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Sedimentary Rocks

water, materials, called, material, angular, fragments, residual and rock

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SEDIMENTARY ROCKS.

Rock Disintegration.— Granite is one of the most abundant and widespread of igneous rocks, and is a most important species among the numerous other primitive rocks of the lithosphere. The changes which take place in it in the process of its decay can be taken as illustrative of those which take place in the dis integration of all other primitive rocks. Gran ite consists of quartz, orthoclase feldspar (with some soda-orthoclase microcline or plagioclase), and light or dark mica, or both, or perhaps hornblende in place of the micas.

On decomposition under the influence of at mospheric agencies it falls to a more or less rusty; clayey mass of sandy or gravelly mate rial, the sandy or gravelly part consisting of angular fragments of the original quartz (which is practically unaffected by atmospheric agen cies), and fragments of still undecomposed feldspar. The clayey portion of the alteration products results from the decomposition of the feldspar and consists largely of kaolin, which in a pure condition is a white powdery or plastic material, according to whether it is dry or wet. It is usually stained rusty brown by iron oxide, which results from the decomposi tion of any ferromagnesian constituent con tained in the rock or from small particles of some one of the ores which are quite certain to have been present in small amounts. Cer tain constituents of the original minerals are carried off in solution; the alkalies and a part of the silica of the feldspars; also a portion of the iron of the ferromagnesian minerals, to gether with some of the magnesia and much of the lime that may have been present as a minor constituent.

This mass of loose, more or less rusty, in coherent material remaining behind is termed residual granite. Rocks of whatever nature are in like manner subject to decomposition by at mospheric agencies. and their residual materials everywhere cover the greater portion of the surface of the underlying rocks, and constitute what is termed mantle-rock or detritus. A part of this mantle-rock consists not of residual ma terial in the strict sense, but is made up of angular fragments of various sizes, which have been broken, from the rock-masses by the action of frost. At the foot of nearly every steep cliff is to be found an accumulation of angular rock fragments called talus, rock-slide or breccia.

Removal of Rock-Must. and Its Dego& tion in the Form of Sediments.— Most of the various materials of the mantle-rock, whether residual or fragmental, find their way sooner or later, chiefly by the action of rain or frost, to the neighboring streams, and are borne by them to the rivers, which in turn transport than, after numerous halting-periods, to the sea or to smaller bodies of salt or fresh water. In this

process of transportation the angular rock fragments are reduced by attrition to rounded pebbles. The angular grains of quartz also be come rounded and water-worn, while much of the material becomes reduced to an impalpable mud. These more or less finely comminuted and abraded materials are distributed along the shores of lake or sea. Gradually the finer, more easily suspended and transportable ma terials are carried out into deep water, the finest and most impalpable muds being trans ported farthest from the shore; so that, broadly speaking, the washings from the land surface become distributed over sea and lake bottom in order of fineness, beginning with the coarsest gravelly materials at the shore-line, and grow ing successively finer toward the deep water. where finally only the impalpable silts and muds are deposited.

Stratification.— In addition to this more or less gradual horizontal change from coarse to fine material brought about by the transporting power of water, there is always to be observed a much more sudden and abrupt change vertically, by which materials of various sorts and degrees of fineness are arranged by the sorting power of the same medium (water) into horizontal beds of varying thickness, which are separated from each other by sharply defined planes of demar cation called sedimentation or bedding planes. Beds which consist of the same material min eralogically are called strata. But a stratum may be made up of thinner beds, owing to a difference in the fineness of the material which composes it. These subdivisions of a stratum are called layers, and layers in their turn may be composed of extremely thin beds, only the very small fraction of an inch in thickness, called lamina. Strata vary in thickness from a few inches to several feet. All materials de posited in water are called sediments; and all sediments were originally deposited in nearly horizontal position. These nearly horizontal strata slope or dip slightly toward deep water, and this slight inclination is known as initial dip. All materials deposited in water are stratified, and this is the chief characteristic of all rocks so formed. Rocks of this character are called sedimentary.

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