i-5o 



University of California. 



[Vol. i. 



differential movement arched the peninsular region in such a way 

 that the portion of it which lies to the southwest of the present line 

 of demarkation was elevated above sea level and subjected to a long, 

 continuous, and vigorous erosion, while the part to the northeast of 

 the line remained below sea level, and was thus protected, being 

 only lifted to its present position by the much more recent epeiro- 

 genic uplift of the coast. It is now being removed with astonishing 

 rapidity. 



Development of Montara Mountain. — The orogenic event here 

 described, by which the Merced series was lifted into its present 

 position as the limb of a great anticlinal arch, implies unavoidably 

 the post-Pliocene upthrust of the granite mass whose summit is 

 Montara Mountain. This mass resembles in its structure an 

 acutely domed laccolite. The strata of all ages dip away from it 

 quaquaversally. In this respect it closely resembles Mount Diablo, 

 whose structure has been studied by Turner,* with this exception, 

 that the igneous rocks in the core of Diablo are basic. Its upthrust 

 corresponds, also, in age with that of Diablo, as determined by 

 Turner. In neither Montara nor Diablo, however, is the structure 

 laccolitic. The granite in the one case, and the basic igneous rocks 

 (diabase) in the other, were both in place, as the rocks we now 

 know them, beneath the seat of the present mountain masses prior 

 to the upthrust. In both cases the mountains have been formed 

 by a sort of upward telescopic thrust, which may, of course, have 

 been due to tangential forces. In neither case was there any 

 apparent development of igneous rock during the movement. The 

 upthrust tilted the strata quaquaversally from the rising central 

 mass. In the case of Montara Mountain the resemblance to the 

 laccolitic structure is the more striking because the central core of 

 the mountain is an extensive granite mass. This mass, however, 

 antedates the oldest sedimentary strata on its flanks, as is well 

 attested by basal conglomerates, composed of bowlders and pebbles 

 of the granite. This type of mountain structure must be carefully 

 discriminated from the true laccolite. 



One other point requires to be noticed in connection with the 

 differential movements of the San Francisco Peninsula. The line 



*Bull. Geol. Soc. Am, Vol. II, pp. 383-414. 



