THE BED-ROCK SURFACE AND THE CHANNELS. 347 
Portions of the erupted material were almost entirely indestructible, and 
remain to the-present time very little affected by weathering, protecting the 
various underlying detrital materials from being swept away, while they 
themselves stand out in the well-known “ table mountain” form, often rising 
high above the surrounding country. The cause of such topographical 
features is easily enough comprehended ; and were all the volcanic materials 
as solid and indestructible as that of the Tuolumne Table Mountain, for 
instance, there would be no difficulty in understanding how it is that these 
lavas occupy so high a position. But, as Mr. Goodyear has shown in his 
review of his notes, much of the erupted material appears to be of a kind 
readily acted on by water, and more likely to be eroded away than the 
average bed-rock itself. This is particularly the case in the region which 
was the field of his special investigation ; but both north and south of his 
district there is much solid basaltic lava, forming “table mountains,” like 
that of which the one in Tuolumne County offers a typical example.* 
The great difficulty in regard to the formation of the present river caiions 
is to explain how they were started in the position which they now occupy. 
Having once been begun, their present character can easily be shown to be 
the result of the peculiar climatological and geological character of the region. 
The gradually diminishing quantity of water which the streams carry pre- 
vents their rising above the channel in which they are confined; they must 
therefore continue to excavate along one line. But as the volume of water 
has decreased, so their erosive power has diminished; and the cafons, 
although gradually deepened, have become narrower, their whole form 
being that which must result from the action of a force continually 
diminishing in intensity. The sides of these cations have been kept free 
from débris, as it appears, through the agency of the occasional sudden and 
heavy “ freshets,” which still take place, as previously described, although 
with, on the whole, gradually lessening intensity. 
At the present time the excavating power of the streams seems to have 
diminished almost to nothing. Only the finer portions of the débris resulting 
from the hydraulic washings is carried away by the current; so that where 
the bottoms of the cafions have become filled to great depth with tailings, the 
streams, under ordinary circumstances, hardly disturb them. It is only dur- 
ing such winters as that of 1861-1862, when very extraordinary rises take 
place, that portions of this detrital material are swept down into the Great 
* See Geology of California, Vol. I. pp. 219 - 211, for sections and descriptions of some of these. 
