MOUNTAIN SLIDES. 
265 
rock was produced by the denudation of the surface. It may 
have been occasioned by this cause, or by the construction of 
the road through the Notch, the excavations for which, per¬ 
haps, cut through the buttresses that supported the sloping 
strata above. 
Not to speak of the fall of earth when the roots which held 
it together, and the bed of leaves and mould which sheltered 
it both from disintegrating frost and from sudden drenching 
-and dissolution by heavy showers, are gone, it is easy to see 
that, in a climate with severe winters, the removal of the for¬ 
est, and, consecpiently, of the soil it had contributed to form, 
might cause the displacement and descent of great masses of 
rock. The woods, the vegetable mould, and the soil beneath 
protect the rocks they cover from the direct action of heat and 
cold, and from the expansion and contraction which accom¬ 
pany them. Most rocks, while covered with earth, contain a 
considerable quantity of water.* A fragment of rock per¬ 
vaded with moisture cracks and splits, if thrown into a fur¬ 
nace, and sometimes with a loud detonation; and it is a familiar 
observation that the fire, in burning over newly cleared lands, 
breaks up and sometimes almost pulverizes the stones. This 
effect is due partly to the unequal expansion of the stone, partly 
to the action of heat on the water it contains in its pores. The 
sun, suddenly let in upon rock which had been covered with 
moist earth for centuries, produces more or less disintegration 
in the same way, and the stone is also exposed to chemical 
influences from which it was sheltered before. But in the 
climate of the United States as well as of the Alps, frost is a 
still more powerful agent in breaking up mountain masses. 
* Rock is permeable by water to a greater extent than is generally sup¬ 
posed. Freshly quarried marble, and even granite, as well as most other 
stones, are sensibly heavier, as well as softer and more easily wrought, 
than after they are dried and hardened by air-seasoning. Many sand¬ 
stones are porous enough to serve as filters for liquids, and much of that 
of Upper Egypt and Nubia hisses audibly when thrown into water, from 
the escape of the air forced out of it by hydrostatic pressure and the 
capillary attraction of the pores for water. 
