88 
These patches must belong to a third bed of conglomerate, and 
since they are parallel with the second or underlying conglom- 
rate, the separation of the two beds measures, approximately, 
ils thickness of the porphyrite, which cannot well be less than 
fifty feet and may be more. I have, however, observed no dis- 
tinet, indications that it is a composite flow ; but it is from bot- 
tom to top one solid and essentially homogeneous mass. The 
underlying conglomerate must be nearly as thick as the por- 
phyrite. 
West Porphyrite Hill.—This approximately isolated and 
well-defined rock-mass is, in both form and structure, an al- 
most exact repetition of East Porphyrite Hill, except that the 
structure is but slightly complicated by faults and dikes. In 
short it is a solid bed of conglomerate underlain by melaphyr 
and overlain by porphyrite ; and none of the Nantasket ledges 
present a clearer or more instructive section (Fig. E): The 
underlying melaphyr is very compact for the most part, green- 
ish and, purplish, non-amygdaloidal, and not conspicuously 
breceiated or ве 'oriaceous, except near the original surface of the 
flow, where it is in contact with the conglomerate. It shows 
many minute crystallizations and some whitish veinlets of 
epidote, and numerous small, irregular segregations of bright 
red jasper. In other words, it is essentially identical with the 
melaphyr which we have found overlying the basal conglomer- 
ate on Cliff Plateau and passing beneath the second conglom- 
erate оп East Porphyrite Hill. The outcrop begins on the 
north at the end of the beach leading to Melaphyr Peninsula 
and skirts the base of the hill to the middle of the west end. 
The contact with the overlying conglomerate is exposed con- 
tinuously for several hundred feet, but the base of the mel: aphyr 
is not seen, the thickness exposed in the hill not exceeding 15 
feet. A greater thickness is proved, however, by the outlying 
half-tide le lees, which extend nearly half-way across Weir River 
Day. These were carefully examined; and all that are marked 
