Law son ~| 

 palacheJ 



The Berkeley Hills. 



43 5 



Rhyolitie Tuffs and Agglomerates. — In the conglomerates and 

 tuffs which constitute the third stratigraphic member of the Lower 

 Berkeleyan, the most important beds on the Frowning Ridge 

 escarpment are whitish tuffs of rhyolitie material. They are more 

 or less distinctly stratified, and weather with rough and pitted sur- 

 faces, owing to the heterogeneity of the material. The acid char- 

 acter of the rock may be recognized in hand specimens from the 

 numerous fragments of quartz which appear in it, this mineral, 

 together with feldspar, being large enough for easy identification. 



Under the microscope it is clearly seen to be fragmental, the 

 larger component grains being angular or rounded fragments of 

 quartz and feldspar, and occasional rock fragments mostly of a 

 b.isic character. These are imbedded in a cement of varying com- 

 position ; glass is usually more or less abundant in shreds and 

 wisps of white or yellowish color; kaolin or a cloudy kaolin-like 

 substance is common, and there is always a certain amount of 

 chalcedony, in some cases so abundant as to replace all the other 

 constituents. Grains of magnetite, hydrous iron oxide, and ocher- 

 ous serpentine occur irregularly in the interstices, together with 

 undeterminable opaque matter. Both orthoclase and plagioclase 

 feldspars are present in fresh and water-clear crystal fragments; 

 these and the quartzes are frequently rounded and embayed as if 

 by magmatic corrosion, a process which could only have taken place 

 before they were ejected and deposited in the present rock. Such 

 rock fragments as were determinable were of the andesitic char- 

 acter. The chalcedony forms characteristic radial aggregates 

 showing the black extinction cross in parallel polarized light. It 

 occasionally forms compact veins and bunches of greenish color 

 flinty aspect. 



A thick lens of very similar rhyolite tuff occurs at a very differ- 

 ent horizon in the Campan series in the tuff belt to the northwest 

 of the basalt quarry, and the description just given would apply to 

 it as well as to that of the Lower Berkeleyan. 



The tuffs and agglomerates which form the highest member of 

 the Campan series differ from the tuffs just described chiefly in 

 the coarser character of the fragments of which they are composed. 

 In addition to the fragments of quartz and feldspar there are 



