IJOUN r.LION 1 )E M I( A ANDESITE. 365 



fonn must liavo Ix-cu (iiiitr roiiiulcil ;is if pnrtially t'liscd. Tlu^ cU-ava},')' in tlu-sc Icld- 

 S]»ars is vei-y iiiii>crt'ect, and is lor the most part waiitiiifj-. the. crystals lieiii^ iiicgii- 

 larly cracked like. sauidiu«\ Tlic itDlysyiithetic twiimiii};: after alhite and iH-ricliiic is 

 very unevenly developed. Tiie latter, never repeated to any great extt^it, is present 

 in nuuiy individuals, the lamelhe seldom traversing the entire width of the crystal; 

 those produced by the former twinning vary greatly both in breadth and h'ligtli in 

 the sam(^ individual, as well as in ditFerent ones, the feldspars in general being <-har- 

 aetei'ized by a ))aucity of sti-ialions. This is well shown in ''"igs. ."> and 4, PI. V, and 

 Fig. 2. PI. VI, the two figures on Pi. v, also illustrating the characteristicr ditterenco 

 between the largest of the i)henocrysts (Fig. .'5, PI. v), and the medium sized oiu's 

 (Fig. t, PI. V), botli being niagnilied to the same extent, 'M> diameters. The laigest 

 have ([uite irregular outlint^s and an abundance of stria-, while the medium sized feld- 

 spars are very sharply (crystallized and are poorly striated. Besides the multiple 

 twiuuing, nearly every individual is twinned in halves, either after albite or in a 

 unmner corresponding to that of Carlsbad in orthotdase; and freipiently several indi- 

 vidiuils have Ibrmed in parallel orientatiou with the brachyi)inacoid as the i)lane of 

 coutact (I'Mg. -, PI. VI)- 'I'he angle of extinction averages about the same in each of 

 the thin sections studied. By far the larger number of readings give angles ranging 

 from 15° to 31°, some being lower and a very few being higher; for example, in thiu 

 section 35 the observed angles in the zone p(!rpendi(!ular to the bra(rliyi)itnicoid are 

 7°, 15°, 20°, 2(P, 2lo, 2lo, 28^, :iO^, 31o, 320, 3.5o, 3.5°, 4()o. In the other thin se.'tions 

 the higher angles are even .scarcer and belong to very perfectly rectangular sections 

 with few striations. Anorthite is jtrobably presc-ut only in small amounts, the greater 

 number of the porphyritieal feldspars being labradorite. The lath-shaped microlites 

 of feldspar m the grouudniass are fibrous and twinned, aud have angles of extinction 

 varying only a few degrees from zero aud are for the most part oligoclase; the species 

 of the ill defined patches or grains of feldspar in the groundmass is optically indeter- 

 minable. The large crystals of feldspar contain numerous small colorless glass int-lu- 

 sions, each with a single gas bubble; what sometimes appear like particles of du.st 

 are tVmud under a high jtower to be aggregates of these sharply defined inclusions 

 0'.02'""' in diameter; they are occasionally arranged in systematic order, but nu)re com- 

 monly are scatt(M-ed irregularly througli the crystal; some are attached to needles of 

 apatite. There are also inclosed a few microlites of apatite, .sonn- of which in turn 

 contain glass inclusions, aud rarely small crystals of zircon. The substance of the 

 feldsi)ar in the thiu sections studied is absolutely fresh. 



The hornblende is fouud only in macroscopic crystals affording the character- 

 istic six sided cross sections and having in every case a bhick border, excejit in the 

 green variety of hornbh!nde-[mica|-an(lesite at the east base of Hoosac M<uintain (42, 

 42((). The substanc(^ of the hornblende is entirely decomposed, not a single unaltered 

 fragment having bciMi found in any of the tliin scctiiuis. The icsulting procbut is a 



