1876.] 219 [BouvS. 



dikes occurred, and he believed that many of our porphyritic rocks 

 were formed from the conglomerate. These views I sustained by ■ 

 referring to my own observations, expressing myself as satisfied that 

 the porphyries of our vicinity, as well as the amygdaloids, were 

 altered conglomerates. 



Dr. Hunt closed the discussion by saying he was confident that at 

 Marblehead these rocks were not altered conglomerates. They were 

 derived rocks, but from the primitive parent rock on which they 

 rested. 



As Dr. Hunt has recently said that he should take issue with me 

 upon the point, that the porphyries of Marblehead were derived from 

 the conglomerate, I presume he has not altered his opinions in respect 

 to any of the felsites of our neighborhood. I refer particularly to 

 Dr. Hunt's expressions because of the very great respect that I have 

 for his views, based as they are upon extended observation and a 

 more thorough knowledge of the chemistry of rocks, and of rock 

 formations than many can attain. They could not but have some 

 influence in leading me to distrust my own conclusions without fur- 

 ther examination. But such examination having only confirmed 

 my original thoughts, I have sought to bring before you such evi- 

 dence as hand specimens may exhibit. I have therefore brought 

 here, not only specimens illustrating the variety and beauty of our 

 porphyries, but such as may be serviceable in showing their origin. 

 [A fine series of specimens was then exhibited.] 



In conclusion, I wish not only to re-express my belief in the deriva- 

 tion of these felsites from conglomerates, but to go one step further, 

 and include among the rocks having the same origin, some at least of 

 the underlying sienites. I know that chemical questions can be asked 

 that may not be easily answered, discountenancing this view, such 

 as were asked by Dr. C. T. Jackson, at a meeting of the Society in 

 December, 1869, who inquired, when Professor Shaler expressed 

 the opinion that the sienites of our vicinity were of sediment- 

 ary origin, how the sienite was made, what sediments were so 

 strangely metamorphosed into a crystalline salt like feldspar, and 

 where did the rock get its potash and soda ? Possibly if we knew 

 more of the aqueous menstruum that permeated all these rocks 

 when they were metamorphosed, these questions might be satisfac- 

 torily answered. That some of our sienitic rocks exhibit conglom- 

 erate structure, will not be denied after the very instructive instances 

 cited by Hitchcock. But I refrain from expressing more on this 



