ANDES1T1O PEAKL1TE AND DACITE. 371 



their proper connection. At the top of the table stands the quartz-bearing horn- 

 blende-mica-andesite (52) found in Sierra Canyon, forming the connecting link that 

 unites by its microscopic structure the hornblende-mica-andesites and andesitic 

 pcarlites. The groundinass of this rock is completely crystalline, exactly as in the 

 typical hornblende-mica-andesite of the district (35). In the thin section besides the 

 plagioclase there are two or three unstriated sections which may possibly belong to 

 sauidiue. The fresh hornblende is without dark border, a few individuals having a 

 slight aggregation of magnetite grains around them, which is also noticeable around 

 the biotite. There is no pyroxene present, but some well developed quartz crystals. 

 The nearest approach to crystalline andesite in the Dry Lake series is thin section 



53, whose gray grouudmass is microspherulitic. The spherulites are composed of 

 radiating colorless needles, besides which are multitudes of transparent globulites 

 and trichites, straight and curved, some black and opaque, others red and referable 

 to mica, and some formed of a string of transparent grains which are also found in 

 short, stout, interpenetrating microlites, which appear to belong to augite. The 

 whole shows a marked flow structure and bears phenocrysts of labradorite, biotite 

 and hornblende crowded with magnetite grains and no longer fresh; besides com- 

 pletely altered pyroxene [hypersthene] ; zircon occurs in good crystals. There is no 

 macroscopic quartz, but small aggregations of colorless plates appear to be tridymite. 

 Thin section 61 is more highly crystalline and illustrates the first stages of the forma- 

 tion of the feldspathic grains in the groundmass of the hornblende-mica-audesite; they 

 are seen forming around the phenocrysts as centers, which are the same as those 

 in 53 with the addition of macroscopic quartz. 



A modification common to four separate localities is represented by thin section 



54, and approaches closely to the pyroxene-andesite of the district; the silver gray 

 grouudmass has a satin-like sheen in transmitted light, produced by fibrous feldspar 

 microlites in nearly parallel arrangement in a colorless glass base, having a marked 

 flow structure, with a felt-like appearance in the thicker parts of the section ; there 

 are also grains of magnetite and a little hyperstheue. The larger phenocrysts are 

 well developed and the inclusions are very fine. Feldspar is in excess of the other 

 constituents, and hornblende and hyperstheue occur in about equal proportions, 

 biotite being scarce. The corresponding varieties (62, 63, 71, 73) are almost identical. 

 In 62 the feldspar microlites are more delicate, biotite is wanting and quartz occurs 

 in macroscopic grains; 63 is richer in glass and poorer in large crystals and has a 

 little brown mica in the groundmass. 



In 71 the glassy grouudmass is richer in augite microlites, and also contains some 

 of hornblende and biotite. It very closely resembles the pyroxeue-andesite of Rich- 

 mond Mountain; 73 is remarkable for the abundance of biotite in hexagonal plates in 

 the groundmass. This variety of the pearlite is further characterized by the presence 

 of feldspars with very high angles of extinction, several of which reach 40 and 4f>, 



