J, LeConte—Old River-beds of California. 187 
Why the modern rivers have cut to a lower level.—I have 
already in a previous article* given reason to believe that the 
great lava flood of the Northwest came not from craters but 
from great fissures, and that the force of eruption was not the 
pressure of elastic gases merely, but also the lateral squeezing 
y which mountain ranges are elevated. It is almost certain, 
then, that coincident with the outflow of lava in California 
there was an increase in the elevation of the Sierra range. The 
inevitable effect of this would be the cutting of the new chan- 
nels below the level of the old, and thus finally the singular 
relation between the old and the new channels which now exists. 
There is a certain definite relation between the slope and the 
amount of detritus which determines the depth of the cafions. 
sion of the whole Cretaceous and Tertiary times, if the general 
Sierra slope were as high then as it is now, viz: 100 to 200 feet 
Proper relation was again established. 
The elevation which I suppose took — in the Sierra range 
of the continent was probably along the valley of Mississippi 
River; the axis on this side was the crest of the Sierra, where 
it gave rise to fracture and outflow. : 
ie places where the lava emerged have not been found with 
Certainty. It was probably along or near the crest, where the 
* This Journal, vol. vii, p. 177, 1874. 
