THE BUD-ROCK SURFACE AND THE CHANNELS. 



349 





In a 



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.* 

 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 beflrinninff the erosive work, the final results of which are visible in 

 the grand dimensions and depth of the canon 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 debris 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 

 case 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. 



