CHAPTER V. 

 THE LOWER CAMBRIAN GNEISSES. 



THE BECKET CONGLOMERArE-G]SrEISS. 



This o-neiss skirts the western liorder of Humpshire and IIainj)den 

 counties, inclosing- narrow strips of Algonkian rocks in Middletield and 

 ToUand, and stretches westward across the first two tiers of towns in 

 Berkshire County, around many pre-Canibrian areas, to enter into most 

 complex and obscure relations to the Stockbridge limestone and associated 

 rocks of western Berkshire. It is thus much more amply developed beyond 

 the boundaries of the river counties than within them, and I have for 

 convenience given it a name from the town in Berkshire where it may be 

 best studied. It rests upon the older gneiss in great beds of highly altered 

 (juartz-congloraerate, as at the Hoosac Tunnel central shaft and at the 

 Dakon Clubhouse, and graduates iii its upper portion into the Cheshire 

 quartzites, so largely used for glass-making. The rock is unconformalde 

 upon the lower series. 



With many exceptions, especially where it folds round the older rock, 

 as given in detail below, the strike is the prevailing one of the region, 

 varying but little from north and south, and the dips are high. 



CONTACT UPON THE WASHINGTON GNEISS BELOW. 



As it passes down the eastern side of the area of older rock in Hins- 

 dale it dips away from it with some irregularity, which is confined to the 

 immediate vicinity of the contact; farther away it regains the normal north- 

 south strike and a dip which varies but little from verticality for long dis- 

 tances. As it swings around the southern end of the underlying gneiss it 

 dips away from it with low angles, changing from east thi'ough south to 

 west, and it is at the same time so far affected by the strong east-west com- 

 pression which has molded the whole region that it is thrown into a series 

 of subordinate folds with axes radiating outward and pitching from the old 

 gneiss, which has thus assumed the role of a foreign and more resistant 



