AGENTS THAT DESTROY ROCKS. 135 



Land-slides occur frequently on the north coast 

 of Long Island. In some places they are caused 

 by the sea undermining the cliffs, so that the super- 

 incumbent masses crack off at a short distance from 

 their edges, and slide down to a lower level, carry- 

 ing with them trees and shrubs, and often changing 

 their relative position. These slides are the most 

 numerous where there are beds of clay. A land- 

 slide occurred at Troy in January, 1837, caused by 

 a spring of water rendering the clay slippery. The 

 hill was about 227 feet in height, from which a large 

 mass slid off, and, carrying everything before it, 

 swept down a distance of 800 feet, crossing one 

 street, crushing three houses and two barns, and de- 

 stroying the lives of several persons, who were 

 buried beneath the ruins. The avalanche was ac- 

 companied by torrents of water and mud, and the 

 materials now cover a space 200 yards in length by 

 100 in breadth, and from 10 to 40 feet deep. It is 

 calculated that 200,000 tons of earth were thus trans- 

 ported to a distance of 200 yards. 



The atmosphere may be considered as a mechani- 

 cal agent, which has more or less influence in the 

 destruction of rocks, especially when violently agi- 

 tated, as during storms and tempests. We see hur- 

 ricanes uprooting the largest trees, throwing rocks 

 from their native beds, and scattering their frag- 

 ments over the surface. 



Action of Running Water. — Lands elevated above 

 the sea attract, in proportion to their volume and 

 density, a larger quantity of that aqueous vapour 

 which the heated atmosphere continually absorbs 

 from the surface of lakes and the ocean. By these 

 means, the higher regions become perpetual reser- 

 voirs of water, which descend and irrigate the low- 

 er valleys and plains. The water is consequently 

 carried to the higher regions, and is then made to 

 descend by steep declivities towards the sea; so 

 that it acquires superior velocity, and removes a 



