DRAINAGE SYSTEM— MIOCENE EROSION. 17 



can be clearer than the fact that the structural deformations (unless older 

 than Tertiary time) never determined the present courses of the drainage. 

 The rivers are where they are in spite of faults, flexures, and swells, in 

 spite of mountains and plateaus. As these irregularities rose up the streams 

 turned neither to the right nor to the left, but cut their way through in the 

 same old places. It is needless to multiply instances. The whole province 

 is a vast category of instances of river channels running where they never 

 could have run if the structural features had in any manner influenced them. 

 What, then, determined the present distribution of the drainage ? The 

 answer is that they were determined by the configuration of the old Eocene 

 lake bottom at the time it was drained. Then, surely, the water-courses ran 

 in conformity with the surface of the uppermost Tertiary stratum. Soon 

 afterward that surface began to be deformed by unequal displacement, but 

 the rivers had fastened themselves to their places and refused to be diverted. 

 Many of the smaller streams have dried up and perished through the fail- 

 ure of their springs and the advent of an arid climate. These have left 

 traces here and there in the shape of dry canons and gulches. Many more 

 are still perishing. But the larger streams heading far up in moist Alpine 

 highlands still meander through the desert, and have never ceased to flow 

 from the beginning. 



In order to comprehend the relations of the High Plateaus to the 

 province at large, it is necessary to advert to some of the salient features 

 of the general erosion of the Plateau Country which followed the desicca- 

 tion of the great lake, and which continued without interruption during 

 Miocene time and down to the present day. Its history during Miocene time 

 must be spoken of only in general terms. In truth, during that great age 

 there is no evidence of the occurrence of any critical event aside from the 

 general processes of uplifting and erosion which affected the province as a 

 whole. What forms and what topography were sculptured we know not- 

 Of its climatal condition we can only suppose that it was similar to that of 

 neighboring regions similarly situated — moist and subtropical. The vast 

 erosion of the region has swept away so much of its mass, that most of the 

 evidence as to details has vanished with its rocks. But the more important 

 features of the work, its general plan in outline, have left well-marked 



2 H P 



