356 



University of California. 



rvoi.. 2. 



Eruptive and Intrusive Rocks. — There are also certain basic 

 eruptive rocks, 'which were laid down as lavas and tuffs at the time 

 of the accumulation of the sandstones, and a great abundance of 

 intrusive rocks, which invaded the series after its accumulation. 

 These intrusive rocks are of two general kinds: (i) Diabases and 

 basalts, very frequently showing a spheroidal structure, and some- 

 times variolitic on their margins. (2) Peridotites and pyroxenites, 

 now almost completely serpentinized and not uncommonly asso- 

 ciated with gabbros, which appear to have been differentiation 

 products from the same general magma as that which yielded the 

 peridotites. Both of these classes of intrusives are to be seen 

 within the limits of our map in the bed of Strawberry Creek, 

 between College Way and the mouth of the canon, and a small 

 area of serpentine occurs in Hamilton Gulch, as is indicated on the 

 map. It is to be noted that, while the diabase and basalt intrusives 

 are confined to the Franciscan rocks, there are occurrences of ser- 

 pentine in various parts of the Coast Ranges which seem to indi- 

 cate that certain of these peridotite intrusions took place at a date 

 so late as to pierce also the overlying Knoxville series. 



While the occurrences of serpentine and basaltic rocks within 

 the limits of the map are too limited and too much decomposed for 

 advantageous study, much more extensive and satisfactory expos- 

 ures of these rocks may be seen to advantage both to the north 

 and to the south of Berkeley, along the same general line of out- 

 crop of the Franciscan. From a well ninety feet in depth sunk on 

 the west side of Prospect Street, on the J. F. Sims property, there 

 were brought up fragments of a dark porphyritic igneous rock, 

 unlike any other rock known in the district. It was in a thor- 

 oughly sheared and decomposed condition, and is referred without 

 question to the eruptives associated with the Franciscan. The 

 deformed condition of the rock, taken in connection with the sim- 

 ilarly-deformed phyllites, above referred to, at the mouth of Straw- 

 berry Canon, indicate the presence of a shear zone of considerable 

 movement along the immediate front of the Berkeley Hills. In the 

 well in question this rock was mantled by not less than forty feet 

 of alluvium and sandy deposits. 



