292 Sloping Terraces of Gravel paet ii. 



solid rock has been reached, it has been cut into deep 

 and narrow gorges. Still higher up the valleys, the 

 terraces gradually become more and more broken, 

 narrower, and less thick, until, at a height of from 

 7.000 to 9,000 feet, they become lost, and blended with 

 the piles of fallen detritus. 



I carefully examined in many places the state of the 

 gravel, and almost everywhere found the pebbles equally 

 and perfectly rounded, occasionally with great blocks 

 of rock, and generally distinctly stratified, often with 

 parting seams of sand. The pebbles were sometimes 

 coated with a white aluminous, and less, frequently with 

 a calcareous, crust. At great heights up the valleys, 

 the pebbles become less rounded : and as the terraces 

 become obliterated, the whole mass passes into the 

 nature of ordinary detritus. I was repeatedly struck 

 with the great difference between this detritus high up 

 the valleys, and the gravel of the terraces low down, 

 namely, in the greater number of the quite angular 

 fragments in the detritus, — in the unequal degree to 

 which the other fragments have been rounded, — in the 

 quantity of associated earth, — in the absence of stratifi- 

 cation, — and in the irregularity of the upper surfaces. 

 This difference was likewise well shown at points low 

 down the valleys, where precipitous ravines, cutting 

 through mountains of highly coloured rock, have thrown 

 down wide, fan-shaped accumulations of detritus on the 

 terraces : in such cases, the line of separation between 

 the detritus and the terrace could be pointed out to 

 within an inch or two ; the detritus consisting entirely 

 of angular and only partially rounded fragments of the 

 adjoining coloured rocks; the stratified shingle (as I 

 ascertained by close inspection, especially in one case, 

 in the valley of the E. Mendoza) containing only a 



