87 
the bay east of East Porphyrite Hill the section is continuous 
and complete, all five of the beds being represented (Fig. 10) ; 
but the evidence along this line is greatly strengthened at the 
weakest point by the very clear and simple section afforded by 
West Porphyrite Hill (Fig. 11). The lithological comparisons 
are almost perfect and certainly safe for so small an area ; and 
the succession may be said to rest at every step on the approved 
canons of stratigraphy. 
The general structural relations of the rocks in those portions 
of the western area already deseribed are clearly exhibited in 
the two accompanying sections: one (Fig. 10) east and 
west, and the other (Fig. 11) north and south. For the sake 
of greater clearness the east-west dikes are not shown in the 
first section. 
Conglomerate Hill.—This completely isolated mass of con- 
glomerate is, perhaps, the simplest of all the structural blocks 
composing the Nantasket area. It is all conglomerate, with 
the exception of occasional thin layers of hard red sandstone, 
which show a dip of not more than from 5° to 10° to the east or 
alittle south of east. Some parts of the conglomerate are very 
coarse, holding pebbles from six to twelve inches in diameter ; 
while other parts are very fine and arenaceous. The red 
jaspery porphyrite of East Porphyrite Hill is a very conspicuous 
constituent ; but the red jasper is also, to a very limited extent, 
an indigenous constituent, forming irregular segregations in the 
cement of the conglomerate. The hill appears to be entirely 
unbroken by either faults or dikes; and the most diligent 
search has failed to reveal a vestige of eruptive rock of any 
kind, except in the form of pebbles. "There are no overlying 
rocks; and the base of the conglomerate is not exposed. 
Hence the total thickness of this bed is a matter of conjecture ; 
but since the eastern slope of the hill is approximately parallel 
with the dip, the exposed thickness must be at least fifty feet. 
The hill is surrounded on all sides by grass land and marsh ; 
