. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 70.—Fissure-Spring. with c, and then by hydrostatic pressure moves laterally until it emerges at a. Sometimes this is a geological agent of considerable importance, modifying even the forms of mountains, and producing land-slips, etc. Thus the Lookout and Eaccoon Mountains, in Tennessee, are table- mountains of nearly horizontal strata, separated by erosion-valleys. These mountains are all of them capped by a sandstone stratum about 100 feet thick, underlaid by shale. The water which falls upon the mount
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 70.—Fissure-Spring. with c, and then by hydrostatic pressure moves laterally until it emerges at a. Sometimes this is a geological agent of considerable importance, modifying even the forms of mountains, and producing land-slips, etc. Thus the Lookout and Eaccoon Mountains, in Tennessee, are table- mountains of nearly horizontal strata, separated by erosion-valleys. These mountains are all of them capped by a sandstone stratum about 100 feet thick, underlaid by shale. The water which falls upon the mountain emerges in numerous springs all around where the sandstone cap comes in contact with the underlying shale. The sandstone is gradually undermined, and falls from time to time, and thus the cliff remains always perpendic- ular (Fig. 69). Large springs generally is- sue from fissures. Water pass- ing along the porous stratum b, perhaps from great distance, and prevented from rising by the over- lying impervious stratum c, coming in contact with a fissure, immedi- ately rises through it to the surface at a (Fig. 70). Artesian Wells.—If subterranean streams have their origin in an elevated region, a d, composed of regular strata dipping under a lower flat country, c, then the subterranean waters passing along any porous stratum as a (Fig. 71), and confined by two impermeable strata, b and d, will be under powerful hydrostatic pressure, and will, therefore, rise to the surface, perhaps with con- siderable force, if the stream be tapped by boring at c. Borings by which water is obtained in this manner are called Artesian wells, from the French province Artois, where they were first successfully attempted. The source of the water may be 100 miles or more distant from the well. Some of these wells are very deep. The Grenelle Artesian in Paris is 2,000 feet deep. At the moment of tapping the stream, a powerful jet was thrown 112 feet high. One in West- phalia, Germany, is 2,385 f
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
