91 
posed fault is really an overlap, the conglomerate having 
covered a boss of granite whieh rose like an island through the 
first or underlying melaphyr. 
Following the fault-contact across to the southeast corner 
of the hill, we again find the conglomerate on the north under- 
lain by the compact melaphyr; and the granite outcrops at a 
little distance on the south without any capping of conglomer- 
ate, erosion having cut deeper here. It is evident now that 
Great Hill must be on the upthrow side of the fault bounding 
it on the northwest; and that the displacement is more than 
equal to the combined thickness of the porphyrite and third con- 
glomerate, or more than 100 feet. 
Correlation of the First Melaphyr.— Passing down 
the long southwestern slope of Great Hill, we come at last, 
in the ledges of granite isolated by the western marsh, {о 
the large dike of melaphyr, No. 2 of the map and the deseriptive 
list of dikes. This dike, which is strongly contrasted in 
character with all the diabase dikes of this district and certainly 
is not, as it appears to be at first glance, a southward continu- 
ation of the composite dike (66) separating Cliff Plateau and 
East Porphyrite Hill, demands our attention at this time on ac- 
count of its important and interesting relations to the first mel- 
aphyr, the only melaphyr of the western area. The microscopic 
resemblance is certainly very marked, and in his brief descrip- 
tion of the microscopic characters (page 37), Mr. Merrill 
has cited no fact inconsistent with the view that the dike is a 
more crystalline and less altered form of the same original rock 
as the flow. Attention was first attracted to this dike, as re- 
gards its relations to the effusive melaphyr, in seeking an ex- 
planation of the melaphyr which is mingled irregularly with the 
first or basal conglomerate on the eastern edge of Granite 
Plateau. This ledge of conglomerate and melaphyr is only a 
few feet west of the direct course of the dike ; and the question 
naturally arose, Why may not these irregular intrusions be 
