Vol. 4| Osniont. — Geological Section of Const Ranges. 



71 



The area east of Petaluma is largely composed of St. Helena 

 rhyolite, and at certain places very laminated phases of it. occur. 

 Near Kelly's Ranch, six miles southeast of Petaluma, a beautiful 

 laminated variety occurs. 



Microscopically this is a light colored, almost white rock, 

 marked with numerous parallel reddish lines giving it a beauti- 

 ful laminated appearance. 



Microscopically it is seen to he composed of sanidine and 

 quartz phenocrysts with no definite crystal boundaries, which 

 have been drawn out into parallel lines by the movement of the 

 molten magma. They were rolled over and over, and fractured 

 and rounded before the magma finally solidified as a glass around 

 them. The ground mass now folds around the phenocrysts and 

 broken fragments in eye-shaped form, the flow structure being 

 beautifully brought out by numerous wavy parallel lines of 

 hematite dust. It contains 72.24% SiO,. This rock is an excel- 

 lent example of a banded rhyolite. 



Similar banded rhyolites have recently been observed by Ch. 

 E. Weaver farther to the south, on the ridge between Petaluma 

 and Napa, and at Glen Ellen. In both cases they lie above the 

 tuff. 



Becker's " Asperite." — Becker* refers to these rocks as fol- 

 lows: "Extensive areas of andesite occur to the southward of 

 Clear Lake. Mt. Cobb and Mt. St. Helena and, indeed, a great 

 part of the range of which the latter forms the culminating 

 peak, known as the Mayacmas Mountains, are andesitic. The 

 andesites extend clown almost continuously to within a few miles 

 of Vallejo, at the head of the Bay of San Francisco. 

 Both dense andesites of the earlier type and the asperites are 

 represented. . . . The andesite is for the most part, glassy 

 when fresh, though asperites are also found. This rock consti- 

 tuted the greater part of the mass of St. Helena, and covers large 

 areas to the north, east and southeast of that mountain. The 

 summit of Mt. Cobb is also andesitic. Tuffaceous forms of ande- 

 site, usually much decomposed, are also abundant, especially to 

 the south. 



"Quicksilver Deposits of the Pacific Coast, Mon. XIII, U. S. G. S. 



