384 OSCAR H. HERSHEY 
Clear Creek volcanic series, both relatively resistant. In the 
Weaver basin it entered an area of the Abrams mica schist, which 
observation elsewhere has shown to be one of the less resistant 
formations, and here the valley widened out to three and four 
miles. When it entered the much harder hornblende schist west 
of Weaverville, it contracted very rapidly to less than one mile. 
I do not see how we can get around this evidence that this is a 
simple valley of erosion, and not a depression resulting from the 
sinking of a fault block. 
The other hypothesis has an objection also. It is another 
unusual feature of these Neocene valleys that they had few trib- 
utaries. Probably the small streams came down from the 
uplands in short cafions with steep gradients. Pleistocene ero- 
sion, in developing the gulches, has largely obscured these ear- 
lier ravines. Outside of the Sierra Costa range it was a region 
not unlike that of the Sierra Nevada today, with the smaller 
streams flowing in shallow valleys on the uplands, and only the 
trunk streams in deep cafions. The topography seems to have 
been young, so far as that particular cycle of erosion was con- 
cerned. Naturally we would suppose that all the streams were 
flowing far above a baselevel of erosion, but in the flat bottoms 
of the old valleys and their width of nowhere much less than a 
mile and in places as much as four miles, we seem to have evi- 
dence that the trunk streams at least were approaching a base- 
level of erosion. I will acknowledge that there is something 
apparently contradictory and unnatural about this, and I am 
unable to give a satisfactory explanation of it. However, if the 
Neocene valleys are the product of erosion, the fact remains 
that some deep cafions were excavated beneath the general level 
of the country. The evidence seems to me to indicate a sharp 
uplift of most if not all of the Klamath region in the Neocene, 
preceding the accumulation of the old channel deposits. The 
elevation may have had a maximum of several thousand feet in 
Trinity county and probably died out to zero on the borders of 
the Klamath province. 
Following this elevation there seems to have been a slow 
