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University of California. 



[Vol. 1. 



way. The metamorphism is remarkable alike for its intensity and 

 for the mineralogical simplicity of the resulting rocks. It will be 

 shown that the latter, in their typical development, consist almost 

 wholly of holocrystalline glaucophane schists, and, furthermore, 

 that these schists appear to present no essential differences, when 

 derived from the sandstone, from those formed by the metamor- 

 phism of the chert. Lastly, the schist produced by contact meta- 

 morphism alongside the serpentine, has not been found to present 

 any real distinctive feature to differentiate it from that adjacent to 

 the fourchite. The conclusion from this fact is, that the unknown 

 causes that have led to the development of glaucophane as the 

 most conspicuous contact mineral are not confined to any single 

 rock, but must be dependent upon the common properties of at least 

 two of them, — the chert and the sandstone, or the serpentine and 

 the fourchite, — while it is quite probable that if either of these two 

 binary groups had been replaced by another rock or group of 

 rocks, the resulting metamorphism would have been of a different 

 character. 



THE SEDIMENTARY FORMATIONS. 



The San Francisco Sandstone. — The San Francisco sandstone 

 has been described as occupying the larger portion of the island. 

 At Quarry Point, where considerable quantities have been taken 

 out for building purposes and for use in government harbor improve- 

 ments, it is a hard stone of fairly homogeneous texture, and bright 

 gray color, the latter weathering to a yellowish brown on exposed 

 outcrops. The most noticeable constituents to the unaided eye are 

 occasional flakes of mica, and numerous sharply angular fragments 

 of a very fine black shale. Under the microscope the rock appears 

 as a wholly unaltered sandstone. A close study of the component 

 grains reveals their very heterogeneous character. Besides the 

 quartz grains, plagioclase occurs in great abundance, usually in 

 fairly fresh granules showing numerous polysynthetic lamellae. 

 Then follow, in about the order of their frequency, fragments of 

 chert, small scales of allogenic muscovite, angular splinters of black 

 shale with fine quartz-filled cracks, bits of some glassy basic erup- 

 tive, and, lastly, of some finer grained sandstone. A chemical 



