ALLUVIAL OR TORRENTIAL CONGLOMERATES. 219 



is a process which has often awakened the surprise of engineers who are 

 called for the first time to deal with the problems of harbor protection and 

 is ever revealing wonderful things. Not only does the finer loose material 

 move in grand procession under the influence of unseen, though still com- 

 prehensible, agencies, but very coarse detritus is carried slowly with it. 

 The tendency of the process, however, is not towards an indiscriminate 

 mixing of all sorts and sizes, but towards the grouping into layers, here of 

 coarser, there of finer, stuff, according to the variations in the power of the 

 moving water. 



But there is another class of conglomerates which claims our special 

 attention. These are of alluvial origin, formed, not beneath the surface of 

 the sea nor of lakes, but on the land itself. They do not seem to have 

 received from investigators all the attention and study which they merit. 

 They are usually called gravels — perhaps are sometimes or even frequently 

 mistaken for glacial drift — but their homology to the ordinary stratified 

 conglomerates of the systematic strata is not always recognized. Through- 

 out great portions of the Rocky Mountain region they are accumulating 

 to-day upon a grand scale and have accumulated very extensively in the 

 .past. 



The processes of degradation are far more energetic and effective in 

 mountains than upon plains. The agents which disintegrate rocks — frost, 

 rain, chemical solution — have the greatest freedom of action upon the steep 

 slopes of the numberless ravines, and are continuously breaking off frag- 

 ments and reducing them to sand, gi-avel, and clay. Not only is the greater 

 part of the finer mold gathered up by the swift rills and torrents, but frag- 

 ments of considerable size, attaining, under favorable circumstances, the 

 weight of several tons, are caught and urged downward in rushing- rapids 

 with an energy which must be seen in order to be realized. The many 

 streamlets and filaments of a mountain amphitheater gradually unite, as we 

 descend from the crest of the mountains, generating a creek, which attains 

 its greatest flood near the mountain base, and when the snows melt in the 

 spring its swollen current sweeps onward a mass of clastic material of every 

 description from impalpable clay to bowlders. Within the mountain masses 

 the descents are rapid and the streams are torrents. Reaching the valleys 



