WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
147 
My thanks are due to Prof. W. O. Crosby for enabling me, under 
his guidance and that of Mr. Thos. A. Watson, of Weymouth, to 
visit, in a few days, the most noteworthy exposures, contacts, and 
outcrops illustrating stages of transition among the rocks of the 
Blue Hills, which he has spent years in searching out and mapping, 
and for his valued criticisms in the prosecution of the research. My 
thanks are also due to Prof. J. F. Kemp of Columbia University for 
many valuable suggestions in the course of this petrographic study. 
With the increasing interest recently developed in the petro¬ 
graphic examination of these Pre-Cambrian volcanics, especially of 
the Atlantic coast, it is to be hoped that in a few years we shall be 
able to correlate them, and more fully to determine their most 
interesting relations, than is possible with the limited published 
results now at our disposal. 
List of thin sections of the rocks examined. 
Slide. 
1. Cambrian slate. 
Hayward Creek. 11 
Mill Cove — (altered slate). C15 
Slate — Eldridge Hill. 16 
2. Newer slates and conglomerates. 
Slate. El 
Conglomerate, north of North Common Hill quarries, PI. 2, Fig. 7 204 
“ “ “ “ “ “ “ P1.4, Fig.13 212 
Sheared conglomerate; cut on Old Colony R. R. just north of 
Black Creek. .... 210 
Shaly conglomerate ; same locality. 211 
3. Contact phenomena. 
Contact slate adjacent to coarse granite ; West Quincy . . 61 
Garnetiferous hornfels; North Common Hill quarries . . 60a 
Garnetiferous and pyritiferous slate; same locality ... 60 
Contact of slate and granite ; same locality, PI. 1, Fig. 1 . . C 14 
Slate adjacent to granite, north quarry, same locality . . 56 
Secondary minerals in hornfels ; same locality .... 57 
Tourmaline crystals in hornfels; same locality . . . 57a 
Hornfels contact between slate and granite; North Weymouth 
R. R. cut. 52 
Felsite near the contact with the Cambrian slates, Hayward 
Creek.. 15 
4. Diorite and dioritic granite. 
Ledges on south side of Whitman Pond, East Weymouth . 216 
East of Whitman Pond. 220 
