HOllNHLKNDK MICA ANDESITE. :i(37 



qiiiii'tz grains Ibiiii a twistitueiit of tlio f^roiiiidiuass in the. crystal I i no forms of the 

 rock and are (leteriniiiable as such in tliin section -41 : they are more nnnierous in tliin 

 section 35. They also occur in small aggregates around the sides of cavities reseni- 

 liling 'jhalcedony. The ahunchmce and intimate association of this niodilication of 

 ijuartz with the grouudmass of the rock, and the al)un(hiuce of macroscopic (|uartz 

 in the rock near Pinto Road makes it an iutt^rmediate varit^y between andcsiti' and 

 dacite. 



JMagnetite in macroscopic grains and in microseoiiic crystals is very evenly dis- 

 seminated through the grouudmass, but is not nearly so abundant as in tlu; pyroxene 

 andesite. It is everywhere coated with red oxide and in tiiin section .JT it has been 

 converted into the yellow hydrous oxide. 



Apatite is especially well developed in stout hexagonal prisms with a pyramidal 

 te-mination and occasionally the base, a beautiful example being found in thin sec- 

 tion 37, Fig. 4, ri. III. They are dusted gray in the center and show the customary 

 ideochroism. Cross sections show iniilusions parallel to the sides of the prism. 

 There are also glass inclusions in negative crystals, Fig. 1, PI. iii. Apatite is asso- 

 ciated with hornblende and biotite and also occurs isolated in the grouudmass; it is 

 specially noticeable in thin section 37. It has a tine red color in thin sections 35, 30. 

 Zircon is a constant ingredient, though in very small quantities. It is in niicroscoi)ic 

 crystals of a yellow color easily recognizable by their sharp outline, high index of 

 refraction, and consequent brilliant display of interference colors between crossed 

 nicols. They are rather more frequent in thin sections 35, 43. 



The gronndmass of the typical hornblende-(mica]-andesite of this district (35, 41) 

 is microcrystalline without glass. It is composed of microlites of plagioclase, largely 

 oligoclase, in an aggi-egate of feldspar and quartz grains of irregular outline, that are 

 nearly free fi'om microlites ac the center, especially in thin .section 41. Besides these 

 minerals are minute crystals of magnetite and in thin secti<ni 35 opacpie microlites, 

 which are seen to be made up of o])a(iue and transparent yellow grains and correspond 

 to the shreds of browni mica that occur in thin section 41. This is more abundant in 

 the fine grained andesitic breccia (38), where it also occurs in well defined hexagonal 

 plates. A flow structure is evident in the arrangement of the lath-shaped feldspar 

 microlites. The grouudmass in thin sections 42, 42a, 39 presents a less advanced 

 stage of crystallization, the lath-shaped microlites being accompanied by smaller and 

 fev»'er faintly polarizing feldspar grains in a relatively small amount of colorless glass. 

 Through this in the green variety from the east base of Hoosac Mountain (42, 42a) is 

 disseminated yellowish green fibrous chlorite, resulting from the decomposition of the 

 hornblende. 



Thin sections 45, 46, and 48 are from highly decomposed rock, whose original 



