258 



looking south from Slate Island we are facing a syncline or an 

 anticline. The former is, of course, indicated by the l)eds on 

 the island, :ind the latter by those on the shore, and it may 

 very well be that several axes intervene. 



THE DIKES OF IIINGHAM. 



It is intended, of course, to include here only those intru- 

 sive masses which are either contemporaneous with or newer 

 than the sedimentary rocks ; and the granitic rocks — diorite, 

 o-ranite, and felsite — are thus wholly excluded. No dikes of 

 porphyrite have been certainly identified in Hingham ; but, as 

 previously stated, the dikes associated with the slate and con- 

 o-lomerate are all of more basic character and are certainly nearly 

 all true diabase. It is possible, however, that several of the dikes 

 which have l)een observed should be classed as melaphyr. One 

 of these, which is colored as mela})hyr on the map, crosses the 

 small field in the angle between Downer and Grove Avenues, 

 south of the pond. The rock has the texture and general appear- 

 ance of melaphyr ; but the outcrops are insufficient to show clearly 

 its width, exact trend, or relations to other rocks, except that it 

 appears to be cut, as ma[)ped, by a dike of normal diabase. Itis, 

 however, probably intrusive in the conglomerate ; and hence 

 suggests a flow of melaphyr nuich liigher up in the stratigraphic 

 scale than any that we have recognized. A dike of similar litho- 

 loo-ical character, and possibly a continuation of this, although it 

 has been inadvertently colored as diabase, breaks through the 

 conglomerate near Otis Street, in the south part of Melville 

 (Jarden. It is about 15 feet wide, irregular in outline, and 

 iindes to the north. 



A glance at the maps suffices to sliow that the diabase dikes, 

 which we may suppose are Avholly subsequent to the sedimen- 

 tary deposits, dating from the period of distui'bance when 

 the strata were folded and fniihed, have, with few exceptions, 

 a general east-west trend : and it is highly probable that the 



