58 
melaphyr and conglomerate and thus owe its exposure to exten- 
sivefaulting. The map, it will be observed, has been constructed 
in accordance with this view ; and reference to the table of Nan- 
tasket strata on page 24 will afford some idea of the magnitude 
of the displacements which it involves, —at least 50 feet on the 
side toward the conglomerate and 400 feet on the side toward 
the melaphyr. 
Indications are not wanting that Black Rock is near the point 
or centre of emission of the Nantasket porphyrites. One evi- 
denee of this is the seemingly great thickness of the porphyrite 
in the Black Rock area, and its more crystalline character ав 
compared with the porphyrite in the western part of Nantasket. 
The most important fact pointing to this conclusion, however, 
is the series of porphyrite dikes on the Cohasset Shore east of 
Green Hill. The most casual observation shows that the ap- 
proximately north-south dikes on the Cohasset Shore сап not 
all be referred to the third system of diabase dikes ; but besides 
the three well-defined systems of dark-colored, finely crystalline 
diabase dikes, there is evidently a fourth system having a gen- 
eral north-south trend, consisting of a distinctly greenish rock, 
which varies in texture from apparently felsitic and porphyritic 
to visibly holoerystalline or nearly so. The dikes of this kind 
are not only lithologically but chronologically distinct, for they 
are repeatedly cut by both the east-west and north-south dia- 
base dikes; and hence, although agreeing approximately in 
trend with the newest series of dikes, they must be regarded as 
the oldest system exposed on this shore. 
My interest in this oldest system of dikes was not aroused so 
much by the clear proof of their age as by the marked litholog- 
ical resemblance which they bear to the more acid lavas or 
porphyrites of Nantasket, and especially to the porphyrite of 
the Black Rock area. Except that the porphyrite flows, al- 
though often visibly epidotie, are rather rarely distinctly green- 
ish in color, while the dikes of this system are always so ; and 
that the dikes are more crystalline than the effusive rocks, as 
we should naturally expect; while the latter exhibit commonly 
