INTRODUCTION. 39 



folding and distortion. A line of ocean bed, at least for two or three 

 hundred miles, and probably much more, which had not been reached 

 by the sediments of the Niagara and Onondaga-salt pei'iods, presented 

 a set of strata more or less inclined, upon Avhich the calcareous sedi- 

 ments of the Lower Helderberg group wei-e deposited*. 



Parallel to and near the line of the strongest current of tlie preceding 

 period, the ocean had now become quiescent ; and instead of the trans- 

 portation of coarse sediments, which had spread out the conglomerates 

 of the Shawangunk mountains and the Blue ridge, we have in the 

 Lower Helderberg group evidences, along the same line, of quiet waters 

 depositing calcareous mud, and marked near its beginning by a broad 

 belt or reef of slow-growing corals, and in its central portions by the finer 

 argillaceous mud which supported myriads of brachiopoda and of fragile 

 gasteropoda, showing the most quiescent condition for a long period of 

 time. 



This epoch of calcareous accumulation was followed by an almost 

 purely arenaceous deposit, which, mingling with the later sediments of 

 the preceding formation, produced, near the junction of the two, a 

 calcareous sandstone of a peculiar character which we find in the 

 Oriskany sandstone. This formation, known as far to the northeast as 

 the region of Gaspe, stretches to the southwest almost coincident to the 

 line of the Lower Helderberg rocks ; spreading continuously very little 



•I tuve, on a former occasion, expressed some doubt regarding the asserted nonconformity of the 

 Lower Helderberg rocks to those below. In Becrafl's mountain, on the east side of the Hudson river, 

 the strata of this age lie inclined above the Hudson-river group, and their appears no positive evidence 

 of their unconformability. On the west side of the Hudson river, the highly inclined strata of the 

 Hudson-river group are succeeded by the Lower Helderberg rocks at a very different inclination; 

 and there are some thin intermediate inconspicuous layers, which, from the presence of some obscure 

 corals, may be referred to the age of the Coralline limestone or to the Niagara group. Farther west, 

 within fifteen miles of the Hudson river, and extending many miles westward, the Lower Helder- 

 berg strata lie above the Hudson-river rocks, as seen in the northern esc.irpincnt of the Helderberg 

 mountains, and the inclination is entirely conformable. Finally, the thinning civstern edge of the 

 Onondaga-salt group comes in between the two formations; and still farther west, the Niagara and 

 Clinton groups intervene before the disappearance of the Lower Helderberg by thinning to the 

 westward. 



