WATER. 649 



supplies an abundance of water, though a little brackish. Several have been 

 made in New York City connected with manufactories. In California they have 

 been resorted to successfully for agricultural purposes. 



Borings are often successful in alluvial regions fifty or one hundred miles 

 from*any high land. A second boring in the same region sometimes seriously 

 lessens the amount of water afforded by the first, by giving the same subterra- 

 nean stream a new place of exit. The layer from which the boring and jet 

 rise may be gradually worn through by the flow, and the water, or part of it, 

 become lost by being thus let off to a lower level. 



The mechanical effects of subterranean waters are — (1) Erosion 

 and the consequent undermining of strata ; (2) Land-slides. 



1. Erosion. — Running water will wear rocks under ground as well 

 as above, and may excavate a channel in the same way. Caverns 

 are made partly by erosion and partly by the dissolving action of 

 water. A common effect bf such excavations is the production of 

 subsidences of the soil and overlying rocks, and the formation 

 of sink-holes. Small shakings of the earth may be a consequence 

 of the fractures of undermined strata. 



2. Land-slides. — Land-slides are of three kinds : — 



(1.) The mass of earth on a side-hill, having over its surface, it 

 may be, a growth of forest-trees, and, 'below, beds of gravel and 

 stones, may become so weighted with the waters of a heavy rain, 

 and so loosened below by the same means, as to slide down the 

 slope by gravity. 



A slide of this kind occurred during a dark, stormy night in August, 1826, 

 in the White Mountains, back of the AVilley House. It carried rocks, earth, and 

 trees from the heights to the valley, and left a deluge of stones over the country. 

 The frightened Willey family fled from the house, and were destroyed : the 

 house remains, as on an island in the rocky stream. 



(2.) A clayey layer overlaid by other horizontal strata some- 

 times becomes so softened by water from springs or rains that the 

 superincumbent mass by its weight alone presses it out laterally, 

 provided its escape is possible, and, sinking down, takes its place. 



Near Tivoli, on the Hudson River, a subsidence of this kind took place in 

 April, 1862. The land sunk down perpendicularly, leaving a straight wall 

 around the sunken area sixty or eighty feet in height. An equal area of 

 clay was forced out laterally underneath the shore of the river, forming a 

 point about an eighth of a mile in circuit, projecting into the cove. Part 

 of the surface remained as level as before, with the trees all standing. Three 

 days afterwards, the slide extended, partially breaking up the surface of the 

 region which had previously subsided, and making it appear as if an earth- 

 quake had passed. The whole area measured three or four acres. 



(3.) When the rocks are tilted and form the slope of a mountain, 

 the softening of a clayey or other layer underneath, in the manner 



