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BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
that the heavy conglomerates of the northern border and the arkoses | 
of the southern border were formed at the same time. The basal 
character of the arkoses cannot be doubted, since they rest upon the 
granitite from which they were derived. ‘The basal character of the 
heavy conglomerates at the north, on the other hand, does not seem 
to the writer to be clearly established, though it must be admitted 
that they lie beneath a great mass of grits and sandstones, which, 
from the attitude of included ripple-marks and mud-cracks, show that 
there has been no inyersion of the strata. The writer is therefore 
inclined to place the conglomerate higher in the section than at the 
base. Under this interpretation the structure of the sediments along 
the northern bordermay be regarded as monoclinal but faulted down 
„along that line. It is highly significant in this connection that the 
first sediments encountered on the north side of. the Blue Hill Range 
toward the west are arkoses overlain by slates of somewhat similar 
appearance to those that overlie the arkose at Pondville. Under this 
interpretation it may be conceived that the arkoses and overlying 
sediments of the Narragansett Basin were once continuous over the 
u u en 
explained as due to drag, with a slight overturning on the north. 
On the other hand, the coarseness and known distribution of the 
conglomerate seems to indicate a local source for the materials and it 
is natural to turn to the hills immediately at hand as the most probable | 
region from which the boulders could have been derived. Crosby, | 
in his description of the Streamside Ledge, states positively that f 
pebbles of the Blue Hills porphyry occur in the conglomerate of that | 
locality and that the contact of the conglomerate with the igneous 
rocks on the north is sedimentary. According to his view the existing 
basins now occupied by Carboniferous rocks were not outlined at the | 
time when deposition began (n, p. 464) but boundary faults developed | 
during the progress of deposition (ibid., p. 501). He conceives that : 
deep faults occurred along the north side of the Blue Hills Range and _ 
farther south along the boundary between the sediments and the 
granites; that the block thus formed comprised the mass of the Blue 
Hills and the area now occupied by the sediments, and that this block $ 
was gradually tilted southward so that a growing depression was 
formed, in which the sediments were deposited (ibid., loc. cit.). The | 
giant conglomerate was formed by the action of vigorous surf on the 
present site of the Blue Hill Range and that the basins became sepa- j 
rated by the down-faulting of the Norfolk sediments along the south y 
side of the present range. The steep northerly. dip, changing to 
southeily, of the rocks along the northern border of the basin may be 
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