Decomposition of Sedimentary Rocks and Formation of Deposits of Residual 28. When sedimentary rocks, such as limestones, sandstones and shales, which were formed in the bed of the ocean, are elevated into dry land, they are immediately attacked by erosive forces which seek to break them down and transport their debris again into the sea. In the case of sandstones and shales, earth-water dissolves the Cement which helps to hold the grains together, and, aided by alterna tions of heat and cold, frost and winds, reduces them to masses of loose material, which are picked up and carried away by running water to be assorted and redeposited as beds of gravel, sand and clay. In protected locations where washing goes on but slowly, beds of shale are often trans formed into clay without removal. Earth-water passing through these beds may, when conditions are favorable, dissolve the soluble salts which they contain and remove them, thus purifying the deposit. Valuable deposits of residual clays may in this way be formed from relatively im pure shales. It must be understood, however, that under other condi tions this same earth-water may carry impurities into the shales and so make them or the clays derived from them of lower grade than before.
In the case of limestones, rain-water carrying carbonic acid in solu tion enters the rock. The acid attacks the carbonate of lime, converting it into the bicarbonate which is far more soluble, and is consequently dissolved and carried away by the water when conditions favoring drain age prevail. If the limestone is pure it will be entirely removed by this process, but if it contains sand or clay these will be left and accumulate into beds whose thickness sometimes aggregates hundreds of feet. Most limestones contain more less of clay which was deposited with the lime-sand when it was accumulating on the ocean bed, and so the de composition of a limestone usually leads to the formation of a bed of clay of greater or less thickness, depending on the clay content of the rock and protection from erosion during decomposition. If the clay
deposited with the limestone was pure and conditions during the break ing down of the rock were unfavorable to the introduction of impurities from outside, or if conditions during decomposition were such as to cause the impurities to dissolve and leach away, these clays may be of exceptionally high grade. Some of our very best deposits have origin ated in this way and will be described later. While deposits of this character are occasionally formed, it is usually the case that clays which result from the decomposition of limestones contain so much lime as to unfit them for anything but the coarsest wares.
Clays which have been derived from the decomposition of sedimentary rocks are classed with residual clays but differ from them in that they were at one time transported clays which have been built into these rocks and again recovered from them through decomposition without having undergone any material change in composition or structure. These resemble transported clays more than they do residual. When such clays are carried from their place of origin by running water they form de posits which do not differ in any way from those derived from crystalline rocks. In the case, however, where sedimentary rocks are metamorphosed into crystalline rocks, as explained in § 27, and then broken down, the clays undergo great chemical and physical changes during the process, and when recovered are in every day like those derived from original granitoid rocks.