2 6 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [BulL 



where intrusions are common, will show a greater proportion of 

 such changes. 



The Berkshire Schist 



The Berkshire schist is a widespread formation in northwestern 

 Connecticut and extends north into Massachusetts and southwest 

 into New York. It varies considerably in its mineralogy and in 

 the amount of metamorphism that it has undergone. The Berk- 

 shire that extends into this map area is the same general type as 

 that which forms Canaan Mountain — i. e., a coarse quartz, biotite 

 schist with locally sufficient feldspar to be a gneiss.^ It is cut off 

 from the Canaan Mountain mass in the north by a narrow belt of 

 gneisses belonging to the Becket formation but is almost certainly 

 a continuation of the Canaan Mountain rock. It is intruded by 

 a large stock of Thomaston granite gneiss in the northwest corner 

 of this area and is cut by many dikes of pegmatite. 



The areal distribution of the formation as mapped in the present 

 report dififers considerably from that on the former map.^ The 

 whole region about the Shepaug River west of the Brookfield 

 diorite and north of Mount Tom is mapped as Berkshire, and the 

 Becket, shown here on the former map, is omitted altogether. The 

 first three thousand feet of the tunnel pass through a contorted, 

 variable mica gneiss and open up a section not hitherto observable. 

 Reasons are given below for regarding this rock as a part of the 

 Berkshire formation, though such an assignment is certainly sub- 

 ject to change when a more detailed study of that whole formation 

 is completed. 



The Becket gneiss as originally mapped and described^ includes 

 a great variety of metamorphic rocks in part originally igneous, 

 in part sedimentary. Some parts are undoubtedly pre-Cambrian 

 and some are of questionable age. The writer has begun to remap 

 those areas and finds the following three main types: i. The 

 Becket granite gneiss typified by the slightly gneissoid rock at 

 Becket, Massachusetts, and Norfolk, Connecticut.* This is a 

 white or grey quartz, microcline, oligoclase, biotite and (or) 

 muscovite granite grading into a strongly gneissoid type with a 

 cataclastic texture. 2. A paragneiss consisting of the Becket 

 injected into some pre-existing biotite schist series. 3. An older 

 series of metamorphosed sediments — the Hinsdale gneiss of B. K. 

 Emerson^ made up of quartzite, altered limestone layers, and a 

 siliceous biotite gneiss. The gneiss is very variable but is charac- 

 terized by large amounts of microcline, dark brown biotite with 



1 See bottom of page 17 of this report. 



2 Preliminary Geological Map of Connecticut, H, E. Gregory and H. H. Robinson. 



3 Manual of the Geology of Connecticut, W. N. Rice and H. E. Gregory, Conn. Geol. 

 and Nat. Hist. Survey Bull. No. 6, page 93, and Preliminary Geological Map of 

 Connecticut by H. E. Gregory and H. H. Robinson. 



* Geology of Massachusetts and Rhode Island, B. K. Emerson, U.S.G.S. Bull. 597, 

 pp. 154-155, 1917- 

 5 Op. cit., page 10. 



