46 THEORY OF THE EARTH. 



their want of solidity subjects them also to slips, in 

 consequence of being undermined by the waters 

 of rivulets. On these occasions, towns and rich 

 populous districts are sometimes buried under the 

 ruins of a mountain; the courses of rivers are 

 stopped up, and lakes are formed in places which 

 were before the abodes of fertility and cheerful- 

 ness. Fortunately such great slips occur but sel- 

 dom; and the principal use of these hills, compos- 

 ed of fragments and ruins of the high mountains, is 

 to furnish materials for the ravages of the torrents 

 to operate upon.* 



10. Of Alluvial Formations.^ 



. 



The rains which fall upon the ridges and sum- 

 mits of the mountains, the vapours which are con- 

 densed there, and the snow which is melted, de- 

 scend by an infinite number of rills along their 

 slopes, carrying off some portions of the materials 

 of which these ridges and summits are compos- 

 ed, and marking their courses by numerous gut- 

 ters. In their progress downwards, these small 

 rills soon unite in the deeper furrows with which 

 the surface of all mountains is ploughed up, run off 

 through the deep valleys which intersect the bot- 

 toms of the mountains, and at length form the 

 streams and rivers which restore to the sea the 

 waters that it had formerly supplied to the atmos- 

 phere. 



/ *NoteE. fNotcF. 



