MANSFIELD: ROXBURY CONGLOMERATE. 195 
River. This is the view held by LaForge. A somewhat similar view 
is held by Crosby, except that the latter believes that the continuity 
of the conglomerate is interrupted by a fault, with downthrow on the 
south along the smooth escarpment (Crosby, g, p, 21). In his dis- 
cussion of the probability of a boundary fault LaForge (p. 89) presents 
two facts that militate, as he thinks, against the hypothesis. First, 
the great Medford dike extends from the sedimentary rocks of Somer- 
ville well into the igneous highlands without evidence of being faulted. 
Second, the conglomerate and arkose of West Medford appear to pass 
into the granite next to which they occur without evidence of faulting. 
As regards the first objection it may be said that though no direct 
evidence of the faulting of the dike is at hand it is by no means certain 
that no faulting has occurred. The dike cannot be traced by con- 
tinuous outcrops from Somerville into Medford; furthermore, it is 
‘crossed by the valley of the Mystic just south of the escarpment. 
But, even if the dike is not faulted, it cannot be assumed that the 
escarpment does not represent a fault, for if the dike were younger 
than the period of faulting, which is not unlikely, it might readily 
pass across the previously formed fault plane without itself becoming 
faulted, though in that case branches of the dike might be expected 
to traverse the fault for a certain distance in either direction. The 
‚second objection does not hold, for the conglomerate and arkose 
might be supposed to represent a remnant preserved in a depression 
of the crystallines and uplifted with them by the faulting, so that the 
upper members of the series were carried above the level of erosion. 
According to LaForge’s view the escarpment forms an old shore line 
‚against which the slates and conglomerates were deposited (p. 90). 
This is indeed the narrow terrane view. The relations of the rocks 
in the Boston and adjoining basins seem to warrant the supposition 
that the area of deposition was far more extensive and that disloca- 
tions subsequently occurred on so large a scale as to justify the 
hypothesis of a fault of the dimensions required by the appearance 
of the escarpment. The present topographic form is probably the 
result of differential erosion and does not in itself express displace- 
ment. Thus from the above discussion it appears that while it is. 
true that no conclusive evidence has been adduced in favor of the 
boundary fault hypothesis it is also true that no valid arguments 
have been advanced against it. 
——:—The Somerville-Cambridge Slate. According to LaForge (p. 
-38-39) the slates of Somerville and Cambridge are generally considered 
‚as forming a great syncline, the axis of which runs along the valley 
