390 OSCAR H. HERSHEY 
later system of valleys often coalesce with the slopes of the 
older valleys so as to make the whole trench, perhaps five thou- 
sand feet in depth, appear as the work of a single cycle of ero- 
sion. Usually, however, there is a shoulder high up on the 
slopes which is not always explained by the structure or by 
landslides, and the concurrence of a series of these shoulders at 
about the same elevation on both sides of one of these valleys 
raises the suspicion that they represent the bottom of the 
Neocene valley. 
So far as is now known the Sierra Costa range is the only por- 
tion of the Klamath region or indeed of any part of northern Cali- 
fornia which possessed a rugged, sierra-like topography during 
the late Neocene, and it should be awarded the distinction of 
being the oldest prominent mountain range within the state, 
unless such exist south of the Tehachapai range. 
Correlation.— Rhyolite tuff has been found by Diller in the 
Indian Creek basin, and I believe also in Hyampour valley, and 
by Anderson and the writer in Hay Fork valley. The first 
investigator thought the material had been showered over the 
hills as fine ashes and then carried down into the streams, and 
thus became interstratified with the alluvial gravels. It was 
largely because of the presence of this tuff that he provisionally 
correlated the Neocene gravels in Trinity county with the Ione 
formation. At the same time he recognized the possibility that 
the tuff may have been derived from the group of volcanoes in 
the Coast Range region, in Lake county, and may not belong to 
the epoch of rhyolitic extrusions in the Sierra Nevada region. 
The latter represent a distinct epoch of the period of vulcan- 
ism, and this rhyolite epoch was contemporaneous with the 
accumulation of the auriferous gravels proper or high-level 
channels. The latter are now thought to be the chronologic 
equivalent of the San Pablo formation, presumably of Lower 
Pliocene age. If the rhyolite tuff of the Klamath region was 
derived from the Sierra Nevada or Lassen Peak volcanoes, it 
would imply that the tuff-bearing portion of the Neocene gravels 
is probably Lower Pliocene in age. The presence of lignite in 
