DOMAIN II. SILICEOUS. 



seems harder than the pebbles themselves, which 

 . are apt to drop out entire, the circumference of 

 crystallisation having been as exactly denned by 

 the laws of attraction, as in the detached peas, or 

 little geods of iron, already mentioned. Patrin 

 supposes that they were formed separately, and 

 afterwards cemented by siliceous matter ; but as 

 many other crystals are easily detached from the 

 gangart, there seems to be no necessity for this 

 supposition. 



Saussure, 1943, has treated the utility of the 

 Pebbles, study of pebbles. In the glens of high mountains 

 they are of the same stones with these mountains; 

 but in the plains, and the large adjoining valleys, 

 they are pf quite a different nature, and seem to 

 have been transported by some great revolution. 



" It is an important observation for the theory 

 of the earth, that in the upper parts of valleys 

 surrounded with high mountains, no rolled pebbles 

 are found, which are foreign to the valley itself in 

 which they are met with ; those observed are never 

 other than spoils of the neighbouring mountains. 

 In the plains, on the contrary, and at the openings 

 of valleys which adjoin the plains, and even some 

 way up the sides of the mountains which border 

 on these plains, pebbles and blocks are found, 

 which might be said to have fallen from the hea- 



