128 GEOLOGY OF THE COMSTOCK LODE. 



in a true colorless glass. A few small hornblendes and some magnetite are 

 the subsidiary minerals. The feldspars show little tendency to elongation; 

 they appear to be all triclinic, and the maximum angles of extinction ob- 

 tained correspond to labradorite. The augites are not very sharply crys- 

 tallized, are largely massed in bunches, and are more than ordinarily 

 dichroitic. There are a few hornblendes which are bright brown in color, 

 and, like those in the glassy hornblende-andesite of the District, without 

 black borders. The glass which forms a large part of this rock is colorless, 

 and shows in places perlitic cracks. Many microlites and trichites are dis- 

 tributed through it, some of them transparent and very likely feldspathic; 

 others opaque. Some of the trichites show a beaded structure. Embedded 

 in the glass are many curious spots of rounded shape, which are yellowish- 

 white by reflected light and feebly transmit yellow rays. In polarized light 

 they are seen to be wholly or partly crystalline; they are not sufficiently 

 diaphanous to say which. They are evidently irregularly radial in structure, 

 and in some favorable instances give a broad ill-defined cross between 

 crossed Nicols. The pseudo-spherolitic structure is further marked by moie 

 or less curved opaque trichites which, starting from the center, preserve an 

 approximately radial direction, branching like twigs at short interval-s -One 

 of these masses is shown in Fig. 20, Plate III. 



Slide 125. Above the Ophir grade, due west of Belcher hoisting-works. 



Granitoid variety. — This is a bluisli-gray granular rock, looking almost like 

 an older cr3'stalline species. Under the microscope, however, it reveals 

 itself as merely an unusually coarse-grained augite-andesite. The ground- 

 mass is granular. A portion of the augites are fresh, the remainder con- 

 verted into chlorite. 



Slide 465. Crown Point Ravine; on tlume, near drainage. 



Specimen showing stages of decomposition. MaCrOSCOpically a blulsh-gray, COUl- 



pact, and rather granular rock, without macroscopically visible bisilicates. 

 The slide affords an unusually fine opportunity of studying the decom- 

 position of augite-andesite. It happens to contain a large proportion of 

 augites in octagonal sections, the outlines of which have been but little dis- 



