WATERS, Underground. A certain amount of the water that falls on the earth's surface passes underground, the percentage varying with the porosity of the surface ma terials. This absorption of water is due to the fact that all rocks are somewhat porous and considerably fissured. Sand and gravel deposits are very porous and can store from 15 to 35 per cent of their bulk of water. Sand stones have space between their sand grains, their porosity varies greatly with sue of grata and especially with the amount of ce menting material filling the interspaces which, in the case of quartzite and some highly calcareous sandstone, fill these spaces entirely. Most sand stones, however, are porous and many hold from 5 to 10 per cent of their bulk of water. Limestones are only slightly porous, but they are always traversed by joint planes and usu ally, toward the surface, by channels and caverns. Clays, shales and slates have but little capacity for water and the crystalline rocks, such as granite, have very slight porosity. Crystalline rocks, however, are usually traversed by many joint planes and often by zones of de composition along which surface waters descend for a greater or less distance. In many dis tricts, also, the crystalline rocks are deeply de composed by the solution of some of their components, and the resulting 'rotten rock' is usually as porous as many sandstones, Many lavas are porous and they usually are exten sively fissured. Water passes underground in various ways. of which the following are the most prominent: direct inhibition of rainfall; the sinking of surface flows in passing over zones of porous rock; the spreading of streams laterally into the porous deposits of their val leys; and the percolation of water laterally from the ocean or lakes into the materials of the shore. In all regions it is found that the surface run-off and evaporation do not equal the rainfall, which is evidence of the general in hibition of water. Many streams are observed to diminish in volume and even to disappear entirely in running over areas of porous sand stones, cavernous limestones or especially per meable portions of their beds. In many and re gitns waters flow out of the mountains in rocky beds and at once sink in the sands of the valleys. In nearly all river valleys there are alluvial deposits in which the water extends laterally from the main stream and some water courses have water only in the sands and of their beds for the greater part of travel of underground water is slow, averaging about one mile a year in moderately porous sandstone.
In the accompanying figures are shown some Ater..., tebicb bate Waters present various conditions under ground, in some cases flowing to lower levels, through permeable rocks, caverns in limestone or crevices in the harder rocks, to emerge as springs in hillsides, valley bottoms or even out tinder the ocean, as off the east coast of Florida.
Usually the water supply fills an underground reservoir of sand, gravel, porous sandstone. de composed crystalline rock or fissures. Water under this condition is usually designated 'ground water' and its surface level is known as the 'water plane.' In some cases ground water may occupy strata or follow down crev ices for a depth of several hundred feet. Waters which extend far underground are mainly contained in sandstones, and some of these water-bearers are of vast extent and often descend to great depths. Two conditions frequently existing are shown in accompanying cuts. Fig. 1. Cross-sections showing conditions of underground water in permeable rocks. In the first of these sections a bed of sandstones reaches the surface at A A where it receives water from rainfall or sinking of streams. This water passes underground below relatively impermeable clay or shale, and an artesian basin is formed which would yield a flowing well at I and at other points where the land is less elevated than at A and A. In the second sec tion the conditions are somewhat similar, the water entering at C but escaping in springs at 0. so there is a constant flow from C toward D. Oa account of this flow to a lower level there es gradual diminution of 'head' of the water from C to D. known as the 'hydraulic grade.* This condition is found in the Central Great Plums of the United States, where there is a bed of porous sandstone with an average thick ness of 200 feet, underlying more than 500,000 square miles and in places lying 5,000 feet deep, and apparently filled with water throughout. Many wells draw artesian supplies from this sandstone and in some areas the flows have a =dace pressure of over 200 pounds to the square inch. Hundreds of billions of gallons nett cantatned in this stratum. The rate of been referred to above. Fig. 2. Sections of a river valley, showing the relations of under ground waters. This valley is cut in rock but partly filled with sand and gravel, as shuwn by the stipple. The river flows on the surface at A but the waters also flow slowly underground in an "undertlowl and pass laterally into the sands, tilling them to the °water plane' 13 13 at the level of the river. Fig. 3. Fissured and de composed crystalline rock. Surface waters often sink deeply and occur in considerable volume under the conditions shown in this figure. ,4 is the zone of decomposition and disintegration of the rock. H A are fissures down which the decomposition has progressed still deeper. This condition is found over wide areas in the Piedmont region of the eastern United States.