Hyatt.] 220 [January 19, 



point, simply because my own observations in the field have been so 

 limited, but will ask if the reputed succession of our rock deposits is 

 not itself very suggestive. 



Conglomerate. 



Compact Feldspar, gradually passing into Porphyry. 

 Porphyry, gradually passing into a rock intermediate between 

 Porphyry and Sienite. 

 Rock intermediate between Porphyry and Sienite. 

 Sienite. 



Now if this gives the true succession of our rocks, and I believe it 

 does from the observations of others and not from my own, I ask if it 

 be not a fair inference, that the causes that led to the changes in the 

 higher portions of the series, affected all, only to a much greater 

 degree the lower ; that the heat and aqueous menstruum that soft- 

 ened and partly changed some of the conglomerates of the upper 

 portion forming the felsite conglomerate, as it may be called, repre- 

 sented by the large specimen exhibited, and which melted the suc- 

 ceeding strata so as to produce first felsites without crystals, and 

 below these the true porphyries, may not also by its greater in- 

 tensity so thoroughly have melted down still lower strata of sedi- 

 mentary rocks, (conglomerates and slates perhaps), as to entirely 

 resolve them into their original elements, recrystallize them and thus 

 have formed sienites, some of which may have even subsequently 

 played the role of eruptive rocks; for it by no means follows that 

 because a rock has been sedimentary that it may not also have 

 become likewise eruptive by being forced upward when in a semi- 

 fluid state. 



Prof. A. Hyatt made some remarks in support of the theory 

 advanced by Mr. Bouve, and exhibited a map of Marblehead 

 Neck, made some years back by the aid of the Plane Table 

 Map of the United States Coast Survey, and also largely 

 from observations made by the class of 1871, of the Mass. 

 Institute of Technology. 



The outlines of the porphyritic, granitoid, and micaceous rocks 

 were pointed out, and the first named rocks more particularly de- 

 scribed. The porphyries are the underlying rocks and occupy the 



