No. 40] GEOLOGY OF SHEPAUG TUNNEL 1 9 



region as dikes and stocks of granite or its gneissic equivalent 

 and cuts all of the other rocks, excepting the pegmatite dikes. 



The relations of the strata south of the granite and west of the 

 diorite are very complicated. The geological map of Connect- 

 icut shows several disconnected areas of Becket gneiss (referred 

 to the pre-Cambrian) and Berkshire schist. According to that 

 interpretation the first several thousand feet of the tunnel pass 

 through the Becket gneiss. Reasons will be given in this 

 bulletin for including all of this complex within the Berkshire 

 formation and omitting the Becket altogether. 



The remaining rocks of the region mapped include the pegma- 

 tite dikes and the relatively much rarer quartz veins. These do 

 not appear very frequently on the surface though there is a large 

 pegmatite dike forming a bare knob in a field one mile northeast 

 of Woodville, but they are very common in the tunnel, and imme- 

 diately east of this area the Hartland schist is disrupted by a great 

 quantity of pegmatites ranging from large-sized dikes to minute 

 injections. Underground the pegmatites are found in large num- 

 bers in both the Berkshire and the Hartland schists and in much 

 smaller numbers in the Brookfield diorite. The quartz veins have 

 about the same relative distribution but are much less common. 



The foliation of the metamorphic rocks throughout the area 

 strikes N. 50° E. to N. 10° W., generally about N. 20° E. ; and 

 the dips, which are nearly always towards the west, vary from 

 vertical to about 30°. The only exception to this uniform strike 

 and dip is in the easterly part of the diorite area where the folia- 

 tion of the gneissoid diorite and the included fragment of schist 

 strikes northwest and dips east. 



The tunnel exposes the contact between the Brookfield diorite 

 and the Berkshire schist on the west and between the Hartland 

 schist and the diorite on the east. The westernmost contact is 

 not very definite. The igneous rock has penetrated the schist as 

 sills parallel to the foliation and two areas of the schist occur some 

 distance within the igneous rock. The eastern contact is sharp 

 although there are many fragments of the Hartland series included 

 in the diorite far from the contact. 



The fragments of included schist from both formations are 

 somewhat different from their corresponding types but there is no 

 marked amount of contact metamorphism to be noted in either 

 case. 



The Hartland Schist 



The Hartland schist is a widespread formation that extends in 

 relatively narrow belts from northern Massachusetts, where it is 

 known as the Hoosac schist, through the western part of Con- 

 necticut to Long Island Sound. The small area considered in this 

 report cannot be regarded as including all the varieties present in 

 the formation but the tunnel exposes a section two and three- 



