A i 
MANSFIELD: ROXBURY CONGLOMERATE. 205 
to the slates of Braintree, and they are believed to be Precarboniferous 
because they are represented by pebbles in the conglomerate which is 
Supposably of Carboniferous age. 
The relations of the slates at Hyde Park to the conglomerate series 
are not clear. The small slate areas at Quincy seem to be merely 
inclusions or outliers. The sandstone or quartzite at Green Lodge, 
teferred to on page 98, resembles to some extent the quartzite at 
Somerville and Everett, but its st ratigraphic relations are not known. 
he area is so covered with drift and alluvium that boundaries are 
Merely conjectural. 
On the supposition that the Neponset slate is Carboniferous these 
Precarboniferous areas must be regarded as outliers or inliers of 
Cambrian sediments resting on the underlying granitic rocks. ‘There 
still remains the difficulty of fixing the relations between the Neponset 
slate on the north and the Weymouth and Braintree slates on the 
South. Crosby has met this difficulty by assuming a fault along the 
northern boundary of the granitic area, extending eastward some- 
where beneath the drift between Hough’s Neck and the Lower Cam- 
brian beds at Mill Cove and East Quincy (n, p. 508). The supposi- 
tion that the slates are Cambrian involves even greater difficulties, 
for the field evidence, so far as it may be obtained, appears to favor 
conformable passage upward from conglomerate to slate. If the 
latter is r eally Cambrian its presence apparently above the conglom- 
erate must indicate an inversion of the strata, or perhaps an over- 
thrust fault, the whole complicated by minor folds or faults to account 
for such features as the Hough’s Neck Conglomerate. It must be 
admitted that there is no evidence in favor of such conditions. On 
the contrary, the occurrence of southward dipping, ripple-marked 
Sandstones in normal position in ledges by the Neponset River (Boston 
VL Y 26) is opposed to the idea of inversion of the strata but is favor- 
able to the supposition of anticlinal structure. 
——:— The Southern Boundary. The southern boundary of the 
Boston Basin sediments is believed by Crosby to be a fault of perhaps 
2,500 feet displacement, with the downthrow on the north (n, p. 509). 
he facts upon which this view is based are the following: the cutting 
Out of a great body of Cambrian slate, the brecciation of the granite, 
the rapid narrowing of the conglomerate and the exceptional indura- 
tion of the latter as if by hydrothermal action (ibid., p- 428). The 
Conglomerate here mentioned is the western extension of the supposed 
anticline, of which the conglomerate and the associated rocks at Rock 
Island, Hough’s Neck, form the southern limb. This anticline is 
believed to be cut diagonally by the boundary fault (ibid., p- 498). 
