86 



University of California Publications. 



[Geology 



During the gradual subsidence thus indicated, the sea prob- 

 ably encroached on the land from the west at Santa Rosa Valley 

 to form marine deposits. 



So far as the writer's personal observation goes, the last flow 

 of lava, in this area was the St. Helena Rhyolite, which in the 

 vicinity of Mt. St. Helena has a thickness of at least 2,000 feet. 

 This flow seems to have occurred at the end of the Merced, since 

 it everywhere overlies the Sonoma Tuff and the Cache Lake Beds, 

 and no pebbles of it are found in the Wilson Ranch Beds. Since 

 it is folded conformably with the Sonoma Tuff, and hence pre- 

 ceded the uplift and the great period of erosion which followed, 

 it is probably not later than the very latest Merced. Becker* 

 states that a later flow of basalt took place to the north, in the 

 Clear Lake region. 



The great uplift which raised and folded all the Merced 

 strata is supposed by Lawsonf to have taken place just at the 

 beginning of the Pleistocene. More recent work by Arnold! on 

 the upper portion of the Merced, at Seven Mile Beach, points to 

 its somewhat later age. The fact that the St. Helena Rhyolite, 

 fully 2,000 feet thick in places, rests conformably upon the Mer- 

 ced sedimentaries, and is folded with them, would seem to sug- 

 gest this view, if we could be sure that the Wilson Ranch Beds 

 represent the whole of the Merced, but at present the palaeon- 

 tological evidence is not strong enough to settle this point. 



At Wilson's Ranch area trilineata is very common, and Ash- 

 ley states that these large areas are common toward the base of 

 the Merced, but disappear before the top is reached. Hence the 

 Wilson Ranch Beds may represent only the lower part of the 

 Seven Mile Beach Beds. 



Probaby during the time of this folding the St. Helena fault 

 occurred, by which the strata were displaced nearly 3,000 feet, 

 and the high ridge formed of which Mt. St. Helena and Cobb 

 Mountain are the culminating points. Probably about the same 

 time the San Bruno Fault on San Francisco Peninsula, and 

 its northern extension, the Tomales Fault, occurred, although, 



*Mon. XIII, U. S. G. S., pp. 157 and 223. 



fPost Pliocene Diastrophism of the Coast of Southern California. Bull. 

 Dept. Geol., Univ. Cal., Vol. I, No. 4, p. 157. 

 t Memoirs Acad, of Sciences, Vol. Ill, p. 13. 



