2 BULLETIN OF THE 



ago described from the Granite Street locality in Somerville "by J. F. 

 and S. L. Dana/ and by J. W, Webster,^ as made up essentially of the 

 minerals feldspar and hornblende. This was supported by Professor 

 Hitchcock, in his Survey of Massachusetts.' The mistake, which con- 

 sisted in taking augite to be hornblende, was farther repeated by W. W. 

 Dodge ^ and W. 0. Crosby.^ Professor "Wadsworth^ was the first to 

 apply the microscope to the study of this region, and was enabled to 

 determine correctly the general composition of tlie rocks. His study 

 included all the eruptives exposed in Somerville, and part of those out- 

 cropping in Medford, Maiden, Brighton, and Brookline. He pointed out 

 the connection of the outcrops, and indicated their general trend. He 

 showed that the feldspar is plagioclase, and that the prevailing non- 

 feldspathic constituent is augite. On the basis of an observed identity 

 in mineral composition he concluded that the fine-grained " greenstone " 

 is identical with the coarser " diorite." Professor Crosby, in his " Con- 

 tributions to the Geology of Eastern Massachusetts,"'' has quite accu- 

 rately outlined on his map the area in which these rocks are exposed. 



The present article, being restricted mainly to the coarsely crystalline 

 diabase, repeats to some extent the work of Professor Wadsworth. 

 "While the results are in the main confirmatory, there are still important 

 points of difference, and some new facts have been determined. 



Leaving out of consideration the vicinity of tlie Old Powder House 

 in Somerville, the field study has yielded but little. The attempt has 

 been mainly to add something to our knowledge of the petrographical 

 characters of the so-called " Mesozoic diabase of the Atlantic border." 

 Those who desire a full literature of the subject treated in this paper 

 should refer to the above-cited work of Professor Wadsworth. 



Field Notes. 



The dike of diabase which is here considered extends from Granite 

 Street in Somerville to Spot Pond in Stoneham, and probably beyond, 

 although no examination in the field was carried beyond that point. 



1 Memoirs Amer. Acad., 1st series, IV. 16-3 (1818). 

 '^ Boston Jour. Pliil. and Arts, XL 282 (1824). 

 8 final Kep. Geol. of Massachusetts, 640-663 (1841). 

 * Proc. Boston Soc. Nat. Hist., XVIL 415 (1875). 

 ^ Occasional Papers of Boston Soc. Nat. Hist., Vol. III. 



^ Notes on the Petrography of Boston and Vicinity, by M. E. "Wadsworth. Proc. 

 Boston Soc. Nat. Hist., XIX. 217 (1877). 



^ Occasional Papers of Boston Soc. Nat. Hist., Vol. III. 



