258 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



sense, are extremely ephemeral, yet they are clearly legible at the present 

 da}', thus indicating the recency of their origin. The time that the terraces 

 liave been exposed to subaerial erosion must evidently be extremely brief 

 in comparison to the ages required for the accumulation of the vast debris 

 piles on which they were made. 



Another, although less definite proof of the aridity of the time preced- 

 ing the rise of Lake Lahoutan is furnished by the canons of the streams 

 that enter the basin. In many instances these were excavated to their 

 present depth before the existence of the lake, which subsequently occupied 

 their channels for many miles. An illustration of this phenomenon is fur- 

 nished by the canons of Smoke and Buffalo creeks, which were eroded to 

 the depth of 250 or 300 feet through compact basalt before the rise of Lake 

 Lahontan. When the lake had its greatest extension it occupied the lower 

 portions of these gorges and filled them deeply with marly-clays and delta 

 deposits^ at the same time that their walls became loaded with tufa. When 

 the lake retired the streams reclaimed their ancient channels and com- 

 menced the removal of the lacustral strata. The creeks are now flowing 

 over their ancient beds of basalt, but the recent corrasion of the volcanic 

 rock is scarcely perceptible. The amount by which the canons have been 

 deepened during the present arid period, as compared with the work accom- 

 plished in pre- Lahontan times, is certainly in the proportion of one to many 

 thousand. Parallel illustrations of the same phenomena are furnished l)y 

 the rivers which enter the basin from the west, all of which flow in channels 

 of 25re- Lahontan date, and became partially filled with lacustrine strata and 

 subsequently re-excavated as in the previous instances. Each of these 

 streams now flows tin-ough a canon within a canon, in the manner illus- 

 trated by the diagram on page 44. 



It might be said that when these canons were formed, the basin to 

 which they are now tributary had a free drainage to the sea. It is impos- 

 sible to prove or disprove this hypothesis, but in general, canons of the 

 character of those in question may be considered as characteristic of arid 

 regions. Besides, we know, from the great depth of marl and gravel in 

 many of the valleys of the Great Basin, that they have been regions of 

 accumulation for long periods. The weight of evidence is such, in our 



