Consolidation of Sediments Into Coherent Rock-masses.— Sediments have accumulated to a very great thickness during geologic time. Those sediments which have been deeply buried and have been subjected to great pressure have been made to cohere either as a direct result of this pressure or by cementing together the particles by mineral substances in solution; namely, silica, carbonate of lime, or some com pound of iron. These rocks of sedimentary origin may be further classified as aqueous, eohan, or glacial, depending on whether they are formed by water, wind, or glaciers.
Rocks of Aqueous Origin.— Rocks depos ited in water are usually stratified. In this respect they are distinguished from typical igneous rocks, which are unstratified and mass ive. Aqueous rocks can be classified as fol lows: (1) Mechanical sediments of terrigenous origin; (2) sediments of organic origin; (3) chemical precipitates.
Mechanical Sediments of Terrigenous Origin.— Included here are all those sub stances which have been washed from land sur faces and deposited in the seas or other bodies of water, as above explained. The principal varieties are: (1) Conglomerates, otherwise known as pudding-stones. These are solidified gravels, in which coherence is due to some cementing principle (lime, iron, or silica), to pressure, or to both. Where the fragments of rock are angular, as though freshly broken, the rock is called breccia.. Breccias are not all of water origin. They are occasionally terrestrial. In either case they have not been transported far from the parent ledge, as their sharp cor ners indicate. (2) Sandstones, consolidated sand-beds. Where the cement is calcareous they form calcareous sandstones. They are also called silicious or ferrugenous, according to the character of the cement. Argillaceous sand stones contain an admixture of clay. Coarse sandstones containing grains of undecomposed. feldspar are called arkose. (3) Shales, solidi fied clays or muds. They may contain some sand, and are then said to be arenaceous; they may contain lime in the form of shell-. fragments or impalpable limy material, and are lacustrine; and those made by rivers are alluvial or fluvial.
Mechanical sedimentaof the character above, described accumulate alike in seas, in lakes, and on river flood plains. Those deposits made in the sea are called marine; those made in lakes, lacustrine; and those made by rivers are alluvial or fluvial.
Sediments of Organic Origin.— These may be the result of the accumulation of either am-, mal or plant remains. The accumulation of hard parts of animals, such as the shells of: mollusks and other mollusk-like forms, the framework of coral polyps and echinoderzeto and the microscopic skeletons of some of the. protozoans, form the bulk of limestone and: other chemically similar deposits. Coquina, the Florida limestone, is composed of a mass of mollusk-shells; crinoidal limestones of the frag mentary skeletons of those marine forms so• abundant during Paleozoic time. Coral lime- stones are formed by the building up of coral. reefs and by the accumulation of fragmentary. coral materials and finely divided coral muds, which may be distributed widely over the floor of the ocean by currents, forming compact cal-, careous rocks. Calcareous accumulation of this particular sort are considered to have been changed to dolomites by the action of the salts of magnesium in the sea-water, on carbonate of lime, replacing, part of the calcium by magne sium and forming (Ca,Mg) C0. out of CaC0... This imperfectly understood process, by which limestones are changed to dolomites, is called dolomitization. Dolomites and limestones grade into each other and together are among the. most common and important rocks of the lithosphere.
Chalk is au accumulation of minute calcare ous shells secreted by the lowest forms of ani mal life, the protozoans, certain forms of which abound in the surface waters of the oceans.
When they die their shells rain down through the water and accumulate on the floor of the high seas, oozes. The chalks are con sidered to have originated in a similar manner during past geologic time.
Chief among the accumulations of plant re mains is peat, an accumulation of vegetable ma terials in swamps or moist places. It is a dark brown or blackish residuum left by the partial decomposition of mosses and other vegetable accumulations, not always made strictly in the water, but always in moist places, and usually in the waters of swamps. Similar vegetable accu mulations on a most extensive scale have been formed during past geologic time, and now con stitute beds of lignite and bituminous and an thracite coaL Diatomaceous or infusorial earth is a silt-like deposit consisting mostly of the microscopic silicious shells of that low order of plants called the Diatomacecr.