390 H. W. TURNER — GEOLOGY OF MOUNT DIABLO. 



comparatively rare may be seen by referring to J. D. Dana's " Manual ot 

 mineralogy." 



The dike nature of this serpentine area is best shown on the western side 

 of the mountain east of the eastern fork of Pine creek, where it is cut across 

 by the Arroyo del Cerro and its branches. It is here from a few feet to 

 about 150 feet in width, enclosed in dark, calcareous shales containing, at 

 several points near the serpentine, Auce/hi mosquensis. Von Buch, a fossil 

 characteristic of the Knoxville beds (lower Cretaceous) ; and near the 

 northern end of this narroAV dike both Aucella and Belemnites occur in lime- 

 stone. 



The strike and dip of the dike is in general about that of the enclosing 

 shales, the strike being nearly north-and-south, and the dip about vertical. 

 The serpentine of this narrow dike has an imperfect fissile structure, and at 

 one point only did I find the dark bastitic variety. 



The eruptive serpentine of Mount Diablo does not differ materially in 

 chemical composition from the metamorphic serpentine described by Becker.* 

 A specimen from one of the small serpentine areas on the southern slope of 

 the mountain contains abundant remains of pyroxenes and olivines. The 

 area is thought to have an origin similar to that of the large serpentine dike 

 on the northern slope above described.f 



As to the diabase and serpentine occurring in little patches throughout 

 the metamorphic areas of the Coast ranges of California, and mixed in the 

 most confusing manner with unquestionably clastic rocks of early Cretaceous 

 age, it might be held by one seeking to prove their igneous origin that a 

 molten magma injected into a mass of rocks so thoroughly and irregularly 

 fractured as are the rocks of the metamorphic areas would on consolidation 

 form irregular areas rather than definite areas and dikes,' there being few 

 regular fissures into which a molten magma could be injected. This is shown 

 likewise in the irregular and generally non-continuous character of the quartz 

 veins of the Coast ranges, giving positive testimony as to the character of 

 the fissures which they have filled. Professor Whitney says : % 



" The rocks of the Coast mountains are especially distinguished by the fact that the 

 movements to which they have been subjected, and which have originated the com- 

 plex of alternating elevations and depressions making up the system of chains known 

 as the Coast ranges, have been apparently sudden and sharp, so that the result may 

 be called a crushing and breaking rather than an uplifting and folding. -5^ * * 

 Often, and especially in the central and northern portions of the state, the rocks for 

 long distances are so broken up that a recognition of their real structural relations is 

 entirely impossible." 



In marked contrast to these are the fissures of the Sierra Nevada, contin- 



* Quicksilver Deposits, 1888, p. 110-111. 



fMr. G. P. Merrill has called my attention to a serpentine collected by himself at San Francisco 

 (the area that extends out to Fort Point), in a slide of which pyroxenes and olivines are plainly to be 

 noted. 



X Auriferous Gravels of the Sierra Nevada, 1880, p. 16. 



