WHITE : PETROGRAPHY OF THE BOSTON BASIN. 145 
rather more abundantly along the contact line, while the granite for 
a few millimeters is a trifle finer grained. The contact metamor- 
phism often noted is not shown, however. Contacts are especially 
well shown in the sections from North Common Hill quarries 
(PI. 1, Fig. 1). A few sections show bits of the slate included in 
the granite, near the contact. 
In the North Common Hill quarries there is on the contact with 
the granite a very compact dense hornfels, so baked that pieces 
emit a metallic sound when struck with a hammer, and the zone 
appears almost like a dike. 
The contact of the Paradoxides slate and felsite in the Hayward 
creek locality has been so much altered that the felsite has assumed 
somewhat the appearance of a gneiss, as described by Shaler (’71). 
In other cases the felsite, which is a contact phase of the granite, 
according to Wadsworth (’82) has along this juncture with the 
slate lost all its distinctive granitic characters and “ been trans¬ 
formed into a spherulitic quartz porphyry along the contact. ” 
SUMMARY. 
From the foregoing it will be seen that there is in the district 
treated a complex series of rocks which have been intruded through 
the Cambrian and later slates and conglomerates. The con¬ 
glomerate occurring near the North Common Hill quarries in 
Quincy, although containing rounded pebbles, which indicate its 
sedimentary character, nevertheless microscopically resembles a 
volcanic tuff, since it seems to have lath-shaped plagioclases in the 
ground mass. There are many evidences of crushing here as well 
as in the conglomerate on Hough’s Neck. In the upper conglom¬ 
erate of the latter locality many fragments of the various neighbor¬ 
ing volcanics are contained. 
Four granites, besides the diorite, are described; but the liorn- 
blendic granitite is omitted from this summary: — The dioritic 
granite occurring on Mount Pleasant and near Whitman Pond, as 
a phase near the contact in which patches of diorite of various 
sizes have segregated out in the granite, until finally the diorite 
itself assumes prominence, as the prevailing type in the vicinity of 
Whitman Pond, as the peripheral phase of the collected dark sili¬ 
cates of the great intrusion of the complex. 
