MANSFIELD: ROXBURY CONGLOMERATE. 223 
The section north of Canton Junction contains fossiliferous shales and 
a band of limestone (Woodworth, b, p. 147). 
General Structure. According to Crosby there is no repetition of 
the basal conglomerate and the dips are all southerly, so that the 
structure of the eastern portion of the basin is a monocline (n, p. 469). 
He considers that the western part of the Blue Hill Range, together 
with the region occupied by the Norfolk Basin sediments, constitute 
a fault block that has been tilted south, the boundary faults lying 
along the north side of the hills and the south side of the sediments 
respectively (n, p. 500-501). The writer's observations have led him 
to propose some modifications of these views. A quarter of a mile 
southeast of Ponkapoag Pond occurs a low ridge-like mass of conglom- 
erate about forty feet long, with an east-west trend, and composed of 
rounded and subangular pebbles ranging in diameter from six inches 
to two feet. This is the outcrop above mentioned (page 221) where the 
pebbles appeared to resemble the Blue Hills porphyry. North of 
this exposure and east of the pond there are numerous outcrops of 
red sandy shale with fine conglomerate that intervene between the 
conglomerate immediately south of the Blue Hills at Houghton’s 
Pond and the rock in question. If the latter be considered as really 
in place it must indicate a repetition of the strata by faulting or fold- 
Ing, or else it belongs in a different horizon from the other coarse con- 
glomerate — an unlikely supposition in view of the gradation of the 
sediments displayed by the rocky ridge at the west. The apparently 
uniform southerly dips would perhaps justify the idea of a monocline, 
broken by strike faults, in the eastern part of the area, but the struc- 
tures indicated at East Walpole and at Pondville show that such is cer- 
tainly not the case in the southwestern part of the basin. The limited 
outcrops at the former locality show intense folding with some faulting 
while at the latter place the same features are more clearly displayed. 
Moreover, the basal sediments exposed at Pondville are arkoses and 
none of the immediately overlying beds can be compared in coarseness 
with the heavy conglomerates of the northern border. In the Narra- 
gansett Basin arkoses occur near the base of the series and heavy 
Conglomerates appear high in the section. 
_ As in the case of the Boston Basin the question of climatic condi- 
tions is raised by the occurrence of arkose at Pondville. The discus- 
Slon of its climatic significance is reserved for the succeeding chapter 
but it may be stated here that the conditions favorable for the pro- 
duction of arkose are unlikely to be favorable for the production of 
heavy, water-worn comglomerates. It therefore seems improbable 
