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genuine pebble that can be clearly identified with any part of 

 the slate formation now exposed to observation. Many seem- 

 ing slate pebbles are merely very limited, lenticular layers of 

 slate intercalated in the conglomerate. Good examples of 

 such false pebbles resulting from irregular sedimentation 

 occur in North Quincy, one-half mile north-east of Atlantic 

 Station, and on the north side of North Beacon Street, in 

 Brighton. The conglomerate of the last-named locality is well 

 known, and several observers have noted the occurrence in it 

 of the " slate pebbles." A careful study, however, has satisfied 

 me that the pebble appearance is illusory, these masses being 

 indigenous in their present positions, and not imported. In 

 most cases the material seems far too shaly and fragile for the 

 formation of transportable pebbles of the sizes observed ; the 

 largest being a yard or more in diameter, and yet only two or 

 three inches thick. And the rare occurrence of the so-called 

 slate pebbles, except along particular planes in the rock, where 

 they are very numerous , is decidedly a suspicious circumstance ; 

 while peculiarities of form in many cases, as where they envelop 

 pebbles of other material or enclose arenaceous strings which 

 are continuous with the general paste of the conglomerate, 

 seem to complete the proof. 



The conglomerate is in many places largely composed of 

 pebbles of the Shawmut amygdaloid ; and among these there 

 exists, just as maybe observed in the yetuneroded portion of that 

 formation, a perfect series, having as extreme terms an almost 

 entirely unaltered slate and a typical amygdaloid. The pebbles 

 derived from the Shawmut group, however, appear to include a 

 larger proportion of the slaty rock than now exists in the parent 

 ledges, indicating that this older series of rocks experienced a por- 

 tion of its metamorphism after the deposition of the Primordial 

 beds. In many instances, among the pebbles of the conglomerate, 

 partially decomposed petrosilex or felsite may be easily mistaken 

 for slate. I am not prepared to deny positively the possible i 



existence in the conglomerate of genuine " slate pebbles," and 

 yet I must insist that the occurrence of such has not been proved. 



