EOOSAC MOUNTAIN. 47 



spar, sometimes not. In some localities the feldspar contains little round 

 red garnets. Flakes of biotite and muscovite and octahedra of magnetite 

 are common inclusions. 



The quartz masses show cataclastic changes in the same way; the 

 original cores of the blue quartz, themselves somewhat strained (seen by 

 using polarized light), are surrounded by masses of broken quartz, the 

 derivation of which from the parent mass can easily be traced. The finer 

 grained portions of the rock are composed of little fragments of microcline 

 broken oft' from the larger pieces, and small simple crystals, often simple 

 twins, of a feldspar which shows but rarely the multiple twinning of pla- 

 gioclase, and which resembles the albite of the schists. The layers of mica 

 are composed of muscovite, often with a greenish color like talc, but easily 

 identified by the large axial angle, flakes of dark brown biotite, rarely 

 altered to chlorite, crystals of magnetite, and the omnipresent epidote in 

 prisms or small grains mixed with the micas or inclosed in them. There 

 are occasional imperfect crystals of apatite and prisms of zircon. Some of 

 the magnetite grains are titaniferous, as can be seen by the yellow border of 

 titanite derived from them. In many slides there are quite large crystals of 

 feldspar which have no multiple twinning, extinguish parallel to the cleav- 

 age, and are perhaps orthoclase. 



Slides of the large porphyritic Carlsbad twins show that they are micro- 

 cline, filled with irregular bands of a feldspar which extinguishes parallel 

 to coP do, and is filled with epidote crystals. Aggregates of biotite plates 

 associated with hornblende crystals are common. There are also masses of 

 ilmenite altered in part to titanite. Sometimes circles of hornblende crys- 

 tals and biotite plates, which inclose a core of aggregate quartz, by their 

 shape and occurrence suggest a possible replacement of garnets. Grains of 

 quartz and crystals of zircon are common, so that nearly all the constitu- 

 ents of the rock occur within these crystals. 



What may have been the origin of this rock it is impossible to say with 

 certainty; it is evident that crushing and the development of mica, quartz, 

 and feldspar, parallel to planes of break and sliding has had a great deal to 

 do with the development of the parallel structure. Viewed from this 

 standpoint it could perfectly well have been an eruptive granite modified 



