No. 40] GEOLOGY OF SHEPAUG TUNNEL 27 



bent or broken lamellae, quartz with sutured boundaries and 

 undulatory extinction, and considerable sillimanite. Apatite and 

 garnet may be present. 



The Berkshire schist of the eastern belt described above and 

 exclusive of the tunnel section is a very variable rock, but it may 

 be characterized as a biotite schist or gneiss containing much 

 quartz and up to ten per cent of oligoclase. Apatite is always 

 present as small rounded grains and both garnet and staurolite are 

 commonly formed. The biotite is frequently bent and where most 

 disturbed sillimanite has formed and the quartz is partially granu- 

 lated. This rock is intruded and injected by variable numbers of 

 pegmatites and, in the Canaan Mountain section, is full of irregular 

 knots of feldspar and garnet. 



The rock exposed in the first three thousand feet of the tunnel 

 is a highly contorted biotite gneiss containing much quartz and 

 oligoclase. Apatite, garnet and sillimanite may be present. The 

 biotite lamellae are frequently warped but the quartz is only 

 rarely granulated. These three thousand feet contain more than 

 six hundred feet of pegmatite exclusive of the dikes only a few 

 inches wide and the thin layers interlaminated with the schist. 



It is evident, therefore, that this rock does not resemble the 

 Hinsdale gneiss. It contains no microcline and lacks the quartzite 

 and metamorphosed limestone bands. On the other hand, its 

 microscopic resemblance to the Berkshire is very striking and, 

 bearing in mind what a variable formation the Berkshire is, 

 there appears to be no reason for establishing a new series here. 

 It is tentatively regarded, then, as a representative of the eastern 

 belt of the Berkshire schist somewhat more thoroughly cut up by 

 pegmatites than most of that rock. 



The strike of the foliation in the tunnel varies between north 

 twenty and north thirty degrees east. The dip is prevailingly 

 west at an angle of eighty degrees or more, though it occasionally 

 flattens to forty-five degrees west, or stands vertical, or is very 

 slightly overturned towards the east. Pegmatite dikes cut the 

 schist at all angles and have very irregular contacts. In large 

 part they lie parallel to the foliation but even then their bound- 

 aries are irregular (Plate iv). A pegmatite may cut across 

 several layers of schist as a fine dike only to spread out as a large 

 bulb some distance from the main mass. This igneous or aqueo- 

 igneous material has not replaced the schist to any noticeable 

 extent but has confined its activity to an injection in fine layers 

 and intrusion on a larger scale. 



There is one pegmatite two hundred and fifty feet thick that 

 outcrops at the surface as well as underground. For the most 

 part these dikes are composed of quartz, white microcline, albite 

 and garnet, and exhibit no foliation, but there is one dike two 

 hundred feet wide made up of pink microcline, quartz, some 

 magnetite, garnet, and very little green pleochroic biotite, and 



