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MANSFIELD: ROXBURY CONGLOMERATE. 289 
the sides of the synclines facing the middle area are nearly vertical 
(Shaler et al., p. 27). These main features are indicated in Figure 
9, taken from Shaler’s account. In some parts of the basin there has 
been further folding and some faulting. At Hoppin Hill in North 
Attleboro a small area of profound dislocation has brought the under- 
lying granitite, Cambrian and Lower Carboniferous strata to the 
surface (Woodworth, d, p. 121). The number of great folds in the 
basin is few, but the great mass of sediments is here thrown into folds 
quite equal in dimensions to those of the Appalachian region in Penn- 
sylvania. From axis to axis of the same kind is a distance of upwards 
of six miles. With dips often 45° or more, folds of so great breadth 
indicate a great thickness of strata, of which there cannot be less than 
12,000 feet now remaining (ibid., p. 122-123). 
Relations to Igneous Rocks. In the Narragansett Basin igneous 
rocks appear in association with the Wamsutta group. Dikes of 
diabase occur near North Attleboro which are commonly vesicular 
for a distance of one to three feet from the upper surface, and some- 
times the lower surface is amygdaloidal, but there is no evidence that 
the diabase flowed out as a contemporaneous sheet (ibid., p. 152). 
In the same region an acid series of igneous rocks of felsitic and 
granophyric texture is intimately associated with the Wamsutta group. 
The felsite occurs frequently at stratigraphically higher horizons 
than the intruded diabase, is marked by a definite flow structure, 
and is often accompanied by a crumpling of the layers (ibid., p. 154). 
“The rapid thickening of the sandstones and conglomerates toward 
the northwest corner of the present area, the felsite with definite flow 
structure, the gray ash beds or Attleboro sandstone, the agglomerates 
of felsitie material, and the associated conglomerates composed in 
large part of felsitic pebbles all point to a volcano or volcanoes exist- 
ing in this field in Carboniferous time” (ibid., p. 155). 
Metamorphism. According to the accounts of Shaler and Wood- 
worth the rocks of the Narragansett Basin have suffered less from 
metamorphism than was formerly believed by most geologists. It 
is true that in some parts of the area the rocks have become highly 
metamorphosed but the regions thus affected are, on the whole, rather 
localized. ‘The most extensive of these regions begins near Pawtucket 
and widens as it extends south toward the sea in Narragansett Bay, 
the most pronounced effects being observed along the western bound- 
ary. The transition from this highly metamorphosed zone to the 
comparatively unaltered rocks on the east is so abrupt that one is led 
to believe that an intermediate zone of considerable width has been 
concealed by a fault (Woodworth, d, p. 119). 
