268 PALAEOZOIC TIME — DEVONIAN AGE. 



III. General Observations. 



The Oriskany sandstone is another of the arenaceous rocks ranging 

 from central New York to the southwest along the Appalachian 

 region, and thus serving to define the old Appalachian sand-reef. 

 This it does not only to the south, but also to the northeast at 

 Gaspe, As in other cases, the rock thickens on going from New 

 York to the southward, showing less depth of water and less change 

 of level, or subsidence towards the Azoic, than in the opposite 

 direction. The limits of the formation in the State of New York, 

 and its fossils also, seem to point to the existence at this epoch of a 

 sheltered bay opening to the southeast, — such a New York bay as 

 might have existed if the Green Mountain region (as before in the 

 Upper Silurian era) were raised a dozen feet or more out of water, 

 and if also the Azoic of northern New Jersey (see p. 137), the proper 

 continuation of the Green Mountains, were an island or reef in the 

 sea. The muddy and sandy bottom of the bay would have given 

 the shells a fit place for growth. To the south, as the fossils in 

 Maryland and beyond show, the accumulations were those of an 

 open bay or coast, where there were at least purer waters. 



The thickness of the formation along the Appalachian region 

 indicates a continuation of the series of subsidences that began far 

 back in the Silurian or before. We may hence conclude that the 

 Green Mountain region was a narrow island lying between seas 

 covering more or less of New England and New York, and bounded 

 by the St. Lawrence channel on the north ; for there is no reason 

 to doubt that Devonian as well as Carboniferous strata occur among 

 the now crystalline rocks of New England. The region of Appa- 

 lachian subsidence, instead of including the Green Mountains, as 

 in the early Lower Silurian era, extended northward, in the direct 

 line of the Alleghanies, over the southern half of central New 

 York, as in parts of the Upper Silurian ; for this is indicated by 

 the position of the sandstone. 



The Oriskany period, taking into view the whole range of its life, is more 

 closely related, as Hall states, to the last period of the Silurian than to the 

 following Devonian ; hut in its more common Brachiopods it has rather a De- 

 vonian character. It was fixed upon as the heginning of the American Devonian 

 hy the eminent French geologist M. de Verneuil. There is, however, a more 

 complete change in the American fauna after the Oriskany period than before 

 it : for this reason, and on account of the relations of its fossils to those of the 

 Lower Helderberg, Hall suggests the query whether the Devonian age would 

 not more properly commence with the next or Corniferous period. 



