Hyatt.] 222 [January 19, 



« 



mapped by Prof. C. H. Hitchcock, but whether the Salem syenites 

 belong properly to the next oldest system of Hunt's series or to the 

 Laurentian, seems at present doubtful. This matter, however, as 

 well as the subject of the chemical changes of the porphyries, will, 

 I hope, be fully investigated by one of the Assistants in the Society's 

 Museum, Mr. W. O. Crosby, and fully reported upon at some future 

 time. 



The porphyries appear to overlie the Salem syenites unconform- 

 able, and together with them are cut by at least two series of dioritic 

 dykes, one running nearly north and south, and the other in a north- 

 westerly and south-easterly direction, if indeed any system can be 

 eliminated from the confused lines, which intersect each other in 

 every direction on the surface. The porphyries, though varying 

 greatly in aspect and in composition, are nevertheless but one forma- 

 tion, and derived from a vast conglomerate which appears in Lynn, 

 Saugus, and Marblehead, and is reported to occur under the granites 

 on the Beverly shore. The originally conglomerate nature of the 

 entire deposit is inferred by extensive observations made by myself 

 at Marblehead Neck, and by my assistant, Mr. W. O. Crosby, in 

 Saugus, and the general identity of the purely crystalline porphyries 

 of Lynn with those of Marblehead Neck, which are undoubtedly 

 merely altered conglomerates. In some localities it is possible to. 

 study the various phases of the changes which may take place in the 

 original conglomerate within the circuit of a few yards. Tl»us at one 

 point on the ocean side of Marblehead Neck, the variegated conglom- 

 erate is altered to compact light colored felsite in one direction, in 

 another becomes a dark colored porphyry with crystals of feldspar. 



The change into the felsite is the most instructive, since here it is 

 possible to trace the included pebble of dark colored, banded por- 

 phyry through all stages until it becomes a mere spot in the light 

 colored matrix. During this change the pebble disappears by some 

 process by which the structure is altered from without, the centre 

 being the last point to lose its distinctive coloring or structure. This, 

 and the unaltered form of the pebbles or masses, would appear to 

 militate against the supposition that such a series of changes could 

 only take place in a plastic or semi-fluid mass. But whether this was 

 the case or not, and whatever the condition may have been, the fact 

 seems to me unquestionable, after a review of this locality, that both 

 a felsite and a true porphyry were formed out of a conglomerate, 

 without any perceptible change having been made in the form of the 



