University of California. 



[VOL. 2. 



best seen in the shape of a line of fragments near our line of section, 

 and further down, in the same situation, near the southern limit of 

 the map. 



Thus, while, so far as our examination has extended, the 

 exposures are not sufficient to reveal to us the exact relative pro- 

 portions of these various deposits, we can confidently affirm that 

 the formation which underlies the terrace is made up of soft sand- 

 stones, carbonaceous and lignitic shales, an abundance of clay, and 

 thin beds of limestone and chert. 



These various beds, and others not yet mentioned, make up the 

 Siestan formation, an accumulation of deposits in a fresh-water lake 

 which occupied the region at the close of the epoch of the volcanic 

 activity which is represented by the Grizzly Peak andesites. 



Their Extent. — In order to appreciate th.e essential features of 

 the stratigraphy of the Siestan formation, it will be necessary to 

 digress somewhat from our line of section and follow the distribu- 

 tion of these soft beds. It must be noted, first, however, that the 

 clays of the Siestan formation in their northeasterly dip pass under 

 another set of lavas. These are thick-bedded flows of basalt, the 

 edges of which rise in an abrupt acclivity at the back of the terrace 

 to the culminating line of Frowning Ridge southeast of Grizzly 

 Peak. 



If now we follow the outcrop of the Siestan beds to the north- 

 west between these two sets of massive lavas, andesitic below and 

 basaltic above, we find the terrace becoming gradually narrower and 

 the formation thinner. In the vicinity of Grizzly Peak the terrace 

 is little more than 100 feet wide and the underlying beds are only 

 traceable in occasional fragments of the characteristic chert of this 

 formation, and in the clayey character of the soil. At Grizzly Peak 

 the terrace ends and the outcrop of the formation passes behind 

 the peak on the northeast side. From this point onward the 

 width of the outcrop is much attenuated, partly owing to an 

 increase in the angle of dip of the strata and partly to a rapid 

 thinning out of the formation. It is traceable, however, in a 

 depression behind a line of shoulders on the northeast slope of 

 Grizzly Peak and by occasional small exposures of the rocks 

 themselves, up to the Canon fault, which cuts it off. 



