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first sediments laid down in this part of the Gulf of Maine, and 

 their occurrence in the minor basins above indicated shows that 

 at that early day the gulf had very nearly its present outline. 

 Following these deposits, after a " lost interval" of unknown, 

 but probably great, duration, came the conglomerates and slates 

 now recognized as belonging to the Primordial horizon ; and 

 the Paleozoic era was fairly ushered in. Extensive denudation 

 and the deposition of more recent sediments have conspired to 

 obliterate the surface exposures of the Shawmut group, and 

 hence this formation is found now only in limited and isolated 

 areas, which lie, for the most part, along the common border of 

 the crystallines and the Primordial system ; the stratigraphic 

 relations of the group being well expressed in the geographic 

 distribution. 



With regard to the mutual relations of the two principal 

 constituents of the Shawmut group — the breccia and the 

 amygdaloid — the evidence is not so clear. I have been un- 

 able to fully satisfy myself as to whether the breccia underlies 

 the amygdaloid, or vice versa. These rocks occupy ai-eas which 

 are for the most part quite distinct, and we have little evidence 

 of their synchronism save what is afforded by their relations to 

 the subjacent and superjacent formations.- They both, as al- 

 ready pointed out, represent a horizon above the Huronian and 

 below the Primordial. At several points, which will be spe- 

 cially noticed in the descriptions that follow, I have observed 

 indications, more or less plain, of a gradual lithologic passage 

 between the breccia and amygdaloid, and this inclines me to 

 the view that they are strictly cotemporaneous deposits ; slate 

 in the deeper water and breccia in the more shallow being 

 formed at the same time. On the other hand I find that the 

 amygdaloid has contributed far more material than the breccia 

 to the formation of the Primordial conglomerate, — a circum- 

 stance which appears to favor the view that the amygdaloid is 

 the newer rock ; and in this case we might regard the breccia 

 as the lower and the amygdaloid as the upper part of the same 

 continuous formation, the lithologic passage supposed to exist 



