i6o 



University of California. 



[Vol. i. 



While epeirogenic processes have thus been active, orogenic 

 forces have not been at rest. Important local differential displace- 

 ments of the crust have occurred. The instances noted are prob- 

 ably representative of other similar orogenic displacements not yet 

 known. 



In the case of Santa Catalina Island these are manifest in a de- 

 pression of the island relatively to the land masses on either side. 

 This depression may have been subsequent to the major portion of 

 the epeirogenic movement, or may have been concomitant with it. 



In the more notable case of the San Francisco Peninsula orogenic 

 action is very apparent in the upthrust of Montara Mountain, and 

 the consequent deformation of the Merced series (Pliocene). This 

 upthrust, unlike the Santa Catalina depression, seems to have pre- 

 ceded the general uplift, and the two events may be discriminated. 

 The effect of the upthrust has been to produce a mountain with a 

 granite core and flanking strata dipping quaquaversally from it. 

 There are two types of mountains which have this structure, viz., 

 the laccolitic and the batholitic. Montara Mountain is a type of 

 structure distinct from either of these, since the granite is not 

 intrusive in the flanking strata, but represents the basemenc upon 

 which they were deposited. This upthrust in a locality where the 

 Pliocene strata are excessively thick, suggests interesting specula- 

 tions as to the much discussed relations of accumulation to diastro- 

 phic movements. For the accumulation of the mile of strata repre- 

 sented by the Merced series, the coast must have been locally 

 depressed. A limit seems to have been reached and upthrust to have 

 at once set in. The local character of the subsidence, the excessive 

 accumulation, and the strongly pronounced upthrust, are peculiarly 

 interesting, and invite the attention of students of diastrophism. 

 The post-Pliocene upthrust of Montara Mountain is in harmony 

 with Turner's conclusions as to the post-Pliocene origin of Mount 

 Diablo, and the two events are probably to be correlated. Were 

 similar conditions of excessive accumulation associated with the 

 birth of Diablo ? Can overloading by any possibility be assigned 

 as the cause of the relative or absolute depression of Santa 

 Catalina? 



Geological Laboratory , 



University of California, Nov. 20, iSpj. 



