HORNBLENDE MICA-ANDES1TE. 



quartz grains form a constituent of the groundmass in the crystalline forms of t In- 

 rock and are determiuable as such in thin section 41; they are more numerous in thin 

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

 bling chalcedony. The abundance and intimate association of this modification of 

 quartz with the groundmass of the rock, and the abundance of macroscopic quartz 

 in the rock near Pinto Road makes it an intermediate variety between andesite and 

 (Incite. 



Magnetite in macroscopic grains and in microscopic crystals is very evenly dis- 

 seminated through the groundmass, but is not nearly so abundant as in the pyroxene 

 andesite. It is everywhere coated with red oxide and in thin section 37 it has been 

 converted into the yellow hydrous oxide. 



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

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

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

 pleochroism. Cross sections show inclusions parallel to the sides of the prism. 

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

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

 specially noticeable in thin section 37. It has a flue red color in thin sections 35, 39. 

 Zircon is a constant ingredient, though in very small quantities. It is in microscopic 

 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, 42. 



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

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

 oligoclase, in an aggregate of feldspar and quartz grains of irregular outline, that are 

 nearly free from microlites ac the center, especially in thin section 41. Besides these 

 minerals are minute crystals of magnetite and in thin section 35 opaque microlites, 

 which are seen to be made up of opaque aud transparent yellow grains and correspond 

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

 the fine grained audesitic 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 groundmass in thin sections 42, 42a, 39 presents a less advanced 

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

 fewer 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 tin- 

 hornblende. 



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



