266 



and riiniiliani, which are so largely composed of the debris of 

 these eniiitives ;iiid are seen in several sections to rest directly 

 upon tlicni, nnist represent an horizon above the Paradoxides 

 beds. The conglomerate series is overlain conformably by the 

 great slate series of Hingham, with some interstratification or 

 blending of the two series. We are thns obliged to recognize 

 in the Boston l)asin a thousand feet or more of argillaceous 

 strata above the Paradoxides or Middle Cambrian zone and 

 separated from it by a corresponding or greater thickness of 

 coarser sediments and lavas — the conglomerate series — with a 

 probable unconformity at the base of the latter. This uncon- 

 formity between the Paradoxides beds and the conglomerate 

 series is proved, not alone by the extensive erosion of the 

 o-ranitic rocks, but we also find in the cono-lomerate, at Pluit's 

 Cove and elsewhere, pebbles of slate similar to that of the 

 Paradoxides beds. Of special interest in this connection, as 

 already explained, are the pebbles of limestone in the conglom- 

 erate of Huit's Cove. Limestone is a rare rock in eastern 

 Massachusetts ; and the only beds now known that can be 

 regarded as a probalile source of these pebbles are the limited 

 and impure layers in the Cambrian slates of Nahant and 

 Weymouth, and, possibly of Stoneham and other points outside 

 of the Boston Basin. 



It is apparent from the foregoing that, although we may 

 fairly regard the stratified rocks of Hingham as forming one 

 conformable series from the lowest conglomerate to the highest 

 slate, and this series, which is quite certainly 2,000 and 

 probably, including the Nantasket beds, 3,000 feet in thickness, 

 is newer than the Middle Camln-ian beds and sejjarated from 

 them by an important unconformity, we are still wholly at sea 

 as regards the precise horizon of the Hingham and Nantasket 

 strata. We might, consistently with the facts so far examined, 

 refer them to any horizon between the Middle Cambrian and 

 the top of the Carboniferous. It remains to be noted, however, 

 that but for the contemporaneous lavas and the intersecting 



