DISINTEGRATION OF ROCKS. 



CHAPTER VIII. 



On the Formation of Arable Land. 



The hardest rocks and stones gradually lose their coherence 

 when exposed to the influence of certain agencies. Soils con- 

 sist of the debris of rocks which have suffered this change. 



The disintegration of minerals and rocks is effected partly by 

 mechanical, and partly by chemical means. It has been re- 

 marked in all the mountainous districts of perpetual snow, that 

 the most refractory rocks crumble into fragments,* which are 

 either rounded by the action of glaciers, or are thoroughly pul- 

 verized into dust. The rivers and streams arising out of the 

 glaciers are rendered turbid with this mineral debris which they 

 deposit on reaching the plains and valleys ; thus fertile soils are 

 formed. 



" As often as I have seen beds of mud, sand, and shingle, 

 accumulated to the thickness of many thousand feet, I have felt 

 inclined to exclaim, that causes such as the present rivers and 

 the present beaches could never have ground down such masses. 

 But, on the other hand, when listening to the rattling noise of 

 these torrents, and calling to mind that whole races of animals 

 have passed away from the surface of the globe, during the 

 period throughout which, night and day, these stones have gone 

 rattling onwards- in their course, I have thought to myself, Can 

 any mountains, any continent, withstand such waste V f 



* " I frequently observed, both in Terra del Fuego and within the 

 Andes, that where the rock was covered during the greater part of the 

 year with snow, it was shivered in a very extraordinary manner into small 

 angular fragments. Scoresby has observed the same fact in Spitzbergen ; 

 he says : * The invariably broken state of the rocks appeared to have been 

 the effects of frost.' " — Darwin's Nat. Hist, of the Voyage of the Beagle ; 

 p. 388. 



t Darwin, Nat. Hist, of the Voyage of the Beagle, p. 386 

 5* 



