126 GEOLOGY OF THE COMSTOCK LODE. 



AUGITE-ANDESITE. 



Slide 122. Peak south of Crown Point Ravine, marked 7075. 



Typical variety. — Tliis is a black, ratlicv fine-grained, apparently crystalline 

 rock, with a somewhat pitchy luster. Under the microscope it is seen to be 

 composed of angite and tri clinic feldspar, with apatite and magnetite as 

 accessory constituents. The feldspar is sharply angular, but there is no 

 special tendency in the larger crystals to elongation. The large crystals 

 give very high angles of extinction, many of them exceeding the labradorite 

 limits, and they must all therefore be regarded as anorthite. Among the 

 elongated microlites I noticed many which gave too high an angle for oligo- 

 clase, biit none which exceeded the labi-adorite limits. The feldspars contain 

 partially devitrified glass inclusions and augite microlites. The greater part 

 of the augite is fresh. It is very light brown in color and slightly dichroitic. 

 It is not specially well crystallized, and shapeless masses are more abundant 

 than perfect sections. There is a decided tendency to the development of 

 only one of the prismatic cleavages, and I found no trace of pinacoidal 

 cleavage. There are numerous bubble-bearing glass inclusions. Many of 

 the augites are converted in whole or in part into chlorite, of the same 

 properties mentioned so often in previous descriptions. There is a single 

 bright brown hornblende of small size heavily bordered. The apatite is in 

 part colorless and in part brown. The magnetite shows no peculiai-ities. 

 The groundmass is microlitic and in parts shows a felted structure. 



Slide 137. Bench 400 feet southeast of intersection of Crown Point Eavine and 

 Water Company's flume. 



The same slightly decomposed. — This Is macroscoplcally aud microscopically the 

 same rock as the preceding, being merely somewhat more decomposed. Tlie 

 feldspars contain secondary fluid inclusions; the augite is wholly converted 

 into chlorite, which for the most part retains the augitic forms ; and epidote, 

 quartz, and calcite are developing from the chlorite. 



Slide 416. First peak above Ophir Grade, south of Crown Point Eavine. 



Variety with augitic groundmass. — This is a gTay porphyritic rock, with none of 

 the resinous look which augite rocks usually possess; and though it shows 



