450 STRATIGRAPHICAL GEOLOGV. 



Helclerberg series, it is certain the principal part of the Coos series must 

 go with them. 



1. This Heidelberg series in central New England comprises a large part of the 

 common kinds of metamorphic rocks, gneiss, of several varieties, undistinguishable 

 lithologically from the oldest ; hornblende rock and schist ; sienitic gneiss ; coarse 

 mica slate ; staurolitic slate. 



2. A large part of the rocks that have been distinguished as of the "Montalban" or 

 "White Mountain Series" in New Hampshire, and regarded as of pre-Silurian age, are 

 here included, and are hence nothing but altered Helderberg sediments. It is there- 

 fore far from true that " the crystalline rocks of the Green Mountain Series," and " the 

 whole of our crystalline schists of eastern North America, are not only pre-Silurian, 

 but pre-Cambrian in age." Hunt. 



3. The passage of quartzite into gneiss is exhibited in different ways. The presence 

 of mica in the quartzite is one of the steps; but at the locality marked q [15] on the 

 map, part of the quartzite is very finely banded with white and gray. The white por- 

 tions are feldspathic, as is proved by their fusibility before the blow-pipe ; while the 

 light gray are cjuartz, and some short darker lines are micaceous. Thus this region, 

 like that of Berkshire, demonstrates that gneiss and quartzite are rocks of the most 

 intimate relations, — as intimate as anud and sand along sea-shore flats. Part of the 

 feldspathic layers widen at intervals into forms an inch or more thick at the middle, 

 but they are generally nearly even. 



4. Hornblende rocks of the purest kind and of great extent are here so intercalated 

 with quartzite and mica slate, and so often graduate into one or the other, that all must 

 have been alike a result of the metamorphism of sedimentary beds. It is not possi- 

 ble that these hornblende rocks, constituting part of a Devonian or Upper Silurian 

 formation, should have come from the metamorphism of beds of a pre-Silurian, chemi- 

 cally-deposited, meerschaum-like hydrous silicate. 



5. The Bernardston, South Vernon, and Northfield beds being of Helderberg age, 

 the Coos group, which is but the northern continuation of the same series, is, if cor- 

 rectly traced out, also Helderberg. Hence, in the era of the Lower Devonian, the 

 Connecticut valley, from the latitude of Bernardston northward, was an arm of the 

 sea, extending down from the great Devonian Gulf of St. Lawrence. Crinoids require 

 the best of ocean water, and thus they had it in central New England. 



6. The Bernardston beds have been supposed to be Lower Devonian or Upper 

 Helderberg, on the ground of Hall's opinion, and also that of Billings's, recently ob- 

 tained. But both of these paleontologists speak of it only as the most probable con- 

 clusion from the fossils thus far found ; — they may belong to the Lower Helderberg or 

 later part of the Upper Silurian. According to Prof. Hitchcock, a slate containing 

 imbedded crystals of mica, just like that of Bernardston, occurs near Lake Memphre- 

 rnagog, where there are fossil corals of unquestionable Lower Devonian types ; and, as 

 the same slate follows the Cocis series through to that region, the lithological resem- 



