186 J. LeConte—Old River-beds of California. 
country it does not easily loose its hold. Rivers with deep 
channels like those of California will not change. eir chan- 
nels must be obliterated, and then they make new channels. 
Such obliteration can only take place by submergence and 
prolonged sedimentation or else by a lava-flood. 
We have seen that tufaceous conglomerate usually underlies 
the basaltic lava and covers the detritus even where the lava 
is wanting. It is evident therefore ash eruptions preceded the 
basaltic flow. The washing down of these ashes as mud 
streams completed the filling and then the lava flood covering 
stream beds, and once commenced would continue to cut in 
these places. 
ing* has drawn attention to the fact that in the same locality 
ifornia and especially in Oregon the lava flood is so thick that 
the buried old river system is not revealed by erosion—the 
present rivers are running far above the old rivers. In South- 
ern California on the contrary the rivers have never been dis- 
placed by lava, for the lava flood did not reach so far. If these 
channels were ever filled with detritus, this has not only been 
swept out again, but the rivers have continued to deepen their 
channels even to the present time. The double river system 
of Middle California is the result of the fact that this part lay 
in the extreme skirts of the great lava flood. In British Oo 
lumbia beyond the limits of the lava flood, the relation of the 
new to the old river beds, as I learn from Mr. Amos Bowman, 
is again like those of the Eastern States. The rivers are noW 
cutting into the detritus which fills the broader and deeper 
channels of the old rivers. 
* Exploration of 40th Parallel, vol. i, Systematic Geology, p. 715. 
