164 



everywhere seen to repose unconformably upon the upturned or 

 extravasated Huronian terranes ; and in the case of the breccia, 

 at least, their derivation from the Huronian series is beyond 

 question. Looking in the opposite direction we find the Shaw- 

 mut group, in its turn, overlaid by the Primordial strata of 

 Eastern Massachusetts and contributing very largely to the 

 formation of the conglomerates of that age. In fine, the Shaw- 

 mut group is newer than the Hi^ronian and older than the 

 Primordial ; and that it belongs, in time, nearer the latter than 

 the former we have abundant evidence. It has never been ob- 

 served in contact with Montalban rocks, but I think no doubt 

 can rest upon the view that it is newer than that great and 

 truly crystalline series ; for it is cleaj, as already stated, that 

 the Gulf of Maine was eroded mainly, if not entirely, after the 

 deposition of the rocks of Montalban age, and of course the 

 erosion of this gulf must have preceded the formation of rocks 

 now resting unconformably upon its floor, as do both the Shaw- 

 mut and Primordial groups. The very general absence in the 

 Shawmut group, as will presently appear, of crystalline char- 

 acters, is, too, a strong indication that it post-dates the Huronian 

 and Montalban systems. We have come, apparently, to a zone 

 of debatable ground between those itwo grandest divisions known 

 among the geological formations, — the crystallines and the 

 uncrystallines. 



Subsequently to the erosion of the Gulf of Maine, and after 

 the sea, which stood at a higher level, relatively, than now, 

 feeling its way between the Natick and Cape Ann granite range 

 on the north, and the granite of Cohasset, Hingham, and 

 Quincy on the south, had excavated Boston Harbor with a 

 maximum breadth of nearly twenty miles, and with its head in 

 South Natick or farther west, and at a time when the similarly 

 formed but much smaller basin of the River Parker in Newbury 

 was open to the sea, were deposited the breccias and slaty 

 rocks of the Shawmut group ; the last-named rocks becoming 

 by subsequent alteration the amygdaloid of the present day. 

 According to the evidence afforded by this region these were the 



