J. LeConte—Old River-beds of California. 185 
thick but that subsequent erosion has cut through the thinner 
arts, i. @., on the previous higher ground. Immediately after 
the obliteration of the previous drainage system, the rivers, of 
course, commenced cutting a new system, having the same 
general trend (for this. is determined by the general mountain 
slope), but wholly independent of, and therefore often cutting 
across, the older system. Furthermore the streams in forming 
their new beds seem to avoid the places of the old beds, for 
there the lava would be thickest, and cut their channels on the 
old divides for there the lava was thinnest and therefore soonest 
removed by general erosion, or perhaps was absent altogether. 
Again: we have already seen that the rush of overloaded 
With detritus. Before the melting was completed the ash-erup- 
tons had already commenced, and mud-streams, foliowed by 
lava-streams, completed the work of obliteration. We see pre- 
cisely the same phenomena on a smaill scale, in the destructive 
fi s and mud-streams which precede and accompany the 
fruptions of volcanos like Cotopaxi, whose summits are cap- 
ped with perpetual snow. In the case we are discussing, how- 
ever, instead of a volcanic peak, a great mountain range 
covered with snow, eru 
of all geographical features, river courses in elevated regions 
most permanent. 
Uinta Mountains commenced to rise directly athwart its path- 
°wnward in the same proportion. When once a river, as It 
Were, bites in and gets a grip upon the rocky bones of the 
* Powe lorati iver, p. 152; Dutton, Nature, vol. xix, p. 
247 and Ay “35 on of Colorado river, p. ; ; 
$ 
