﻿J. LeConte — Elevation of the Sierra Nevada. 173 



Geological Survey, that there is a peculiar, almost unique rela- 

 tion of the old to the new river-beds in middle California. At 

 the end of the Tertiary period the then existing river-system 

 was obliterated by an immense flood of lava which, issuing from 

 fissures in the high Sierra, flowed westward down the river- 

 valleys filling them completely and displacing the rivers. The 

 displaced rivers, of course, immediately set to work cutting 

 new channels mainly on the intervals or low divides between 

 the old channels, because there the lava was thinnest or absent 

 altogether. Since that time the rivers have cut beds 2000 to 

 3000 feet below the level of the old beds. The relation between 

 the old and new beds is shown in the figures (figs. 5 and 6) in 

 •which r' r' represents the old Tertiary beds and r r the present 

 beds. Sometimes the lava is hard basalt and erosion then gives 

 rise to table mountain forms as in fig. 5. Sometimes it is 

 softer, as ash, tufa, etc., and thus erosion- forms are rounded as 

 in fig. 6. 



5. 



Ideal sections across river beds in middle California, In fig. 5 the lava is 

 basalt, in fig. 6 tufa, r, Present river bed ; r' r f , old river bed ; I, lava ; si, slate. 



Now what we wish to emphasize, is that the lower position 

 of the present river beds is not the result of longer time of 

 cutting (for the time has been very much shorter), but of more 

 rapid work — that this position is demonstrative proof that a 

 very great increase in the height and angle of slope of the 

 Sierra took place contemporaneously with the lava flow at the 

 end of the Tertiary or beginning of the Quaternary. The old 

 river-beds were probably established at the beginning of the 

 Cretaceous when the Sierra range was born from the sea, and 

 were being cut and shaped throughout the whole Cretaceous 

 and Tertiary. By the end of that time they seem to have been 

 broad, shallow troughs with low divides between as shown by 

 the dotted lines in figs. 5 and 6. The rivers seem to have 

 reached their base-level and had ceased to deepen their chan- 



