Vol. 4] Osmont. — Geological Section of Coast Ranges. 



7:; 



Beyond the fact that the St. Helena Rhyolite followed the 

 Sonoma. Tuff, the age of which is pretty well established, the 

 writer has not been able to gain much evidence as to its aye. It 

 has not been observed in contact with the Wilson Ranch Beds, 

 but is probably later in aye, as the latter do not seem to carry 

 any pebbles of this rock. 



Becker* determined the lower limit of the aye of this rhyo- 

 lite, which he called "asperite," by the fact that it, overlies fresh- 

 water beds near Clear Lake, called by him the Cache Lake Beds, 

 and referred doubtfully by Marsh, on the evidence of very fray- 

 mental vertebrate remains, to the late Pliocene. At every place 

 where seen in contact with the tuff, with the possible exception 

 of one locality in the edye of the foothills east of Sonoma, where 

 the rhyolite may be lying across the eroded edges of the tuff, 

 it appears to be strictly conformable in dip with the underlying 

 tuff. These dips are frequently steep, and this fact, taken in 

 connection with the topographical consideration that deep wide 

 valleys such as those of Santa Rosa, Sonoma and Napa have been 

 formed subsequent to the folding of the rhyolite, and have cut 

 through thousands of feet of if and deep into the underlying 

 formations, makes it seem unlikely that the St. Helena rhyolite 

 can be much later than the end of the Pliocene or, at most, the 

 very early Quaternary. 



THICKNESS. 



The later Pliocene volcanic and sedimentary formations 

 form a series, conformable in dip, of varying thickness, which 

 is shown in the section at St. Helena to be at least 4,000 feet 

 thick, leaving out of account the rhyolite removed by erosion. 

 In Santa Rosa Valley, to the west, the marine sediments and 

 fluviatile deposits are certainly not less than 2,000 feet thick, 

 and the synclinal structure shown in Section AB at Santa Rosa 

 Valley points to their being fully 3,000 feet. In most places the 

 series is gently folded, mantling indifferently over the older 

 formations, but in a few places near the axis of the range, notably 

 near Franz Valley, the axes are steep and the folds numerous. 



*Mon. XIII, U. S. G. S. 



