30 
Its source is undoubtedly to be found chiefly in the alteration 
of these basic lavas; and it occurs even more abundantly in the 
melaphyr itself than in the conglomerate, in the form of irreg- 
ular segregations and veins of jasper, chalcedony and vitreous 
quartz. 
The Older Eruptive or Granitic Rocks. 
Doth the diorite and the older, dark colored granite of the 
Cohasset shore are practically wanting in the Nantasket area; 
the granitic rocks being limited to the coarsely crystalline, acid 
type, excepting a few small, irregular and ill-defined dikes of 
finely crystalline granite (micro-granite) and felsite. The ex- 
traordinary abundance of pebbles of quartz-porphyry and other 
varieties of felsite in the conglomerate is a plain indication that 
these rocks were far more extensively developed over the 
granitie area in early times than now, forming, probably, sur- 
face flows as well as dikes. The coarsely crystalline, acid 
granite of Nantasket is essentially similar to that of Cohasset 
and the South Shore generally ; and little need be added to the 
statements of the preceding pages. Over Granite Plateau and 
Granite Point and on Rocky Neck the relations of the basal 
conglomerate to the granite are very intimate and intricate ; 
but absolutely no facts have been observed which indicate that 
the granite is intrusive in the conglomerate, or that any of the 
granitic rocks of the Nantasket region are more recent than the 
sedimentary rocks. On the contrary, the relations of the two 
series, as will appear later, are, at every point, consistent with 
the view that the conglomerate was deposited over an uneven, 
fissured surface of the granite, such as that rock presents to-day 
at many points. 
