J. LeConte—Old River-beds of California. 181 
some 
cases, however, are found alternate layers of gravel and tufa. 
Where the basaltic lava occurs it always nearly overlies the tufa. 
Nearly all the higher parts of the country are covered both 
With the gravel and with the tufa. The lava is also very 
widely spread. The indications are that these materials formed 
at one time an almost universal covering, but subsequent ero- 
sion has left them in ridges and patches. 
Lixplanation of the phenomena above described. 
There are many difficult and important questions suggested 
. by the phenomena above described which press u 
soluti “How were the old river beds filled with detritus Ze 
How were the streams displaced from their old beds? 
Why the new channels been cut so much deeper than 
these four points in the order mentioned. 
1, Lhe mode of filling of the old river-beds.—There are three 
Possible modes in which we may conceive these beds to have 
* In silicification of wood there is little doubt that the percolating alkaline 
Waters charged with silica are neutralized by the acids of organic decom) 
and the silica thus rendered insoluble is deposited then and there (see author's 
ho yt ae Il the Dardanelle mines near 
n many of the hydraulic mines, especially in the Dardanelle 
Forest Hill, I found in certain parts all the slate and volcanic pebbles while still 
Tetaining their form perfectly, reduced to a soft soapy bluish clay. gone are 
er stones. It is evident their silica has been extracted by alkaline 
