32 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



eighty-five per cent with a very Httle muscovite, pyrite, plagioclase 

 and calcite. 



Close to the western boundary of the diorite there are two areas 

 of mica gneiss included within the main igneous mass. These are 

 undoubtedly related to the Berkshire schist but exhibit a more 

 advanced stage of crushing than that outside of the diorite. The 

 texture is schistose — that is, they have been recrystallized along 

 with the rest of the Berkshire — but the minerals so formed have 

 suffered further deformation to a greater degree than those in the 

 mass of the schist. The quartz is slightly to nearly completely 

 granulated and the biotite is moulded around the more resistant 

 garnet and plagioclase crystals. The difference in the degree of 

 crushing is about the same as that between the Hartland schist 

 and its representatives included within the diorite. 



The Mount Tom Hornblende Gneiss 



The three prominent hills in the southwest corner of the region, 

 Mount Tom, Little Mount Tom, and Mount Rat, are composed 

 of a dark green or green and white hornblende gneiss. The irreg- 

 ular area occupied by this rock is two miles broad on the east, 

 where it abuts against the Brookfield diorite, and projects in two 

 long spurs to the southwest into the Hartland schist. The longest 

 of these spurs is more than three miles long and forms the ridge 

 of Mount Rat. 



The gneiss is composed chiefly of green hornblende and andesine 

 with smaller amounts of quartz and with titanite and magnetite 

 forming rare accessories. The texture is nearly everywhere 

 strongly gneissoid though it is more massive near the center of 

 the area than along the borders. The foliation is due to the very 

 complete granulation of the plagioclase and the parallel orientation 

 of the hornblende laths. Quartz is present in considerable quanti- 

 ties and always in much larger individual grains than the plagio- 

 clase (Plate VIII b). Some of it occurs in lenticular groups of 

 grains which strongly suggest a certain amount of granulation but 

 much less than the pulverized andesine. The hornblende laths 

 sometimes end in fine rods that pass out into the ground-up plagio- 

 clase. They are only in part crushed and do not seem to show as 

 great effects of pressure as it would seem should accompany such 

 complete crushing of the feldspar. The rock has about the same 

 composition as the quartz-bearing types of the Brookfield diorite, 

 with a little more hornblende, but it must not be confused with 

 the gneissic derivatives of the diorite. Those have a cataclastic 

 texture developed by pressure that affected an already solidified 

 rock. The Mount Tom gneiss, on the other hand, has a proto- 

 clastic texture developed through pressure during solidification 

 after the plagioclase had crystallized but before the quartz was 

 formed. The fine, unbroken rods of hornblende appear to show 



