(58 GEEEN MOUNTAINS OF MASSACHUSETTS. 



parts of the rock these grains of hornblende fit in between rounded grains 

 of a twinned plagioclase. In other places in the rock the hornblende is 

 seen to have a narrow fringe of light green pleochroic hornblende (see 

 PL ix, b), massive and not fibrous ; in other grains this entirely replaces the 

 brown hornblende, or only little cores of the latter are left. At the same 

 time the feldspars in those parts of the rock are filled with small acicular 

 crystals of the same green hornblende associated with small grains of pla- 

 gioclase, and minute veins composed of these two minerals often cross the 

 original feldspars by narrow fissures (see PL ix, a). The extreme change 

 consists in the entire replacement in parts of the rock of the feldspar and 

 hornblende by an aggregate of these small secondary feldspars, with a 

 little quartz and epidote in abundance. It is plain that the original plagio- 

 clase and brown hornblende has changed to a new plagioclase, green horn- 

 blende, some quartz, epidote (taking part of the lime from the feldspar), and 

 a little calcite. 



In another form the rock is a fine-grained amphibolite composed of 

 crystalloids of bright green or bluish green hornblende, rarely inclosing 

 small cores of original brown hornblende, and plates of biotite; both these 

 minerals lie in planes, causing" the schistose structure. The remaining 

 space is filled with little plagioclases which are rarely polysynthetically 

 twinned and are filled with grains and prisms of epidote. Grains of 

 titanite Surround small black cores of original titaniferous iron ore and 

 sometimes the titanite grains run out in stringers parallel to the schistosity. 

 These feldspars contain, in addition to epidote, titanite grains, needles of 

 hornblende, biotite flakes, and grains of quartz. In some rocks the little 

 prisms have the characters of zoisite instead of epidote. These feldspars 

 may occur in broad simple twins like the albite of the schists, or may be 

 polysynthetically twinned. The feldspar was isolated from several rocks by 

 the Thoulet solution and found to be always plagioclase, generally toward 

 the albite end of the series. The hornblende contains titanite and epidote; 

 the plates of biotite contain rutile needles. 



A few of these rocks carry irregular masses of red garnet which alter 

 to chlorite; they inclose masses of magnetite and green hornblende with 

 cores of brown hornblende, The garnet seems to be contemporaneous with 

 the feldspar, 



