No. 40] GEOLOGY OF SHEPAUG TUNNEL 33 



that the recrystallization of that mineral had just begun when the 

 pressure ceased. 



On the southwest slope of Mount Rat, northeast of the road that 

 crosses between the two high knobs, the hornblende gneiss is 

 exposed in a field as a series of dikes or sills cutting a silvery 

 white, fine-grained type of the Hartland schist. The foliation of 

 the two rocks is parallel and strikes nearly east and west. One 

 of the intrusions has a very irregular contact zone with fragments 

 of the schist included in the gneiss and a light green epidote schist 

 developed in nodules along its edge. The microscope shows that 

 these nodules are similar to the main gneiss with the addition of 

 large amounts of epidote. 



The protoclastic texture and the igneous contact of the gneiss 

 are of particular interest since they prove the igneous character 

 of the rock and establish something with which to compare the 

 many other hornblende gneisses among the metamorphic rocks of 

 western Connecticut. There are hornblende gneisses in the Berk- 

 shire schist and in the areas now mapped as Becket gneiss that 

 resemble this rock very closely. There are others in the Becket 

 and in the Hartland schist that have been formed out of original 

 sediments. 



The Thomaston Granite Gneiss 



The Thomaston granite gneiss is an intrusive rock that is rather 

 widely developed in the western upland of Connecticut, particularly 

 in the central and southern parts. The texture varies from that 

 of a massive granite with no sign of foliation to a decidedly 

 gneissoid type with a secondary development of mica and con- 

 siderable granulation of the quartz. The least metamorphosed 

 part, as at the Plymouth quarry at Thomaston, was described in 

 the Manual of the Geology of Connecticut, page 109, as follows: 

 .... ''the rock is remarkably white in color, has a medium grain, 

 and is flecked by numerous small scales of mica (biotite)- The 

 v/hite base of the rock is made up of about equal parts of a white 

 feldspar (microcline) and quartz. Locally, as in the Wilton area, 

 it is distinctly porphyritic with phenocrysts of microcline which 

 sometimes reach one-half an inch in length." 



Only one considerable mass of the Thomaston granite gneiss 

 occurs in this map area. It intrudes the Berkshire schist in the 

 northwest corner of the area and projects some distance beyond 

 it to the north and northwest. It is a gneissoid quartz, microcline, 

 muscovite, and biotite granite with considerable oligoclase, micro- 

 pegmatite and some apatite. The biotite is dark brown with 

 strong pleochroic halos around minute zircons. The texture is 

 usually cataclastic and the foliation is due to the parallel orientation 

 of the mica flakes and the granulation of the quartz grains. 



There are two large dikes of exactly this same character cutting 

 the Brookfield diorite just west of the Bantam River opening ia 



