PETROGRAPHY. 



Prof. J. E. Wolff. 



Among the petrographical features in the vicinity of Boston may 

 be mentioned the Quincy granite area on the south, with numerous 

 quarries in the dark gray hornblende granitite. The contact with 

 the slates is well shown in those on the northeast. The Blue Hills 

 are a complex of eruptive rocks comprising various forms of gran- 

 ite-porphyry and quartz -porphyry (aporhyolite), collected between 

 Rattlesnake hill and Wampatuck. 



The Middlesex Fells to the north of Boston include granite, 

 granite-porphyrj', quartz-porphyry, tuffs and trap dikes. The com- 

 plex of igneous rocks of Essex county from Cape Ann to Lynn 

 is of great interest. Beginning with the granitites quarried at 

 Cape Ann, syenite, elaeolite syenite, diorite, essexite and numerous 

 trap dikes are met along the coast. At Marblehead Neck is the 

 original locality for Bostonite, and fine felsite-breccias are found, 

 and granite intrusive into schist. Along the Clifton shore granite, 

 diorite, pegmatite and trap dikes exist in very interesting relations. 



The famous Medford-Somerville coarse diabase, so often de- 

 scribed for its weathering, is well shown at Granite street, Somer- 

 ville, and in the old quarry in the west side of Pine hill, Medford. 



The dikes of the Cambridge-Somerville slates (Kidder avenue, 

 etc.) are of interest. The various amygdaloids (Brighton, Hough's 

 Neck, etc-.) have been the subject of several papers. 



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