191 



I have often wondered if some of the very numerous quartzite 

 pebbles in the conglomerate might not some day afford Scolithus, 

 or Lingula, as have the well-known pebbles of a very similar 

 rock in the Carboniferous conglomerate at Fall River. Of 

 course one bit of evidence of that nature clearly established would 

 invalidate all conclusions inconsistent with the post-Primordial 

 origin of the conglomei'ate, or of a certain part of it at least. 

 Pebbles of conglomerate, too, are known to occur in the con- 

 glomerate, and from these Mr. Dodge^ has inferred the existence 

 of a more ancient formation of that rock. Unlike the slaty 

 fragments, these conglomerate pebbles are rather rare ; and all 

 that I have seen were undoubtedly derived from the Shawmut 

 breccia. I have failed to find that tliis class of pebbles is 

 peculiar to any part or area of the conglomerate. Their rarity 

 is very surprising, considering the abundance of the breccia and 

 the generous manner in which the other member of the Shaw- 

 mut group has contributed to the formation of the conglomerate. 



No one, so far as I am aware, has ever recognized in either 

 the slate or the conglomerate lithological peculiarities which were 

 so localized stratigraphically or geographically as to afford a 

 proper basis for, or even suggest, a chronological division. 

 The most general distinctions are those resulting from the 

 gradual passage already noted, from the base of the conglom- 

 erate upward ; but here the evident gradation destroys the in- 

 ference. The slate, especially, is a very variable rock. The 

 principal colors are black, bluish-black, gray, greenish, brown, 

 and brownish-red. It may be very thin-bedded and beauti- 

 fully laminated, or so massive as to appear unstratified. It 

 may possess a well-developed cleavage or joint structure, or both 

 these may be almost entirely wanting. But none of these dif- 

 ferences admit of correlation with the distribution. Locally 

 this rock is subject to great variation, but to the general view 

 it is decidedly homogeneous. Perhaps the greatest general 

 contrast is between the slate of Somerville and Cambridge, and 

 that of the South Shore district ; and yet most observers have 



iProo, Bost. Soc. Nat. Hist., xvii., 409. 



