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University of California. 



[Vol. 2. 



Crystalline Schists. — On another minor point of the summit was 

 found a micaceous schist that evidently belonged to the same 

 series, though it was not superficially connected with any other 

 member. This rock is probably a representative of the " micaceous 

 slate" mentioned by Whitney as occurring on the peninsula 

 in some connection with the crystalline limestone. 



On the shore of Tomales Bay, just north of the point at which 

 the county road turns westward from the shore, there is a still more 

 interesting exposure of rock that belongs with this series. In its 

 general appearance it might be said to resemble a light-colored 

 siliceous schist, and it may not be incorrect to so term it. It over- 

 lies the granite and extends along the shore of the bay for several 

 hundred feet, dipping generally at an angle of about forty-five 

 degrees toward the northeast. The bed is about thirty feet 

 in thickness and is thoroughly crystalline. It consists of thin 

 layers, ranging from one-fourth of an inch to more than an inch 

 in thickness, which for the most part are alternating leaves of com- 

 pact siliceous material and a mineral which qualitative and micro- 

 scopic tests proved to be mainly wollastonite. The softer layers 

 of the schist dissolve under the action of the weather and leave the 

 siliceous portions standing in high relief. The siliceous part, which 

 perhaps predominates in the exposure, is a fine-grained quartzite, 

 very hard and vitreous, but not sharply separated from the inter- 

 vening layers. Among the grains of quartz of which it is composed 

 are scattered crystals of wollastonite. The grains of quartz are very 

 irregular in shape and are apparently more or less corroded and 

 encroached upon by granular masses of its associate mineral. 

 Under the microscope the quartz grains are seen to be thickly 

 sprinkled with small crystalline inclusions of high refractive power 

 which were not identified. Some of them are hexagonal. 



Wollastonite seems to be more abundant in the more siliceous 

 portions of the schist, and in portions which contain less silica the 

 wollastonite gives place to tremolite. When tremolite occurs 

 it is in small lath-shaped crystals with an extinction angle gener- 

 ally below twenty-three degrees. It has no crystal terminations. 



Age of Pre-granitic Series. — The pre-granite rocks of the penin- 

 sula deserve much more attention than they have yet received. 



