EASTERN MASSACHUSETTS. 423 



" The porphyries appear to overlie the Salem syenites unconformably, and 



together with them are cut by at least two series of dioritic dykes The 



porphyries, though varying greatly in aspect and in composition, are neverthe- 

 less but one formation, and derived from a vast conglomerate which appeai-s in 

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

 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. 0. Crosby, in Saugus, and the general identity of the 

 purely crystalline porphyries of Lynn with those of Marblehead Neck, which 



are undouljtedly merely altered conglomerates The change into the 



felsite is the most instructive, since here it is possible to trace the included 

 pebble of dark colored, banded porphyr\' through all stages until it becomes a 

 mere spot in the light colored matrix. During this change the pebble disap- 

 pears by some process by which the structure is altered from without, the 



centre being the last point to lose its distinctive coloring or structure 



The fact seems to me unquestionable, .... that both a felsite and a true por- 

 phyry were formed out of a conglomerate, without any perceptible change 

 having been made in the form of the contained pebbles." (Proc. Bost. Soc. 

 Nat. Hist., 1875-76, XYIIL, pp. 220-225.) 



Professor Hyatt points out that the felsite pebbles in the conglomer- 

 ate are different from the felsite formed from it, and holds that through 

 pressure or otherwise the pebbles form the thin laminse seen in the 

 banded felsite. The reader is referred to his veiy interesting descrip- 

 tion of the imaginary processes, since it is too lengthy to transcribe 

 here. (Ibid., pp. 223, 224.) 



Mr. W. O. Crosby, In his Report on the Geological Map of Massa- 

 chusetts, 1876, says (pp. 7, 8, 10, 11) : — 



" The Eozoic rocks of Massachusetts may to a large extent at least be divided 

 lithologically and chronologically, into three divisions, which, stated in their 

 order of sequence, are the Norian, the Huronian, and the Mont Alban. I 

 weigh my words well when I describe these divisions as both lithological and 

 chronological; for, .... I do not hesitate to affirm that the lithological char- 

 acters of the di^^sions which have been worked out among the crystallines of 

 this region, — the chronological and geographical distinctness of which I can- 

 not doubt — are as unlike as the faunse of any two successive geological for- 

 mations." 



This statement was in substance again affirmed in 1880. Of the 

 Norian he writes : — 



" The rocks of this formation, though frequently stratified, seem in general to 

 have been somewhat fluent, and usually exhibit more or less extravasation; 

 but doubtless in some cases the metainorphic action has stopped short of this 

 extreme term, though destroying all traces of beddmg. In many places .... 



