270 



University of California. 



[Vol. i. 



probably not confined to these areas of accumulation of Pliocene 

 sediments, but affected also other portions of the Coast Ranges, 

 deforming the peneplain which had been in course of evolution in 

 Pliocene time. Mount Diablo probably owes its upthrust to this 

 epoch of disturbance. The bold ridges and peaks of western 

 Humboldt County probably rose above the coastal peneplain at the 

 same time; and the sag in the coast of middle Mendocino County 

 may possibly be due to the same cause. These mountain-making 

 movements were not, however, adequate to efface the peneplain, and 

 the general altitude of the coast was not apparently affected. The 

 base-leveling process continued and the peneplain was extended in 

 between the bolder masses of the disturbed districts, and completely 

 over the region occupied by the soft rocks of the Wild-cat Series. 

 After this interval of encroachment of the old peneplain between 

 the new orogenic blocks, the general uplift was inaugurated and has 

 proceeded by stages down to the most recent times, the uplift in 

 northern California being from 1,500 to 2,100 feet. This uplift from 

 its extensive character can only be regarded as an epeirogenic move- 

 ment. In so designating it, all allowance must be made for local 

 inequalities of the movement. The ideal conception of an uplift 

 strictly simultaneous in its action and uniform in its measure over 

 any large tract of the earth's surface has probably been rarely or 

 never realized in fact. The term used was probably not coined by 

 its author for an ideal conception, but to describe phenomena which 

 geologists meet with in practical work. The evidence from the 

 geomorphy of the north Californian coast indicates a sufficient 

 approximation to this ideal in so far as the measure of uplift is con- 

 cerned; and the movement has been sufficiently simultaneous to 

 inaugurate a new geomorphic cycle which is in a nearly uniform 

 state of advancement all along the coast. The term epeirogenic 

 seems, therefore, appropriate to the movement, though it may have 

 involved local irregularity of action or of rate of action. 



The Pliocene peneplain which was raised by the epeirogenic 

 uplift seems to have differed in some respects in northern California 

 from that of the southern Coast Ranges. In the latter, the plain 

 was a baselevel partly of delta accumulation and partly of mountain 

 truncation. In the former, it was essentially a plain of truncation. 



