105 
the broad exposure of the conglomerate series at the southern 
end of the peninsula we have only to add one more to the east- 
west displacements with the downthrow on the north which we 
have observed in the Nantasket ledges, the fault crossing the 
beach near the base of Atlantic Hill. 
| THE DIKES OF NANTASKET. 
The dikes of this district, as previously stated, consist almost 
wholly of diabase and are more recent than the bedded rocks. 
The existence of dikes of melaphyr and porphyrite, at least in 
the granite and the older strata, is, however, probable, с priori; 
and two dikes of melaphyr have aetually been recognized in the 
Nantasket ledges; while in the granite ledges along the Co- 
hasset shore there is a fine series of porphyrite dikes. The 
dikes of melaphyr and porphyrite clearly antedate all the dia- 
1 base dikes, dating, probably, from the effusive eruptions to 
| which they correspond in composition ; and they will, therefore, 
| be deseribed first, in each topographic division. Since the 
special map shows so clearly the location and approximate 
trend of each dike, and the descriptive lists give in addition the 
exaet trend and the hade, width, and other features, it is only 
necessary to cite here such additional facts as may be of particular 
interest. 
The Melaphyr Dikes. 
No. Trend. Hade. in Remarks. 
| | E-W. 
2 | 8, 09-809. E. 
Vertica 3-7 | Irregular in form and hade. 
ADM 32 Regular in form and uniform in 
| Width 
| 
| texture. 
The two dikes belonging to this class have been referred to 
in connection with the associated flows of melaphyr. Тһе first 
one (No. 1) breaks through the breceiated and compact green- 
ish gray melaphyr on the north side of Centre ПШ. Except in 
