ANDESITIC I'EAin.ITE AND DACITE. 371 



their pioiier couiiectioii. At the top of tlie table stands the quartz-beariu},' honi- 

 bleude-mica-andesite (52) found in Siena Canyon, forming the connecting Hnk that 

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

 l)eailites. Tho groundniass of tliis rock is completely crystalline, exactly as in the 

 typical horubleude-iriica-andesite of the district (.JS). In the tliin section besides the 

 l>lagi()clase there are two or three unstriated sections which may possibly belong to 

 sauidine. 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 i)yroxene present, but some well developed quartz crystals. 

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



53, whose gray grouiidnuiss 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 foiuid in 

 short, stout, interpenetrating niicrolites, 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 pyroxt^ne [hypersthenoj; zircon occurs in good crystals. Thei"e is no 

 macroscopic quart/,, but small aggiegations of colorless plates appear to be tridymite. 

 Thin section 01 is more highly crystalline and illustrates the first stages of the forma- 

 tion of the feldspatliic grains in the groundmass of the hornblendeuiica-andesite; they 

 are seen forming around the phenocrysts as centei's, which ai'c the same as those 

 in 53 witli the addition of macroscopic (piartz. 



A moditication common to four separate localities is rei)resente<l by thin section 



54, and api)roaches closely to the pyroxeue-andesite of the district; the silver gray 

 groundmass has a satin-like sheen in transmitted light, i)roduced by fibrous feldspar 

 microlites in nearly parallel arrangenu'nt in a colorless glass base, having a marked 

 flow structure, with a felt-like ajjpearance 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 tine. Feldsi)ar is in excess of the other 

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

 biotite being scarce. Tlie corresponding varieties (02, 03, 71, 73) are almost identical. 

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

 in macroscopic grains; 03 is richer in glass and poorer in large crystals and has a 

 little brown mica in the groundmass. 



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

 of hornblende ami biotite. It veiy closely resembles the iiyroxene-andesite of Rich- 

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

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

 of feldspars with \ery liigli angles of extinction, several of wliicli reaeli lO"- and 45°, 



