350 



University of California Publications. 



[Geology 



OBSIDIAN. 



On the Josephine claim, near Versan's cabin, is a small ex- 

 posure, not exceeding 30 feet in diameter, of obsidian, which 

 appears to be intrusive in the porphyry. It is the only occur- 

 rence of the kind which has been observed within the area of 

 the district examined, and is very probably connected with the 

 volcanic activity which gave rise to the rhyolite flows, although 

 there is no direct evidence bearing upon this correlation. In 

 the midst of this exposure a wide pit has been sunk to a depth 

 of perhaps 12 feet, and fresh obsidian is well exposed on the 

 walls of the pit. Surrounding the exposure on all sides at the 

 surface there is nothing but porphyry as far as can be made 

 out from an examination of the soil and hillside wash. As the 

 mantle of debris is here rather heavy, however, it is possible that 

 the obsidian has a much greater extent than is revealed in the 

 exposure. The actual contact with the adjoining porphyry is 

 nowhere exposed. 



The obsidian is a somewhat resinous looking, dark, vitreous 

 rock which might from its luster be called a pitchstone. The 

 most remarkable feature connected with the occurrence is that 

 the rock contains numerous inclusions of dark altered shale 

 ranging from half an inch to 2 or 3 inches in diameter. These 

 inclusions indicate that the intrusion has pierced the White Pine 

 shale in depth, and has derived its inclusions from that for- 

 mation. 



Under the microscope the obsidian appears as a brownish 

 glass with a well marked flow structure containing occasional 

 fragments of crystals of sanidine and quartz. Under high pow- 

 ers the glass appears whitish and the brown color is seen to be 

 due to numerous- brownish, irregularly shaped, ragged or floc- 

 culent inclusions. These are drawn out in lines and give, the 

 rock its flow structure. The glass has a lower refractive index 

 than the balsam as is clearly indicated by the Becke test. 



One of the fragments of shale inclosed in the obsidian was 

 examined in thin section. The fragment was subangular and 

 had dimensions about lXl^X^ inches. Its color was bluish 

 black. The most interesting fact revealed was the evidence of 

 thermal metamorphism shown in the presence of numerous crys- 



