ANDES1TIC PKAHLITK AND DACITE. 373 



quartz, so abundant in the hand specimen in large rounded grains, is scarce in the 

 thin section. The two varieties from Sierra Canyon (68, 69) are denser than that just 

 described. The glass groundmass of 68 is without gas bubbles and is crowded with 

 yellowish translucent particles, which reflect incident light and appear white. It is 

 in places spherulitic and abounds in angular fragments of plagioclase, nine-tenths of 

 the feldspars being striated. There is, besides, quartz, with fine glass inclusions, a 

 very little pyroxene, more hornblende, and much biotite. Thin section 69 is identical 

 with the last under the microscope. Thin section 76, from east of Hoosac Mountain, 

 is similar to the foregoing, but has a cryptocrystalline gronndmass and is somewhat 

 decomposed. Some portions of the groundmass of 70 are crystalline and bear feld- 

 spar microlites, but the whole is the same as 08 and 69. Thin section 60 is more 

 porous, but has the characteristics of the last four sections. Its feldspar is all plagio- 

 clase and gives angles of extinction corresponding to labradorite. This last quartz- 

 bearing group (59, 60, 68, 69, 70) appears to be true dacite. and as such is very 

 interesting. 



It may be well to note at this point some of the characteristic features distin- 

 guishing these closely allied rocks as they are found in this district. The gronndmass 

 of the hornblende-mica-andesite is in general microcrystalline, without glass, having, 

 besides lath-shaped feldspar microlites, which are probably oligoclase, interpenetrating 

 grains of quartz and feldspar. It is freer from magnetite and contains no pyroxene. 

 The 'groundmass of the pyroxene-andesite, on the other hand, is very glassy, with a 

 felt-like structure produced by feldspar and augite microlites, the feldspar being 

 labradorite, with an abundance of magnetite. The pheuocrysts of the former rock 

 are labradorite, dark bordered hornblende in every case decomposed, considerable 

 biotite, and sometimes quartz, but no pyroxene or the remains of any. The pheno- 

 crysts of the pyroxene-andesites are auorthite, hypersthene, augite, dark bordered 

 hornblende, with very little biotite and only an occasional quartz. The andesitic 

 pearlites hold an intermediate position between the two, some of the varieties being 

 quite like the horublende-mica-andesite, while others approach closely to pyroxene- 

 andesite, yet all have features differing from both. The groundmass is a glass more 

 or less full of microlites, and in the greater number of cases is crowded with indeter- 

 minable globulites and pai-ticles. Besides the feldspar phenocrysts, which are for the 

 most part labradorite and possibly a very little orthoclase, with some anorthite, there 

 are hornblende .crystals without dark border, hypersthene, a little angite, biotite, and 

 quartz. The dacites are a modification in which the macroscopic quartz has greatly 

 increased, together with the biotite, while pyroxene has nearly disappeared. They 

 are also the most pumice-like. 



