106 
being more compact and homogeneous, it is very similar, litho- | 
logically, to the enclosing melaphyr ; and hence is quite incon- 
spicuous and not easily found. The outcrop begins on the 
north side of the road immediately east of the Waverly House 
and ean be traced due east several hundred feet to the shore. 
Ft is somewhat irregular in form, and varies in width from 
three feet or less to seven. feet, narrowing eastward. “The hade 
is also variable, but averages about vertical. 
Its compactness, similarity to the enclosing melaphyr, and 
irregular outline are the characters that especially mark this as 
an approximately contemporaneous dike. The melaphyr of the 
dike is firmly welded to that of the walls, and the contacts are 
clearly neither joint-planes nor fault-fractures. Although this 
is the only undoubted dike of melaphyr that I have observed in 
the Nantasket flows, the eye is frequently attracted by sharply 
defined, vertical, dike-like contacts between the ordinary brec- 
ciated, amygdaloidal or scoriaceous melaphyr and more com- | 
paet forms; but as a rule these cannot be traced far, ог only 
one wall can be found, and it is quite clear, in most cases at 
least, that they are not dikes in the ordinary sense. 
'The second dike (2) of the list is much more impressive and 
interesting than the first, being decidedly the largest of all the 
Nantasket dikes. It cuts the granite south of the west marsh 
with a nearly due north-south trend and a width of 32 feet. Its 
interesting relations to the effusive melaphyr of the western 
area, and its marked change in form on passing from the gran- 
ite to the basal conglomerate on the eastern margin of Granite 
Plateau, have been fully described (page 77) ; and in these 
respects, that is, as a clearly exposed volcanic vent the outflows 
from which can be fully identified, it is, perhaps, unique among 
the dikes of the Boston Basin. It is exposed, as the map - 
shows, on two islands in the marsh, forming the western edge 
of one and the eastern edge of the other. It commences on the 
north with a due north-south trend, but on passing to the 
southern ledge it changes to S. 10? E., and then to S. 80° E. 
