photo. by Church.)
Regolith.—The first great product of the surface changes is mantle-rock (regolith), which comprehends all the loose matter that springs from rock decay, wear, fracture, and other forms of disintegration. It lies in an unconsolidated sheet on the face of the land, whether as soil, sand, clay, earth, gravel, or loose rock.
Disrupted products: arkose and wacke.—In dry regions, in cold regions, on mountain heights and precipitous slopes, and under other conditions where sudden changes of temperature and frost action work efficiently, rocks are broken down into fine fragments without much chemical decomposition. Such disaggregated rather than decomposed matter, if derived from granitic and similar crystalline rocks, is termed arkose, or arkose sand, and consists of fragments of quartz, feldspar, mica, etc. Common sand consists essentially of quartz grains. If the fine fragments are derived from the darker igneous rocks, and consist mainly of grains of plagioclase feldspar, and ferromagnesian minerals, it is sometimes called wacke. This term is not widely used in just this sense, but there seems to be an important place for it, and it will be so employed in this work. These disaggregated sands are but special phases of the mantle-rock.
Disintegrated products.—When the surface-rock is chemically decomposed, the residual material is confined mainly to the insoluble portions, i.e., the silicious and clayey parts; while the lime, magnesia, soda, potash, and similar substances are largely dissolved and borne to the ocean. The potash is somewhat more disposed to remain with the clays than the soda, lime, or magnesia; but residues of all are usually present.
Classes of Sedimentary Rocks.
Shales, sandstones, and conglomerates.—As already shown in the discussion of the atmosphere and surface-waters, the mantle-rock is constantly being borne away and redeposited in lodgment spots on the land or in the basins of the sea, while it is constantly being renewed below. It is an evanescent but ever-renewed derivative mantle. In this process of renewal, removal, and redeposition, the mantle material is usually assorted into mud, sand, and gravel; and these several classes of material are laid down more or less separately, and usually take the stratified form, because their deposition depends on different degrees of motion of the transporting waters or wind. When these several classes of material become cemented or otherwise hardened, they give rise to shales (cemented muds), sandstones (cemented sands), and conglomerates (or pudding-stones, cemented gravel, [Fig. 346]). If the coarse material remains angular, they form breccia instead of conglomerates (Figs. [347] and [348]). For the most part, the deposits of mud, sand, and gravel are made under the sea or in lakes and estuaries, but they are also formed on the land in lodgment basins, in low-gradient valleys, and on base-plains. The deposits of sediment on land have received less recognition than they deserve. When formed under the sea or in other life-sustaining waters, shells and other organic material are liable to be entrapped and to form a part of the rock. These organic remains, or fossils, greatly aid in interpreting the deposits in which they occur. Fossils are less liable to be preserved in sedimentary deposits formed on land. There is, therefore, some ground to suspect that great series of sandstones and shales which do not contain marine or fresh-water fossils were formed in lodgment basins on land, though the absence of fossils cannot be regarded as proof of such origin.
Fig. 351.