406 



University of California. 



[Vol. 2. 



This earliest of the three formations in question is essentially a 

 volcanic tuff It is, however, locally admixed with gravel and 

 contains a small lens of limestone and several small lenses of 

 basaltic lava. The tuff is for the most part decomposed and 

 presents usually a yellowish or dark brown appearance. It is 

 generally free from quartz, and is evidently the product of a basic 

 or at least andesitic eruption. This formation presents numerous 

 exposures on the slopes on either side of Gopher Ridge, below 

 Little Grizzly and below Rough Hill. It has a maximum thickness 

 of about 200 feet, and varies from this down to less than twenty 

 feet, the difference in volume being apparently due, as has been 

 above suggested, to the inequalities of the surface upon which it 

 was deposited. In good exposures the formation is seen to be 

 evenly stratified and occasionally is fairly well indurated so as to 

 present outcrops in the form of steep bluffs, as in the section to the 

 northeast of Pie Knob. In this section a considerable fraction of 

 the entire volume of tuffs, towards the upper part, is rhyolitic in 

 composition. 



After the deposition of these tuffs the accumulation of the 

 Campan gravels was resumed, at least in the northwestern part of 

 our area. Here we find a wedge of gravel conglomerate interven- 

 ing between the tuffs and the overlying basalts, while on the slopes 

 of Strawberry Canon and Wildcat Canon the basalts repose directly 

 upon the tuffs. 



The overlying basalts make up the greater part of the surface 

 of Gopher Ridge, and a portion of the lower slopes of Rough Hill 

 in Wildcat Canon. Some outlying patches of the same basalts 

 occur near the county road on both sides of Wildcat Creek below 

 the bridge. The basalts lie usually upon the tuffs, or upon the 

 wedge of gravel conglomerate above referred to, but they also 

 extend beyond the limits of this formation and rest partly on the 

 clays of the Orindan formation and partly upon the Grizzly Peak 

 andesite in situations which may be readily ascertained by a glance 

 at the map. It is probable that the tuffs were subjected to erosion 

 prior to the outflow of the basalts, so that the limits of the tuff 

 formation, where transgressed by the lavas, are not necessarily 

 the limits of the basin in which it was deposited. 



