438 



University of California, 



[Vol. 2. 



tuffs and make a complete list of the andesite and basalt lavas and 

 rhyolite tuffs in the order of their occurrence, from the amygda- 

 loidal andesite reposing upon the Orindan gravels to the rhyolite 

 agglomerate at the summit of the Campan, we discover a very 

 remarkable periodicity in the sequence. 



List of Lavas and Rhyolite Tuffs in the Berkeleyan and Campan 

 Series in Ascending Order of Occurrence , Showing 

 Periodicity of Sequence. 



II. 



III. 



IV. 



C Andesite, amygdaloidal. 



-! Basalt, thin flow in tuffs not separately mapped. 

 ( Rhyolite tuff. 



C Andesite, outcropping only on San Pablo escarpment. 

 1 Basalt, thick flows. 

 ( Rhyolite tuff, thin. 



(Deformation and erosion.) 



! Andesite, Grizzly Peak holocrystalline variety. 

 Basalt, subordinate flow not separately mapped. 

 Rhyolite tuff, thin, superposition on basalt not certain. . 



Lower 

 Berkeleyan. 



Andesite, Grizzly Peak porphyritic variety. 

 (Siestan lake beds.) 

 several flows, 

 formations removed by erosion. 



Upper 

 Berkeleyan. 



~\ Basalts, s« 

 ' ? 



(Deformation and erosion.) 



I Andesite, several flows, with intervening lake beds. 

 Basalt, andesitic. 

 Rhyolite tuff, thick, with other tuffs. f Campan. 



Basalt, heavy flows. 

 Rhyolite agglomerate and tuff, summit of Campan. 

 (Higher beds removed by erosion.) 



The tabulation clearly shows that in the entire succession there 

 is a regularly-recurring sequence in the character of the erupted 

 rocks. There are at least five such recurrences, or periods, and 

 possibly six, if we separate period V into two parts. In four of 

 the five periods the sequence is andesite, basalt, rhyolite. In 

 period IV the rhyolite member is lacking, but may. have once 



