16 GUIDE TO LOCALITIES. 



westward for some distance in the Belmont woods. Its appear- 

 ance suggests a frontal moraine from which the fine material has 

 been washed out, leaving only the coarse blocks. 



MYSTIC RIVER QUARRIES, SOMERVILLE. 1 



Boute. — By electrics, Winter Hill car from Scollay square ; stop at 

 Temple street, Somerville; three miles. Quarries on right, near Mystic 

 river, the largest at eastern end of a low hill, which has been largely 

 removed. 



By wheel, same as to Medforcl, stopping on crest of Winter hill at 

 Temple street, and turning to right. 



These rocks strictly speaking are not slates, but fine-grained pet- 

 ites, devoid of good cleavage. They are intersected by numerous sets 

 of joints, large and small, which divide the strata into polygonal 

 blocks of various sizes, often with the regularity of art. It was 

 largely upou specimens from here that Woodworth based his clas- 

 sification of joint-fractures. The beds are cut by at least two 

 series of dikes — an earlier gray set, now deeply altered, and a later 

 dark group of the basaltic type. The former usually extend in a 

 direction nearly east and west, or parallel with the strike of the 

 strata, and send out several sills. One of these, about four feet 

 thick, may be seen in the southwest corner of the easternmost 

 quarry, where it is cut off by the later black dikes. In the north- 

 ern part of the opening one of the latter shows a remarkable series 

 of included fragments of rocks occurring beneath the Boston 

 Basin. A few annelid or crustacean trails of undeterminable age 

 have been found in the rocks of the quarries facing the Mystic 

 river. Farther west in the open field south of Tufts college this 

 group of pelites is overlain by a small patch of felspathic quartz- 

 ite, which in turn is capped by a few feet of red shale, the whole 

 being compressed closely into an unsymmetrical syncline, with the 

 steeper side on the east. 



Literature. 



Woodworth, J. B.— On traces of a fauna in the Cambridge slates. 

 (Bos. Soc. Nat. Hist., Proc. vol. 26, pp. 125-120.) 



Woodworth, J. B. — On the fracture system of joints. (Bos. Soc. Nat. 

 Hist., Proc, vol. 27, pp. 163-183.) 



1 The notes for this locality were furnished by Mr. J. B. Woodworth. 



