228 PALAEOZOIC TIME — LOWER SILURIAN. 



The Quebec group (Calciferous epoch) and the beds of the Trenton and Hudson 

 periods are represented as having been originally laid down in conformable 

 strata, and as having been involved together in the folding and faulting here 

 illustrated.* 



In Pennsylvania, also, according to H. D. Rogers, the Upper Silurian beds of 

 the Kittatinny Mountain lie unconformably on the upturned Lower Silurian. 



In Tennessee, according to J. M. Safford, there is an area some eighty miles 

 in diameter, about the centre of the State, which was probably raised above the 

 ocean at this same epoch ; for, first, the next bed of rock covering it is a shale 

 of the Upper Devonian, the Upper Silurian and all the Lower Devonian being 

 absent; and, secondly, the absent rocks, where they appear around the area, 

 thin out towards it, while there is no evidence that denudation was the cause. 



In Ohio, about Cincinnati, there is a large region where the Lower Silurian 

 strata are surface-rocks, owing to an uplift ; but it is not yet known whether this 

 exposure was a consequence of a disturbance immediately after the formation 

 of the beds, or whether there were subsequent Silurian and Devonian beds Avhich 

 have been removed by denudation. 



Besides these changes, there was also a general increase of dry- 

 land along the" northern border of the United States and through 

 Canada. The broad band of Hudson River shales shown on the 

 map (p. 170) must have been left to a great extent exposed by an 

 uplift ; for the next deposits do not cover it, and it is not probable 

 that they ever did. The dry land of the continent was gradually 

 expanding southward, and at the commencement of the Lower 

 Silurian its outline in New York lay to a considerable distance 

 south of the Mohawk River. 



Moreover, the great St. Lawrence gulf about Ottawa, where the 

 Trenton and Hudson formations had been accumulated, was pro- 

 bably nearly obliterated at this time ; for no rocks of more recent 

 date occur there, to prove the presence of the sea, until the Post- 

 tertiary period, just before the Age of Man, excepting the small 

 patch of Lower Helderberg near Montreal. This region of dry 

 land spread eastward from Montreal to the Appalachian region 

 in Vermont, which region also was probably above the water-level. 

 Thus, the St. Lawrence channel, which was first a short strait 

 between the Azoic areas of Canada and New York, had become 

 much narrowed and lengthened by the close of the Lower Silu- 

 rian; but it still opened into a broad oceanic basin near the 

 longitude of Quebec ; for both Upper Silurian and Devonian strata 

 were formed over eastern Canada and a large part of New 



* These facts and citations are from the paper of Sir William Logan, pub- 

 lished in the Canadian Naturalist and Geologist, 1861, and also the Amer. Jour. 

 Sci. [2], xxxiii. 



