92 RUNNING WATER 



has been profoundly altered and decomposed by the action of 

 the heated waters which traverse it. 



Except in limestone caverns, underground waters seldom flow 

 with sufficient velocity to accomplish much in the way of direct 

 mechanical erosion, but indirectly they bring about mechanical 

 changes of some importance. Slopes of earth or talus blocks 

 lying on hillsides or mountains, saturated by long-continued, heavy 

 rains, may have their weight so much increased and their friction 

 so reduced, as to glide downward in landslips, which in inhabited 

 regions are sometimes very destructive. Of this kind was the 

 great landslip which occurred in the White Mountains (New 

 Hampshire) in 1826. 



Landslips may also occur when the rocks forming a slope are 

 inclined in the same direction as that slope ; the surface layers 

 weighted with water, and especially if underlaid by clay-beds, 

 which when lubricated with water become very slippery, may 

 glide down the slope into the valley below. Mountain valleys in 

 all parts of the world show plain evidence of such landslips, and 

 the amount of rock thus displaced is sometimes very great. The 

 landslip which occurred at Elm, Switzerland, in 1881, is esti- 

 mated to have Carried down more than 12,000,000 

 cubic yards of rock for a distance of 2000 feet. 



Springs. 



Springs are the openings of un- 

 derground streams upon the 

 surface, and could not be 

 formed were the land 

 perfectly free from 



FlG. 30. — Arrangement of strata which causes hillside . . . 



springs. The lower close-lined bed impervious. irregularities, for 



gravity controls the 

 movement of underground waters, and the source of a spring must 

 be higher than its mouth. It must be remembered, however, that 

 a subterranean stream is often confined as in a pipe, and that the 

 pressure to which it is subjected may seem to make it flow upward, 



