452 STRATIGRAPHICAL GEOLOGY. 



the most prominent epoch of mountain-making on this eastern border of the continent, 

 after the Lower Silurian. Facts are against any earlier epoch ; and the only other 

 probable suggestion is, that the time was after the Carboniferous age, when the AUe- 

 ghanies were made. Hence the various Helderberg rocks of the Connecticut valley 

 have their actual date, as crystalline or metamorphic rocks, either from the close of the 

 Devonian or that of the Carboniferous era. The conclusions which have been pre- 

 sented are based on a foundation of facts, certainly as far as the region of Bernardston, 

 Vernon, Northfield, and Hinsdale is concerned, where my own observations were 

 made ; and, judging from the descriptions of Prof. Hitchcock, they appear to be sus- 

 tained as regards the Coos range to the north. Aiuer. Jour. Science^ iii, vol. vi, pp. 

 348-351. 



Several other conclusions of a hypothetical character, relating more 

 especially to southern New England, follow the above, but do not need 

 to be quoted here. 



Views of J. P. Lesley. Under date of October 27, 1858, Mr. Lesley wrote me, say- 

 ing he had obtained large crinoids and a possible CyathophyllH7ii from this limestone, 

 which had been sent to Prof. Hall for determination. "My Impression, from the 

 whole aspect of the ground, was, that I was on old and familiar ground, namely, V, 

 Vn, and VIII, with us, — Clinton and Helderberg, — but such impressions are never to 

 be trusted. The limestone may be Onondaga, and have a Cyathophylliim.'''' 



Conclusions. 

 Now that no further field-work is possible, I will state fully what con- 

 clusions seem to be warranted in regard to the order of all the formations 

 in this Helderberg area; how far their age can be determined from the 

 presence of the crinoidal limestone ; and what may be learned by a com- 

 parison of the rocks in the Helderberg and Ammonoosuc areas. 



1. The porphyritic gneiss of Winchester is encircled by strata of the 

 Bethlehem group. Therefore the first is the older of the two, and, as we 

 shall see by further comparison, the oldest formation in the Connecticut 

 valley, as well as in all New Hampshire. 



2. The Bethlehem gneiss in Hinsdale and Vernon is encircled and 

 overlaid by hornblende schist. The same has been observed in five or 

 six other areas, so that the relations between these two groups of rocks 

 in the Connecticut basin may be regarded as thoroughly understood. 

 Hence, whenever the hornblende seems to underlie the gneiss, we know 

 that it has been inverted. 



3. Quartzite encircles the hornblende. Hence, as the relations of the 



