190 DISCUSSION OF THE DESICCATION QUESTION. 



itself with those which preceded it, as well as with those by which it was 

 followed, will be, to some extent, set forth in the next chapter. Before 

 advancing another stage in our discussion, however, we have to make it 

 clear that the diminution of the rivers, the disappearance of the lakes, and 

 all the other phenomena indicative of a gradual but persistent tendency to 

 aridity over vast areas once fertile and well watered, do not form a transient 

 phase of a precedent Glacial epoch, but are the residt of some cause which 

 began to act before that period, and is still continuing without any connec- 

 tion with it. 



To throw light on this matter we may again turn to the facts made evi- 

 dent in the course of the investigations of the gravel deposits of the Sierra 

 Nevada, where, as the writer conceives, we find the series of geological 

 events, in so f;ir as they relate to the question now before us, so well marked 

 that it is impossible to mistake their order of succession and their connec- 

 tion with each other. All the points to which attention should be turned, 

 in reference to the diminution of the rivers of the Sierra, cannot be taken 

 up at the present moment, but certain inferences of importance can be 

 drawn. And, in the first place, it is certain that in the gravel region of 

 California there is ample proof that the decrease in volume of the Tertiary 

 rivers had been begun in Tertiary times, and carried so for that the changed 

 condition of things, in this respect, had before the close of that epoch be- 

 come a feature of the utmost importance in the climatology of that region. 

 The whole character of the drainage channels had undergone a change, as 

 described in the Auriferous Gravels;* broad streams, with gradually sloping 

 banks, having given place to narrow ones, confined in deep gorges with ex- 

 tremely precipitous sides. 



Indeed there seems to be little doubt that the topogi\iphy of the western 

 slope of the Sierra had received what may be said with truth to have been 

 essentially its present form and character, before the epoch of the extension 

 of the glaciers over that range took place. And, what is still more impor- 

 tant in this connection, it admits of little doubt that hardly any percejitible 

 change was effected in the topographical features of the range during the 

 time' when ice covered its summits, to the extent and in the manner de- 

 scribed, in considerable detail, in the first chapter of the present volume. 

 A similar statement may be made with truth in regard to the other ranges 

 of the Cordilleras, over which glaciers spread themselves during the Cordil- 



* See Auriferous Gravels, p. 335, and passim. 



