NEW ENGLAND APPALACHIAN SYSTEMS 



185 



Woonsocket Basins 



A section across the southern of the two small basins, here called the 

 Woonsocket, is given in Fig. 11.30. The western margin of the Penn- 

 sylvanian basin dips steeply, although it is a sedimentary contact. The 

 east margin is a high-angle normal fault contact ( Richmond, 1952 ) . The 

 Bellingham conglomerate which fills the small basins generally dips east- 

 ward although it has many small and closely spaced folds. The west 

 margin is a sedimentary overlap. The conglomerate pebbles are stretched 

 in the plane of schistosity and the long axes point down dip. The matrix 

 in places is a mica or chlorite schist which tends to enwrap the pebbles. 

 The conglomerate in the southern basin is more sandy and less meta- 

 morphosed, and contains beds of graywacke, biotite-sericite schist, dark 

 phyllite, and slate. 



Boston Basin 



The strata of the Boston basin comprise the Roxbury conglomerate 

 below, and the Cambridge slate or argillite above. The Roxbury lies un- 

 conformably on the Dedham granodiorite of Precambrian (?) age, and 

 is possibly Pennsylvanian and probably Permian in age, according to 

 Billings et al. (1939). The conglomerate is over 3500 feet thick, and the 

 slate about 3500 feet; both constitute the Boston Bay group. Part of the 

 I Roxbury conglomerate is volcanic and part sedimentary. The volcanic 

 ! rocks include not only effusive lavas but also thick beds of tuff, ag- 

 glomerate, volcanic breccia, and conglomerate. 



The Roxbury conglomerate above most of the volcanics is described by 

 Emerson as consisting of the Brookline conglomerate at the base, the 

 Dorchester slate in the middle, and the Squantum tillite at the top. Ac- 

 cording to La Forge the threefold division does not persist throughout 

 the area occupied by the formation with sufficient definiteness to warrant 

 mapping the members separately. In some areas, beds like the Dorchester 

 ! slate are intercalated in most of the formation below the tillite. The 

 Brookline conglomerate is massive, coarse, and in some areas 1200 feet 

 thick. It contains cobbles and boulders, many of which are of the under- 

 lying Dedham granodiorite or of the volcanic complex. The slate mem- 

 ber is red and purple, and in one place possibly 2000 feet thick. Much of 



Northern 



Soufhrrn 



• Roxbury conglomerate - 



Dec/horri orono - 

 O/orite 



Cambridge*^ „ ' • / 



argillite x /roxbury cong/omerore 



Decfharn grono&iorite 

 zooo rSET 



Fig. 11.31. Cross sections of Boston basin. Upper section from northwest to southeast across entire 

 basin. Cr, Roxbury conglomerate; Cc, Cambridge slate; blank, pre-Carboniferous, mainly igneous 

 (LaForge, 1932). 



Middle section across Nantasket area. Section about 4000 feet in length. After Billings, Loomis, 

 and Stewart, 1939. 



Lower section across the Hingham area. After Billings et al., 1939. 



it is reworked basaltic and andesitic tuff, and layers of purple sandstone 

 and grit are common. The Squantum tillite is exposed in many places in 

 the southern part of the Boston basin, and is about 600 feet thick. It 

 possesses many characteristics of glacial drift and is generally believed to 

 have been deposited by local mountain glaciers. 



The various lithologic types of the Roxbury conglomerate interfinger in 

 a complex fashion in the Nantasket area, according to Billings ( 1939 ) , 

 and the formation consists of numerous lenses of sedimentary and vol- 

 canic materials overlapping one another. See cross sections, Fig. 11.31. 



The Cambridge slate, over the Roxbury conglomerate, underlies nearly 

 all the northern part of the Boston basin and occupies several long belts 

 in the southern part. The rock is practically nowhere a true slate, but it 

 generally has a dominant cleavage parallel with the bedding. It has vari- 



