49 



or unaltered petrosilex breccia, — a gradation due partly to the 

 conditions of deposition of the Shawmut rocks, and partly to 

 the unequal and in some cases extreme metamorphism of the 

 breccia, whereby have been reproduced in this newer formation 

 several types of structure characteristic of the parent beds, — 

 that most observers have failed to distinguish them, considering 

 the whole as of approximately the same age. This manner of 

 regarding the phenomena in question leads naturally to views 

 concerning the origin of these rocks which find their culmina- 

 tion in that proposed by Prof. Alpheus Hyatt and Mr. T. T. 

 Bouv^,^ according to which all these rocks, including the homo- 

 geneous, structureless petrosilex, had originally a conglomerate 

 structure, the present variety being mainly due to the unequal 

 and unlike metamorphism of different portions of the primitive 

 conglomerate. 



Substantially the same idea was subsequently expressed in 

 my report on the Geological Map of Massachusetts, with the 

 important exception that I there dissented from the conglomerate 

 origin of the homogeneous petrosilex and the evenly banded 

 petrosilex, claiming that these were deposited as fine sedi- 

 ments bearing the relation to the other rocks of slate to 

 conglomerate. - 



A more extended and careful study of these rocks -than was 

 possible before the publication of that report, however, has 

 greatly increased my knowledge of their relations and distribu- 

 tion, and led me to the adoption of the conclusion stated on the 

 preceding page ; which, in its essential features, — as regards 

 the separation of the petrosilicious rocks here referred to the 

 Shawmut group from the Huronian petrosilex, and the mutual 

 relations in time and space of the two series thus formed, — 

 is identical with the view previously held by Dr. T. Sterry 

 Hunt.2 



The difficulty of distinguishing the Huronian petrosilex from 



1 Proc. Bost. Soc. Nat. Hist., xviii, 217. 



2 President Hitchcock, also, with his usual sagacity in such matters, expressed the 

 sauie idea, with singular clearness, nearly forty years ago. 



OCCAS. PAPERS B. S. N. H. — in. 4 



