296 Sloping Terraces of Gravel : part ii. 



in every valley will have tended to have removed the 

 matter which just before had been arrested on, or near, 

 the beach-lines ; the torrents, also, having continued 

 to gain in force by the continued elevation increasing 

 their total descent from their sources to the sea. This 

 slow rising of the Cordillera, which explains so well the 

 otherwise inexplicable origin and structure of the ter- 

 races, judging from all known analogies, will probably 

 have been interrupted by many periods of rest ; but we 

 ought not to expect to find any evidence of these periods 

 in the structure of the gravel-terraces : for, as the waves 

 at the heads of deep creeks have little erosive power, so 

 the only effect of the sea having long remained at the 

 same level will be that the upper parts of the creeks 

 will have become filled up at such periods to the level 

 of the water with gravel and sand ; and that afterwards 

 the rivers will have thrown down on the filled-up parts 

 a talus of similar matter, of which the inclination (as 

 at the head of a partially filled-up lake) will have been 

 determined by the supply of detritus, and the force of 

 the stream. 1 Hence, after the final conversion of the 

 creeks into valleys, almost the only difference in the 

 terraces at those points at which the sea stood long, will 

 be a somewhat more gentle inclination, with river-worn 

 instead of sea- worn detritus on the surface. 



I know of only one difficulty on the foregoing view, 

 namely, the far-transported blocks of rock high on the 

 sides of the valley of the Cachapual : I will not attempt 

 any explanation of this phenomenon, but I may state 



present in the valleys of most of the European ranges, had not 

 every trace of them, and all wrecks of sea-action, been swept away 

 by the glaciers which have since occupied them. I have shown that 

 this is the case with the mountains (' London and Edin. Phil. 

 Journal,' vol. xxi. p. 187) of North Wales. 



1 I have attempted to explain this process in a more detailed 

 manner, in a letter to Mr. Maclaren, published in the * Edinburgh 

 New Phil. Journal,' vol. sxxv. p. 288. 



