572 HISTORICAL GEOLOGY. 



is a remarkable feature of the era. Even in the Laramide Range of southern 

 British America, McConnell found the Upper Silurian series only 1500 feet 

 thick ; and in the Wasatch Range, according to King, the thickness of the 

 whole Silurian is but 1000 feet. The era began, as the Medina rocks show, 

 with shallow waters over central New York, and probably large, emerged 

 areas east of the Mississippi as well as west. In its progress through the 

 Clinton epoch there were still shallow waters and emerging lands ; for the 

 extensive beds of iron ore, ranging far south and west to Wisconsin, are 

 evidence of great seashore flats through long intervals over much of the 

 eastern half of the continent. In the Niagara epoch there were somewhat 

 deeper and purer waters over the Interior Continental Seas, but the areas 

 were not of very wide extent, and the epoch closed through the coming on 

 of another period, the Onondaga, in which again great seashore flats pre- 

 vailed, with feeble submergences or emergences where any occurred. 



The length of this period of great briny flats and salt deposits — which 

 were 100 miles or more long in the state of New York, and twice this to 

 Goderich, on Lake Huron — cannot be estimated ; for thinness of rocks 

 means nothing as regards elapsed time where a region is undergoing no 

 oscillations of level, or only those of extreme slowness. 



The prevailing characteristic of the continent during the early and middle 

 Upper Silurian, that of shallow seas and emerging seashore flats, continued 

 on, with little change, through the closing Lower Helderberg period; for the 

 formations are unknown over the Mississippi basin and farther west, and 

 have their greatest extent along the region of the progressing Appalachian 

 geosyncline, and its temporary prolongation northward through the Hudson 

 and Champlain depressions to Montreal. 



The period of briny flats unfavorable to aquatic life. — Only two species of 

 the Niagara fauna, the widely ranging Leptmna rhoraboidalis and Atrypa 

 reticidaris, are known to occur in the Lower Helderberg beds, although the 

 epoch which intervened was only one of muddy, briny flats. But the remark 

 applies only to eastern North America, for nothing has been ascertained with 

 regard to the Onondaga and Lower Helderberg faunas for the larger part 

 of the continent. 



No upturnings at the close of the Upper Silurian. — The era appears to 

 have passed and ended quietly. It had slow and gentle oscillations in level, 

 like other geological eras, but it was marked with no great upturning in its 

 progress, and with none at its close. The Lower Helderberg formation 

 graduates into that of the opening Devonian, and if transferred to the 

 Devonian, the statement would still hold true. 



The eastern continental border related in life to the European. — In Canada 

 and New England the formations of the Upper Silurian have not yet been 

 so fully distinguished and described that the succession of events for this 

 part of the continental border can be deduced. But the fact that the 

 region was distinct from the Interior Continental region has been well made 

 out from the Upper Silurian fossils, by Salter and Billings, who state the 

 following facts : — 



