PETROGRAPHICAL NOTES. 



agrees with that in the specimens already described. It 

 contains many small biotite needles. Orthoclase is perhaps 

 present in very small quantity. Quartz is abundant, filling 

 the interstices between the feldspar grains, and containing 

 very numerous fluid inclusions, with moving bubbles. 



There are no signs of mechanical deformation among the 

 constituents. Hardly any of the quartz grain show undu- 

 lous extinction. 



In order to determine more accurately the character of 

 the feldspars, a separation by specific gravity was made by 

 means of the Thoulet solution. 



A quantity of 4 grms. was used, and reduced to 0.4 mm. 

 grain. The result was as follows: 



Beginning with the solution at the specific gravity of 2.75 

 hornblende and biotite fell ; between 2.75 and 2.66 no 

 appreciable amount was precipitated, while at about 2.655 a 

 large quantity of feldspar, mixed with a little quartz fell. 

 The largest proportion of quartz fell somewhat below this 

 specific gravity. Only a few grains remained swimming 

 with the solution at 2.60-2.59. 



The complete separation of feldspar and quartz was found 

 to be somewhat difficult, their specific gravities being so 

 nearly equal. The percentage of the different constituents 

 would approximately be as follows: 



25 % hornblende and biotite. 

 40 ^f' plagioclase. 

 35 /r quartz. 



The feldspar being so thoroughly fresh, it is safe to con- 

 clude from the specific gravity that it is an oligoclase, per- 

 haps somewhat approaching the andesine. The rock is thus 

 characterized as a quartz-mica diorite. 



4. A fourth typical specimen, collected near the summit 

 of the main range at Hansen's Ranch is a coarse granitic 



