88 
and about the only clue to its stratigraphic position is that 
afforded by the enclosed pebbles of jaspery porphyrite. There 
can be little or no doubt that these represent the flow on East 
Porphyrite Hill; and the inference appears to be justified that 
this is the third conglomerate, a small outlier of which has 
already been observed resting upon the porphyrite. This view 
of its position means that Conglomerate and East Porphyrite 
Hills are separated by an east-west fault with a downthrow on 
the south of fully fifty feet. Following this fault west to the 
valley between these hills and Cliff Plateau, we find that it 
must terminate in a north-south fault, for it cannot be traced 
across the plateau; and this transverse fault must downthrow 
to the east, for otherwise there would be insufficient room 
beneath Conglomerate Hill for the porphyrite and the second 
conglomerate. Conglomerate Hill thus appears as a rectangu- 
lar block which has dropped down, the downthrow being 
greatest at the junction of the two faults, at the northwest cor- 
ner, which explains the fact that the dip is less than in East 
Porphyrite Hill and more easterly. The north-south fault 
probably coincides with dikes 66 and extends with greatly 
diminished throw nearly or quite to the north shore, terminating 
all the minor faults erossing Cliff Plateau. East Porphyrite 
Hill and Cliff Plateau are thus two broken fault blocks which 
have risen relatively to Conglomerate and West Porphyrite 
Hills ; while all these blocks must have dropped with reference 
to Granite Plateau. 
Great Hill.—This is the highest point in the western area, 
rising very abruptly 74 feet above the western marsh, and it is 
the boldest and most impressive topographic feature in all 
Nantasket. With the exception of the granite which supports 
the hill on the southwest, it is another isolated mass of conglom- 
erate; and we are obliged to depend wholly upon internal 
evidence in determining its stratigraphic relations. The entire 
lack of correspondence between Great and Crescent Hills has 
