117 
north as to involve a displacement of improbable magnitude. 
The latter (25) is a good example of a fault-dike (Fig. 5) and 
its northerly hade agrees well with the throw. It is eut on the 
northern edge of Melaphyr Plateau by the most northerly dike 
of the second system (26) ; and near the railroad both of these 
dikes are dislocated by the same transverse fraeture, the hori- 
zontal displacement being 7 feet for dike 26 and, on account of 
its greater hade, 15 feet for dike 25. 
'The fault-dike (27) on East Porphyrite Hill is one of the 
most important in the distriet, marking a vertical slip of about 
50 feet. It is porphyritic and strikingly similar to 25, east of 
the bay ; but since they belong, one to the first and the other to 
the second system, it seems best not to connect them. This is, 
however, one of many indications that the two systems are not 
very distinct, chronologically. The next dike (28), though 
presenting precisely the same characters on Rocky Neck and 
Melaphyr Peninsula, cannot be identified on Kast Porphyrite 
Hill; for it seems best to connect the next dike to the south on 
the hill (29) with the dike south of 28 on Rocky Neck. The 
last-mentioned dike (29), like 27, breaks the dikes of the first 
system on Hast Porphyrite Hill; and it is certainly continued 
on the east side of the bay. In fact it can be identified with 
reasonable certainty, as shown on the map, from east of Hull 
Street to the western shore of Rocky Neck. The outcrop on 
the north side of West Porphyrite Hill, however, is quite un- 
certain; and it may be that what is regarded here as one 
continuous dike may include two or three independent dikes. 
In contrast with this dike of exceptional length, the next dike 
(30) has been recognized only on the west side of Rocky Neck. 
It is interesting chiefly on account of the important displace- 
ment which accompanies it, being the only clear fault-dike on 
Rocky Neck. 
The next dike (31) is especially characterized by its strong 
northerly hade. It probably ends westward against the fault 
between Cliff Plateau and West Porphyrite Hill. At any rate, 
