66 
extension can not be traced far in the direction of the same 
contact in the main area. The conglomerate extension is bound- 
ed on the north by dike 29, which clearly marks a fault with 
the downthrow also, probably, to the south. Hence the mela- 
phyr north of the conglomerate extension appears to be that 
normally underlying the conglomerate, and therefore distinct 
from, and separated by a fault from, that overlying the con- 
glomerate. 
The small meadow or marsh separating the conglomerate 
south of dike 35 from Crescent Hill must cover a fault with the 
downthrow to the north, as indicated on the map; for the bed 
of conglomerate first described and the underlying and over- 
lying melaphyrs are repeated, with a stronger topographic re- 
lief and fewer dikes, in the hill. The conglomerate forms the 
summit of the northwestern part of the hill and extends en- 
tirely around the northern and western slopes, its outcrop thus 
having the form of a crescent. It isthe irregular or mixed 
coarse and fine conglomerate, containing pebbles of amygda- 
loidal melaphyr as well as of granite and felsite. ‘The contact 
with the overlying melaphyr is very plainly exposed on the 
summit. It is a true effusive contact and has a southeasterly 
dip of 10°—20°, being conformable with the bedding-planes of 
the conglomerate. This overlying melaphyr shows several dis- 
tinct zones of amygdules, as well as other indications of flow- 
structure, and altogether exposes a thickness of 25 to 85 
feet. The contact is also clearly exposed for fifteen feet at the 
base of the hill on the east side. The thickness of the con- 
glomerate ean scarcely exceed 30 or 35 feet. The underlying 
melaphyr exposes a thiekness of about fifteen feet at the north- 
west corner of the hill, the contact descending both east and 
south. Near the middle of the west side of the hill the under- 
lying melaphyr appears again, being elevated some ten feet 
above the railroad by an east-west fault with the upthrow to 
the north (Fig. 7). This fault gives the conglomerate and 
underlying melaphyr a distinctly synclinal structure in the 
