PALEOZOIC TIME — UPPER SILUKIAN. 671 



still gave no passage to coarse sediments; for the rocks formed over the 

 channel were mainly limestone. 



So again, over the continental border from New York to Georgia, since 

 Upper Silurian rocks are unknown along the border region, no sediments were 

 supplied to the interior sea across this border from the Atlantic. Upper 

 Silurian beds may exist there beneath the Cretaceous or Tertiary formations, 

 or in the sea bottom outside ; but if so, the broad region of Archaean making 

 the protaxis, without Upper Silurian beds, lies between. The Continental 

 Interior received no Atlantic sediments. 



It has further been shown that the Upper Silurian formations of Kew 

 England and eastern Canada and Newfoundland were in general made, not 

 on the borders of the open ocean, if so at all, but within the limits of 

 channels or bays bounded by Archaean ridges, or ridges of Archaean and 

 Lower Silurian rocks. Of Pacific sea-border beds belonging to the Upper 

 Silurian nothing has come to light. In the Arctic regions the rocks occupy 

 a large basin or area, quite distinct from that of the Continental Interior. 

 Its limits are unknown. 



Influence of the Cincinnati geanticline. — The influence on the eastern 

 interior sea of this barrier of emerged land and shallow seas was strongly 

 marked. Owing to changes in level that were in progress, shifting the areas 

 of deepest water, large changes were made from time to time in the courses 

 of the tidal movements, in the character of the depositions, the clearness or 

 foulness of the water, and accordingly in the character of the life. With 

 clear, deep waters, life of great variety abounded and limestones were 

 formed ; but with sediment-laden waters, or waters half freshened by contri- 

 butions from the land, the living species were only those that could survive 

 under such adverse circumstances. 



Abrupt variations in the rocks and the life become thus intelligible. 

 It is hence easy to understand that a Niagara epoch might be followed, 

 through a wide shallowing of the seas, by a region of immense salt-pans 

 (evaporating sea-border flats) over a large part of New York, making 

 the Salina group of rocks, while to the eastward, southward, and west- 

 ward a tide-washed region existed, — that of the Water-lime group, — free 

 from saline deposits because the tides had access ; and that fresh-water 

 and brackish-water flats, containing species of Eurypterids, might well 

 have been a feature, at the same time, of the sea borders. Then the occur- 

 rence of a slight subsidence would account for clearer seas again, for a 

 restored fauna, and the making of Lower Helderberg limestones, and also 

 for the extension of the limestones over eastern New York to Montreal in 

 the St. Lawrence Channel, and southward over western New Jersey and 

 part of Pennsylvania. Such salt-evaporating basins are due to local condi- 

 tions and cannot be a universal feature of a period. 



Large shallow-water and emerged areas over the continent characteristic of 

 the Upper Silurian era. — The absence of Upper Silurian formations from 

 much of the region west of the Mississippi, and their thinness where present, 



