79 
able fact that the intense hardness of the Nantasket conglomer- 
ates is due to interstitial silica derived from the same source. 
W. 
Fic 9.—SECTION OF THE GRANITE AND OVERLYING CONGLOMERATE 
ON GRANITE PorNT. 
SCALE, I INCH ==10 FEET. 
Cliff Plateau. Cliff Plateau comprises the triangular group 
of conglomerate and melaphyr ledges between Granite Plateau 
and the shore on the north. The average elevation is about 
thirty feet, but two points rise above fifty feet. The name re- 
fers to the well-marked cliff 25 to 40 feet high which bounds it 
on the north, extending from Granite Point to dike 66. Gran- 
ite Point, as already stated, is overlain by the basal conglom- 
erate, with thin beds of sandstone showing nearly horizontal 
dips, the main slope being to the northeast. One bed of sand- 
stone two to three feet thick runs the entire length of the cliff 
facing West Porphyrite Hill, until it meets dike 33, meeting it 
about five feet above the beach and thus sloping approximately 
one foot in ten. It is thrown up about three feet by this dike 
and then continues with the same inclination, but growing less 
distinct, to dike 31, beyond which it cannot be traced, probably 
passing under the water. The eastern half of the cliff, rising 
abruptly from Nantasket Harbor, between dikes 83 and 66, is 
composed entirely of this conglomerate, with the exception of a 
narrow slice of the overlying melaphyr which, toward the east- 
ern end, has been let down by a fault trending N. 80° E. with 
the downthrow to the north. In fact, as the map shows, the 
basal conglomerate forms throughout the main body of the 
