No. 1. — Contributions from the PetrograpMcal Laboratory of the 

 Harvard University Museum^ in Charge of J. E. Wolff. 



On the PetrograpMcal Characters of a Dike of Diabase in the Boston 

 Basin. By William H. Hobbs. 



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, elaeolite-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. 0. Crosby, M. E. Wadsworth, and J. S. 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 coai-se 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. — NO. 1 . 1 



