—_— 
y, 
THE BED-ROCK SURFACE AND THE CHANNELS. 349 
the greatest confidence, to the indestructible character of the basalt by which 
the lower portion of a pre-existing depression was partly filled as the result 
of a volcanic eruption. But only a very short distance above the place 
where all these phenomena are characteristically displayed, we find the 
Stanislaus River breaking directly across the ancient channel, and forming 
a gap or gorge of fully 1,500 feet in depth. From this intersection the 
Pliocene and the present rivers run nearly parallel with each other for 
several miles, when the basaltic mass is again cut through by the present 
river, and from here down to the foot-hills the Stanislaus winds between the 
old lava-flows in a manner for which it seems impossible to account.* Ina 
similar manner the present Yuba cuts directly across the old channel in 
various places, where it appears extremely difficult to reconstruct the former 
topography in such a manner as to give the stream the necessary opportu- 
nities for beginning the erosive work, the final results of which are visible in 
the grand dimensions and depth of-the cafion at the bottom of which the 
river now runs. That there was some reason, however, why the water 
selected the particular course which it followed, no one can doubt, however 
difficult it may be to point it out at the present time. The ways of water 
beneath the surface are enveloped in obscurity ; so much so that the most 
contradictory views are held in regard to this subject by geologists of ability 
and experience. Portions of the lava and gravel deposits must have been 
very permeable to water, much more so than the average bed-rock. This 
might lead to the formation of subterranean currents, the result of which 
would be the diversion of a proportionally very large quantity of water along 
the lines of the former deep depressions. These would thus be deepened at 
a much more rapid rate than those portions of the surface where the bed-rock 
had the thinnest covering of volcanic and other débris upon it. Thus the 
streams finally came to occupy the old depressions, which they have ever 
since been deepening, for reasons which have already been given. In the 
ease of a more rapid wearing away of the bed-rock, in consequence of the 
indestructibility of the lava, there would be usually well-defined rim-rocks 
on both sides of the channel. When the reverse took place, the rim might 
or might not be present, and this would depend, in great measure, on the 
form which the underlying bed-rock surface had before the channel became 
permanently filled up. 
* See diagram of Table Mountain lava-flow, Plate D, opposite page 132. 
