ee u KA AX 
MANSFIELD: ROXBURY. CONGLOMERATE. 225 
bold shore thus produced along the south side of the hills (ibid., p. 500). 
Under this supposition it would seem that at least equally coarse 
conglomerate should have formed on the north side of the hills, where 
the fault scarp was developing. Crosby accounts for its absence by 
Stating that the sediments on the north side were thrown down by the 
fault and that the basal beds are not exposed (ibid., p. 501). ‘This 
idea seems hardly credible, since, in the case of a growing fault of the 
Magnitude required by the hypothesis, a talus of coarser materials, 
Tearranged, perhaps, by aqueous agencies, might be expected to accu- 
mulate along the fault scarp. The successive slips of the growing 
fault might produce fractures or faults in the debris accumulations 
similar to those noted by Gilbert and others in the Great Basin region 
of the United States, but it is difficult to see how such accumulations 
could be carried out of sight by the very faults to which they owed 
their existence. The occurrence of arkose, too, on the north side of 
the hills, indicates that the basal sediments are not concealed, as 
Suggested by Crosby, but that they lie in sedimentary contact upon 
the. adjacent granites. 
If the fault be conceived as occurring on the south side of the range 
the presence of the conglomerate in that particular region may be 
More satisfactorily explained. It may very well be supposed that 
the period of relative quiescence in which the arkosic materials were 
being developed was terminated by movements of uplift or by climatic 
changes of such character as to permit the rapid deposition of these 
Materials without further disintegration. Thus arkoses and over- 
lying slates and conglomerates may have been laid down in order. 
Then the fault may have developed along the southern edge of the 
Blue Hills. If the giant conglomerate does not contain bona fide 
pebbles of the Blue Hills porphyry, it may be considered as part of 
a layer, perhaps eroded away at the north, preserved by down-faulting 
against the porphyry. But if Crosby’s contention be allowed, that 
boulders of the porphyry do occur plentifully in the giant conglom- 
erate, a supposition apparently favored by the doubtful cases noted 
by the writer, the conglomerate may be considered as a talus of coarse 
waste developed along the growing fault scarp and worked over by 
aqueous agencies so that it became more or less. water worn. In 
Such a case it might be expected that, as the fault progressed, the 
Successive slips would cause the growing accumulation to become 
fractured and displaced and perhaps broken into minor blocks. 
Evidence of this character is bountifully supplied by the great blocks 
of conglomerate that lie immediately south of Bear Hill and at the 
