184 J. LeConte—Old River-beds of California. 
the dheioeun of the old river-gravels, as I have 
dekaribed them, are precisely those of deposits made by the 
turbulent action of very swift, shifting, overloaded currents; 
only in this case the currents must have been far swifter and 
more heavily loaded than any existing currents. The detritus 
fore finally the process of filling was probably exceptionally 
rapid. It might have occupied years, or even centuries, but 
was geologically a very rapid process. Now I cannot conceive 
how all these conditions could have been fulfilled, except by 
the rapid melting of extensive fields of ice or snow. But why 
—it will be asked—was the detritus not carried away again? 
I answer: Because immediately after the filling was completed 
the detritus was protected and the rivers rear by the lava- 
flood. This brings me to the next question, 
2. The cause of the displacement of the rivers. ater alre ady shown, 
the mere filling up of the river channels with detritus alone would 
never have displaced the streams. On the contrary, as soon as 
the conditions determining the filling were changed, the rivers 
would immediately have commenced cutting into the detritus 
as they have done on the Eastern coast ; and, on account of the 
high slope of their channels, would ere this ae completely 
swept it all out, as they have done in Southern California. 
The protection of the detritus and the ciienibinceal of the 
streams is due wholly to the lava flood. 
Middle California lies on the southern skirt of the great lava- 
flood of the Northwest.* The center of the great outflow was 
the Cascade and Blue Mountains. In Oregon the lava is 3,000 
feet thick and therefore IRAN conceals the previous sur- 
face configuration of the country. In extreme northern Cali- 
fornia it is still a cenivatran’ mantle several hundred feet thick, 
and therefore the old river beds with few ema es are hope- 
lessly concealed. In Middle California we find it reduced to 
ridges and patches by erosion, but originally it probably was 
even here a nearly universal mantle, covering the whole sur- 
— except some highest points, and substantially obliterating 
e drainage system. But yet this lava mantle was not s0 
a by the writer on this subject in this Journal, vol. vii, p. 167, and P. 
9, 1874. 
