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.2 The mistake, which con- 
sisted in taking augite to be hornblende, was further repeated by W. W. 
Dodge* and W. O. 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 the rocks. His study 
included all the eruptives exposed in Somerville, and part of those out- 
cropping in Medford, Malden, 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,”7 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 the 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, 
althongh no examination in the field was carried beyond that point. 
1 Memoirs Amer. Acad., Ist series, IV. 163 (1818). 
* Boston Jour. Phil. and Arts, II. 282 (1824). 
8 Final Rep. Geol. of Massachusetts, 640-663 (1841). 
* Proc. Boston Soc. Nat. Hist., X VII. 415 (1875). 
§ Occasional Papers of Boston Soc. Nat. Hist., Vol. IIT. 
® Notes on the Petrography of Boston and Vicinity, by M. E. Wadsworth. Proc. 
Boston Soc. Nat. Hist., XIX. 217 (1877). 
7 Occasional Papers of Boston Soc. Nat. Hist., Vol. III. 
