232 



PALEOZOIC TIME. 



level was slow, from the fact that the change in the coast-line in New 

 York, from central New York to the Hudson, demanded the whole 

 of the Medina and Clinton epochs. This change, moreover, was the 

 beginning of a submergence of the east as well as the west side of 

 the Hudson River valley, which continued through the Lower Helder- 

 berg period. 



At the same time that the sea of the Niagara epoch spread over 

 New York and the Interior basin, there was another sea of no small 

 area, over the Eastern Border region, covering the Gulf of St. Law- 

 rence and part of the country south of the St. Lawrence region, — 

 the exact extent not yet ascertained. In the course of these oscilla- 

 tions, from the beginning of the Trenton to the close of the Niagara 

 period, over 12,000 feet of rock were deposited along the Appalachians, 

 indicating a vast subsidence, in slow progress as the accumulations 

 went on. Without the subsidence, great breadth of deposits might 

 have been formed, but not great thickness. The whole change of 

 level over the Interior Continental basin may not have exceeded 

 1,000 feet. 



With regard to the continent beyond the Mississippi, we have small basis for con- 

 clusions. About the Black Hills and the east side of the Laramie Range, the Carbon- 

 iferous strata are stated by Hayden to rest on those of the Lower Silurian, and, there- 

 fore, there is an absence of all the formations of the Upper Silurian and Devonian; 

 but on the east side of the Wind River range Comstock has found some Niagara and 

 Oriskany species. About the El Paso Mountains in New Mexico, between the rivers 

 Pecos and Grande (near lat. 32°), Dr. G. G. Shumard found a limestone of the Trenton 

 or Cincinnati era, containing the fossils Orthis testudinaria, O. occidentalis H., Rhyn- 

 chonella capax Conrad, and others; but to this succeeded the Carboniferous. More 

 investigation is needed to establish the general fact; but if true, as supposed, a part of 

 the region beyond the Mississippi was in no condition for the formation of limestones 

 or sandstones, between the Lower Silurian and the Carboniferous, either because at to« 

 great a depth, or because emerged. 



The Niagara period was, in part at least, one of continental sub- 

 mergence also in Arctic America and Europe. Even Great Britain 

 had its Coral and Crinoidal seas, and thereby its limestone formations 

 in progress, — although the Silurian there contains comparatively 

 little limestone, owing to the fact that the country lies, like the Appa- 

 lachian region, within the mountain-border of a continent. 



2. SALINA PERIOD (6). 

 The Salina is the period of the Onondaga Salt-group, the series of 

 rocks that affords the salt from brines in Central New York. 



I. Rocks: kinds and distribution. 



The Niagara period had covered the sea-bottom in western New 

 York with an extensive formation of limestone. With the opening 

 of the Salina period, there was a change by which shales or marlytes 



