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MANSFIELD: ROXBURY CONGLOMERATE. 227 
possibly nearly 10,000 feet (n, p. 468-469). The lower figure is 
certainly a safe minimum considering the length of exposure and the 
steepness of the dip (90° to 60°). According to Woodworth’s figures 
the thickness of the section exposed at Pondville is 250-300 feet (d, 
p. 136). On the supposition that the giant conglomerate does not 
represent the base of the series the thickness of the Pondville rocks 
may be added to the minimum figure given, making 5,250 to 5,300 
feet. It is probable, however, that the actual thickness of the Norfolk 
Basin sediments greatly exceeds this figure, for at the Canton Junction 
ledges a mile and a half southwest of the southern end of the ridge 
-above mentioned the rocks still maintain the southerly dip and present 
different facies from those farther north. The structure and thick- 
ness of the Norfolk Basin sediments make it probable that they were 
‘continuous, at least in part, with the sediments of the Boston Basin, 
though no definite correlation is now possible. 
The Southern Boundary. On account of the same features, Crosby 
believes that the southern boundary of the Norfolk Basin, in its north- 
eastern part, is a profound fault, with the downthrow on the north, 
that extends northeast and cuts out the gray beds east of the Canton 
Junction locality (Crosby, n, p. 470). So far as the writer’s observa- 
tions have gone they are in harmony with Crosby’s view in this respect. 
There is no locality where the boundary can be observed and its 
position is largely conjectural, but there appears to be no place where 
the lower beds at the north rise again to the surface except at the 
locality southeast of Ponkapoag Pond where the ledge of coarse 
conglomerate was noted (page 221), and here the relations seem to 
indicate faulting rather than folding. In the southwestern part of 
the basin, however,’ the only region that shows the actual southern 
border of the sediments is the Pondville area and there the beds rest 
in sedimentary contact upon the granitite without the intervention of 
a fault. 
Relations to Igneous Rocks. Crosby has pointed out in earlier and 
later papers that the Norfolk Basin sediments are not associated with 
contemporaneous lavas and that they are practically free from dikes 
(Crosby and Barton, p. 417; Crosby, n, p. 499), the only dike thus 
far discovered being the one exposed in the section along the railroad 
north of Canton Junction. Woodworth has suggested that the appar- 
‘ent absence of these features may be due to the extensive covering 
-of drift and alluvium and that they may in reality be more abundant 
than is generally supposed (b, p. 148). The occurrence of the igneous 
impregnations in the conglomerates on the south side of Bear Hill, 
