186 



that there are two zones of conglomerate, the one intercalated 

 with the slates, and the other overlying them. The mass of 

 opinion, however, is in favor of the view distinctly expressed 

 by Mr. W. W. Dodge, ^ that most of the conglomerate, at 

 least, is more recent than most of the slate. The Paradoxides 

 has been found only in connection with the slate, and hence 

 lithological evidence of the Primordial age of these rocks is 

 only indirectly applicable to the conglomerate. The general 

 resemblance of the last-named rock to the Carboniferous con- 

 glomerate of the Narragansett basin has led those believing in 

 its superior position with respect to the slate, to infer that it, 

 too, is of Carboniferous age. That view was entertained by 

 the writer when compiling the Centennial Geological Map of 

 Massachusetts ; but subsequent investigations have convinced 

 me (1) that there is in the Boston basin essentially but one 

 conglomerate and one slate, and (2) that the former under- 

 lies the latter. Whence it follows that, if, as I shall endeavor 

 to show, we are obliged to regard the slate as Primordial, the 

 conglomerate must be of the same age, or older. But the con- 

 formability of the slate and conglomerate, and the probable 

 continuity of their deposition, are shown, not only by a general 

 agreement in strike and dip, but also by the phenomenon, ob- 

 servable at many points, of a gradual lithologic passage between 

 them. Most observers have noted this, audit is generally ad- 

 mitted that such a transition exists, the only question being as 

 to the sequence ; assuming this to be as I have stated, and that 

 these rocks constitute but one series, it may be said that the 

 entire lower half of the formation is merely transitional ; for, 

 taking a general view, the gradual passage often begins at the 

 very base of the conglomerate. 



As a rule, the portions of the conglomerate showing the 

 maximum of coarseness and irregularity in texture are those 

 in immediate contact with the underlying crystallines. Here 

 the conglomerate is sometimes a genuine breccia, and usually 

 holds fragments or boulders of large size ; masses one to two 

 1 Proo. Boat. Soc. Nat. Hist., xtii., 411. 



