ANDESrriC I'KAIJMTK AND DACITE. 373 



(|uartz, so almiidaiit in Ihv IkiikI si.criiiicii In laij;c n.iiii(lc(l jiTaiiis, is scawe ill the 

 tliiii section. The two varieties IVoin Siena Canyon (08. (iO) are denser than that Just 

 described. The s'hiss groundmass of GS is without gas liubbles and is erowded with 

 yellowisli transhieent i)articU'S, which reticct incident light and ai)pear white. It is 

 in places splierulitic and abounds in anyular fragments of i)IagiocIase, nine-tenths of 

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

 very little pyroxene, more hornblende, and ninch biotite. Thin section fiO is identical 

 with the last under the niicroscoiic. Thin srction 7(), from east of Hoosac Mountain, 

 is similar to the tbregoing. but has a cryptoerystalline groundmass and is somewhat 

 decoinjiosed. S(mie portions of the groundmass of 70 are crystalline and bear feld- 

 spar microlites, but the whole is the same as (iS and (ill. 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 (piarcz- 

 beariug group (59, (!0, 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 aie found in this district. The gnmndmass 

 of the liornblende-micaandesite is in general microcrystalline, without glass, having, 

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

 grains of (piartz and feldspar. It is freer tVoni magnetite and contains uo pjToxene. 

 The groundmass of the ])yroxene-andesite. on the other hand, is very glassy, with a 

 felt-like structure iiroduceil by feldspar and angitc microlites, the feldspar being 

 labradorite, with an abundance of magnetite. The iilienocrysts of tbe former rock 

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

 biotite, and sometimes quartz, but uo pyroxene or the remains of any. The i)heno- 

 crysts of the pyroxeiie-ande.sites are anorthite, hypersthene, augite. dark bordered 

 hornblende, with very little biotite and only an occasional (juartz. The andc.sitic 

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

 quite like the horublende-niica-andesite, while others approach closely to pyroxeue- 

 andesite, yet all have features differing from l)otli. The gnmndmass is a glass more 

 or less full of microlites, and in the greater iiiunber ol' cases is ciowded with indeter- 

 minable globnlites and i)articles. Besides the feldspar |)henociysts, which are for the 

 most part labradorite and possibly a very little orthoclase, with some amn-thite, there 

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

 quartz. The dacites are a modilication in which the macro.scopic quartz has greatly 

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

 are also the mo.st pumice-like. 



