page 19


page 19
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Next to unconsolidated deposits, the rocks that present the conditions most favorable for direct absorption are uncemented sandstones and certain porous limestones. In granites, slates, and other compact rocks the direct absorption is very slight. Sandstone is on the whole the best water-bearer among the solid rocks. Under the most favorable conditions the rock is saturated throughout its extent below the regular ground-water level, and water is yielded wherever the sandstone is struck by the drill below this limit. Conglomerates sometimes furnish water in considerable quantity, although as a rule their absorptive capacity is not so great as that of sandstones, and they are much less commonly encountered. Sands and gravels are very porous, the free space between the grains occupying from 30 to 40 per cent of the total volume. A mass of such. materials is saturated below the permanent water level, and when penetrated by wells yields copious supplies. The waters are generally of good quality, but are at some places mineralized, the mineral matter being derived from the more soluble fragments and particles of the deposits. Clay is very impervious to water, and usually contains little or none that can be utilized as a source of supply. The water that is frequently reported in clays usually comes from layers that are more or less sandy. Some sands which approach clay in fineness, and which are at times mistaken for it, yield considerable amounts of water. Clay is of the greatest importance, however, in connection with water supply, not as a direct water-bearer, but as a confining layer to porous sands, from which it prevents water from escaping. Shale, like clay, is a poor medium for the storage and transmission of water, but may yield it from bedding, joint, or cleavage planes. Its most important use is as a confining mass to prevent the escape of water from porous layers which may be interbedded with it. The water derived from limestones occurs mainly in open channels dissolved in the rock by the water itself. Probably the water originally followed joint or bedding planes, which were gradually enlarged by solution into the caverns now found. The occurrence of caverns and passages within limestone is very irregular and their location can not be predicted. Wells that are sunk in limestone only a few feet apart may show very different results, for a difference of a foot or two in the position of the boring may mean the missing of a particular channel. Granites and gneisses are dense and possess small pore spaces, and hence hold very little water. Schists, however, may carry water along the foliation planes, but give it up very slowly, and are not, therefore, important sources of supply. The largest supplies from

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