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 chffs, 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 w^ater 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 
