No. 1. — Contributions from the Petrographical Laboratory of the 
Harvard University Museum, in Charge of J. E. Wourr. 
x 
On the Petrographical Characters of a Dike of Diabase in the Boston 
Basin. By Wivuiam H. Hosss. 
In the region north of Boston occurs a most interesting series of mas- 
sive rocks, which break through the slates and sandstones, and include 
granite, quartz-porphyry, quartzless-porphyry, eleolite-syenite, diorite, 
porphyrite, diabase, augite-porphyrite, and gabbro. They have been 
studied in greater or less detail by many observers, prominent among 
whom are W. O. Crosby, M. E. Wadsworth, and J. 8. Diller. 
The rocks which have afforded material for the present study belong 
to a single dike, and may be seen in a series of exposures in Medford 
and Somerville. They are coarsely crystalline rocks, and have borne 
the names “ syenite” and “ diorite.” They have in general been care- 
fully distinguished from similar rocks of finer texture known as “ green- 
stones,” which were shown by Wadsworth in 1877 to have about the 
same composition as the coarser rock, and were considered by him as 
identical with it. The finely crystalline rock seems to be more widely 
distributed than the coarse variety. In the present paper it has been 
studied only at a few localities, where there was some promise of de- 
ciphering its relations with the ‘ diorite.” 
The age of these rocks has not been accurately determined, though they 
have generally been considered post-triassic on account of their lithologi- 
cal resemblance to the diabase of the Connecticut Valley. The slates 
through which they have broken are probably identical with the Lower 
‘Cambrian argillite of Braintree. Diller has furnished evidence to show 
that the finely crystalline diabase (“‘ greenstone”) is the youngest of the 
eruptives of this region, its dikes cutting those of the other rocks.? 
Many mistakes have been made in determining the composition of 
both the so-called “ diorite” and “greenstone.” The “ diorite” was long 
1 Bull. Mus. Comp. Zool. at Harvard College, VII. 179. 
VOL. XVI. —wNo. 1. 1 
