WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
119 
the present paper for all the rocks of porphyritic or vitrophyric texture 
in which quartz and the alkali feldspars predominate, and in which 
How texture is ordinarily apparent in the microscopic sections; thus 
avoiding the terms felsite, quartz porphyry, etc., which might 
indicate age distinction. 
Professor Crosby’s conclusions (’95) 1 based on the held relations 
are, that “ at the time of its eruption the granite must have been 
deeply buried beneath the Cambrian slates, in which it formed large 
dome-shaped masses or batholites; and the most abundant and 
characteristic type of felsite occurs chiefly or exclusively as a 
contact-zone between the granite and slate. The felsite occurring in 
this way is a highly crystalline quartz porphyry, which passes down¬ 
ward by an insensible gradation into the granite. In walking over 
the main range of the Blue Hills, from the Great Blue Hill to Rattle¬ 
snake Hill and Pine Hill, we find that this quartz porphyry is the 
prevailing and almost the only surface rock. * * * But on closer 
examination we observe that wherever erosion has cut most deeply 
on the flanks or summits of the hills the granite is exposed beneath, 
with a perfect gradation, as already noted, between the two rocks. 
* * * That is, each hill is a dome-shaped batholite of granite, with a 
veneering or outer layer of intensely hard and resisting quartz 
porphyry. * * * During the formation of the batholites, and while 
the interior portions were still in a molten condition, fissures were, 
in some instances, formed in the outer shells, through which, as 
previously stated, considerable volumes of molten rock were 
extravasated, forming by rapid cooling and subsequent devitrification 
dikes and sheets of compact and banded felsite.” 
This compact jaspery type extends from Wampatuck Hill along 
the south side of Rattlesnake Hill to the eastern base of Pine Hill. 
In the field the porphyritic areas are only distinguishable by the 
fact that their feldspar phenocrysts weather more prominently than 
those in the granite and stand in relief on the surface. 
In hand specimens, however, the rocks are often nearly identical, 
so far as the unaided eye can distinguish. Jackson (’71) notes that 
there is an insensible passage from the syenite (old name of the 
Quincy granites) to the greenstone porphyry, while Wadsworth 
(’82), treating of the contact of the granites with the Cambrian 
slates along Hayward Creek, observes that the granite there has 
lost its distinctive characters, being changed to a spherulitic quartz 
1 Compare the earlier accounts by Crosby (’80, p. 90-91, and ’89, p. 5). 
