306 



ral Histoiy took tlic opportunity to explain the general 

 structure of these animals, a fine large specimen being placed 

 upon the table before him. Its cellular structure ; its mode 

 of capturing its prey ; its stomachs and organs of digestion ; 

 its locomotory powers ; the minute proportion of solid matter 

 in comparison with its bulk and other particulars were made 

 interesting to the audience. On conclusion of his remarks 

 Mr. R. assumed the Chair and called upon Mr. James J. H. 

 Gregory, of Marblehead, who furnished the meethig with his 

 views of the GeoloCxY of Marblehead. 



The little peninsula of Marblehead (embracing a territory 

 of about 3500 acres) is mostly of primitive formation. The 

 northern portion of the peninsula is a deposit of Greenstone, 

 intersected at various angles with dikes of the same rock, in 

 which the felspar is more comminuted and the hornblende 

 in greater proportion than in the mass. In the southern 

 sectoin, the Syenite contends with the Greenstone for suprem- 

 acy, and affords a fine proof of the theory of a distinguished 

 Geologist, that the eruption of Greenstone and Syenite were 

 simultaneous, for here these two rocks are thoroughly inter- 

 mingled, here a small patch of Greenstone and there a patch 

 of Syenite occurring in the same ledge, as though the min- 

 eral constituents boiling up together, the quartz had displayed 

 an elective affinity, in its arrangement. 



On the portion of the town, popularly known as the " Neck", 

 a smaller peninsula connected with the mainland by a sea- 

 washed isthmus — we find deposits of Greenstone, Syenite and 

 Porphyry for the most part very distinct, though occasional- 

 ly the Greenstone grades into the Syenite and the Porphyry 

 is somewhat affected by its neighbor rock. The Porphyry 

 occurs under three varieties as regards the structure of the 

 deposit, in ledges having a clevage tendancy, but only de- 

 veloped so far while in process of cooling as to shatter the 

 rock into small angular pieces, whose angles roughen the 

 surface as though studded with spikes. A fine illustration 

 of this form of deposit occurs in the abrupt ledge which 

 makes the termination of the long beach, on the southern 

 side of the isthmus, at the beginning of the Neck. A second 

 form of deposit presents the clevage planes so far advanced 

 that perfect rhomboids arc not unfrequently met with. 



