2 8 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



muscovite. It exhibits a coarse banding parallel to that of the 

 wall rock. 



The contact between the Berkshire schist and the Brookfield 

 diorite is very irregular. Diorite dikes have penetrated the schist 

 several hundred feet from the contact and thick bands of the 

 schist are included within the igneous rock but no intimate mixing 

 of the two formations has taken place. 



It is impossible to draw conclusions concerning the structure 

 and metamorphism of the Berkshire schist as a whole from the 

 small section here described. Certain characteristics may be 

 pointed out, however, that serve to distinguish it from the Hart- 

 land schist with which it comes in contact both north and south 

 of the tunnel. In the first place, there is no constant relation 

 between the foliation and the former bedding planes. The folia- 

 tion is due to coarse bands of oligoclase (in some cases albite) 

 and quartz, rarely somewhat granulated, alternating with relatively 

 thin layers of crumpled and strained biotite and much finer grained 

 quartz. It lacks the simple crystalloblastic texture of the Hart- 

 land schist. The rock was thoroughly recrystallized and injected 

 in fine bands by quartz and feldspar, after which a slight move- 

 ment gave it an incipient cataclastic texture and bent and folded 

 the biotite laths that had formed in response to the first meta- 

 morphism (see Plate vi b). 



Due to the similarity of various parts of this rock, the lack of 

 any sign of original bedding and the number and size of the dikes 

 that cut it, it is impossible even to estimate the thickness of the 

 series. 



The Brookfield Diorite 



The Brookfield diorite forms a nearly circular mass in the center 

 of the area described and separates the Hartland schist on the 

 east from the Berkshire schist on the west. The tunnel passes 

 through this formation from west to east and serves to locate its 

 boundaries in this direction with great accuracy. The line of bore 

 holes bored along the course originally proposed for the tunnel 

 across the broad upper end of Bantam Lake served to locate the 

 Brookfield-Hartland contact under Cranberry Pond and swamp. 

 The contact in the tunnel itself is a little northwest of the angle 

 near the base of Marsh's Point, and the western contact with 

 the Berkshire is located almost beneath the highest point of the 

 first hill in the tunnel line east of the Shepaug River. 



Little Mount Tom, near the southwest corner of the map, is 

 now known to be composed of the same hornblende gneiss as 

 Mount Tom (see next chapter), so that the diorite boundary 

 swings along the northeast foot of both those hills. The rest of 

 the boundary is taken from the Preliminary Geological Map of 

 Connecticut. The contact is everywhere covered and no reason 

 is known to change it. 



