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 periods, presented 
a set of strata more or less inclined, upon which the calcareous sedi¬ 
ments of the Lower Helderberg group were deposited*. 
Parallel to and near the line of the strongest current of the 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 
*1 have, on a former occasion, expressed some doubt regarding the asserted nonconformity of tike 
Lower Helderberg rocks to those below. In Becraft’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 escarpment of the Helderberg 
mountains, and the inclination is entirely conformable. Finally, the thinning eastern 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. 
