SUBTERRANEAN WATERS. 655 



1. Land-slides. — Land-slides are of the 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 Willey 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, to their destruction : the house remains, as on an 

 island in the rocky stream. 



(2.) A clayey layer, overlaid by other horizontal strata, sometimes 

 becomes so softened by water from sj^rings or rains, that the superin- 

 cumbent 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 claj' was forced out laterally un- 

 derneath 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 afterward, the slide extended, partially breaking up the sur- 

 face 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 

 just explained, may lead to a slide of the superincumbent beds down 

 the declivity. 



In 1806, a destructive slide of this kind took place on the Rossberg, near Goldau, in 

 Switzerland, which covered a region several square miles in area with masses of con- 

 glomerate, and overwhelmed a number of villages. The thick outer stratum of the 

 mountain moved bodily downward, and finally broke up and covered the country with 

 ruins, while other portions were buried in the half-liquid clay which had underlaid it 

 and was the cause of the catastrophe. 



Similar subsidences of soil have taken place near Nice, on the Mediterranean. On 

 one occasion, the village of Roccabruna, with its castle, sunk, or rather slid down, with- 

 out destroying or even disturbing the buildings upon the surface. 



Besides (1) the transfer of rocks 

 and earth, land-slides also cause (2) a 

 scratching or planing of slopes, by the 

 moving strata and stones ; (3) the 

 burial of animal and vegetable life ; 

 (4) the folding or crumpling of the 

 clayey layer subjected to the pressure, 

 where the effect does not go so far as 

 to produce its extrusion and destruction ; 

 while the beds between which it lies 



Fie. 1095. 



Plicated clayey layer. 



