1870.] 45 [Hunt. 



Wednesday, October 19, 1870. 



The President in the chair. Forty-two persons present. 



The following papers were presented : — 



On the Geology of the Vicinity or Boston. By Dr. T. 

 Sterry Hunt. 



During the past week I have made several geological excursions in 

 the neighborhood of Boston, in some of which I was accompanied by 

 Prof. N. S. Shaler, and in the others by Prof. Alpheus Hyatt; the 

 familiarity of these gentlemen with the local geology has greatly 

 facilitated my examinations. The rocks which we have seen may be 

 considered in three classes. A, the crystalline stratified rocks; 

 B, the eruptive granites; C, the unaltered slates, sandstones and 

 conglomerates. The former of these may be separated lithologically 

 into two divisions; the first being the quartzo-feld spathic rocks. 

 Among these are included the felsite-porphyrites of Lynn, Saugus 

 andMarblehead, with their associated non-porphyritic and jasper-like 

 varieties, the compact feldspar of Hitchcock, who has well described 

 these rocks in the Geology of Massachusetts, pages 664, 667. Asso- 

 ciated with them is a granular quartzo-feldspathic rock which is often 

 itself porphyri tic, with feldspar crystals, and sometimes appears as a 

 fine grained syenitic or gneissoid rock, often distinctly stratified. 

 This has been described by Hitchcock as intermediate between por- 

 phyry and syenite; his syenites with " a nearly or quite compact feld- 

 spar base " and some of his porphyritic syenites (Geol. Mass., pp. 

 668, 669) will probably be found to belong to these granular eurites, 

 which I connect with the porphyries. These rocks are seen inti- 

 mately associated with the porphyry on Marblehead Neck, also in 

 Marblehead, and underlying the argillites of Braintree and 

 Weymouth. 



The second division of the rocks of class A includes a series of 

 dioritic and chloritic rocks, generally greenish in color, sometimes 

 schistose, and frequently amygdaloidal. They often contain epidote, 

 quartz, and calcite, and occasionally actinolite, amianthus, scaly 

 chlorite, and copper pyrites. This series holds a bed of dolomite at 

 Stoneham, and serpentine in Lynnfield, where bedded serpentines, 

 dipping at a high angle to the N. W., occur apparently in the strike 

 of these dioritic and epidotic rocks, which include the greenstones of 



