DESTRUCTION OF MOUNTAINS. 



315 



Here, placed nearly above the region of vegetable or animal exis- 

 tence, and surrounded by the sublimest objects in nature, the deep 

 silence which prevails around is truly solemn and impressive ; but it 

 is broken from time to time, by sounds like the rolling of distant 

 thunder, or by a nearer and louder crash, which is repeated by the 

 echoes from rock to rock. These sounds proceed from the falling 

 of avalanches, or from glaciers splitting and discharging the loose 

 rocks upon their surface, or from eboulements of rock, detached from 

 I the bare and exposed sides of the pinnacles and aiguilles. The 

 ! fragments generally fall into the elevated mountain valleys, and are 

 scattered over the surface of the higher glaciers, which extend from 

 ! thence into the lower Alpine valleys. As the glaciers in these val- 

 i leys are gradually melting during summer, the ice above progressive- 

 ly moves downward, bearing with it the cargoes of stones on its sur- 

 face, which it discharges in heaps at its feet and sides. These ac- 

 , cumulations of stones are called morains. The destruction of granitic 

 and schistose mountains it has been before observed, is generally 

 effected by water penetrating between the fissures, becoming sudden- 

 ly expanded by frost. The overthrow of calcareous rocks is effec- 

 ted in a different manner ; and the vast eboulements which they oc- 

 1 casion, are more terrific and destructive, than the eboulements from 

 the primary mountains, as they generally take place in more thickly 

 inhabited districts. 



The destruction of the calcareous mountains in the Alps, depends 

 on the peculiar composition and structure of these mountains. In 

 I the year 1821, I passed a great part of the summer in examining 

 the calcareous mountains in Savoy, the structure of which w^as then 

 not generally understood, or at least had not been described, in any 

 j geological work that I had met with. It was generally believed, that 

 j the calcareous mountains were composed entirely of beds of lime- 

 i stone, with lofty mural precipices on the upper part; and that the 

 ! iower parts, sloping from these precipices, were formed of the de- 

 j bris of the limestone. So far from this being the case, the calcare- 

 ous mountains of the Alps, which comprise all the EngHsh forma- 

 tions, from the rnagnesian limestone or chalk, akernate, like the Eng- 

 lish formations, with enormous beds of soft shale and sandstone ; and 

 it is to this alternation, that they owe the frequent destruction of the 

 upper parts of the mountains. 



If all our English secondary formations, were, by some powerful 

 cause, elevated six or seven thousand feet above their present lev^l, 

 and the beds bent into curves, constituting several ranges of imun- 

 tains, we should have precisely what is found in the calcareous ran- 

 ges of the Alps. This arched form of the calcareous mountains is 

 represented, Plate II. fig. 1., and fig. 2. oc, y. Now, if one thick 

 bed of limestone, or a portion of it, be broken off as at ^, fig. 2, the 

 action of continued rains on the soft bed on which it rests, will un- 

 dermine it, until other portions of the limestone will fall down ; and 



