312 Hutit—On the Geological Age of the North Atlantic Ocean. 
from the presence of the sediments of sand, clay, &c., with which those over- 
spreading the regions lying on the borders of the Atlantic Ocean to the westward 
were beclouded. 
(6) North America—Turning now for evidence to the North American continent, 
we find the Lower Silurian beds giving unmistakable evidence of having had their 
origin in a region now submerged beneath the waters of the Atlantic. The base 
of this great group is the Potsdam sandstone,* which has been traced in Canada, 
with an outcrop from the Straits of Belleisle to Bedford, a distance exceeding 
1000 miles. The development and changes in mineral characters of the Potsdam 
group, at the base of the Silurian series of North America, point unmistakably to 
the originating lands as having lain eastwards of the Appalachians. According 
to Professor Dana the laminated sandstones of the State of New York and the 
Canadian borders—only a few hundred feet in thickness—are swollen out by the 
accession of ‘a vast thickness of slate or shale along the Appalachians.” In the 
same way the succeeding calcareous member, called in the Mississippi Valley the 
‘‘Lower Magnesian Limestone,” passes into sandstone, with very thick shales, and 
some beds of limestone, along the course of the Appalachians; and, according to 
the same authority, the beds of ‘the Potsdam group” which, on the borders of 
Western Canada and along the Mississippi Valley, vary in thickness from 30 to 600 
feet, assume along the line of the Appalachians a development estimated from 
2000 to 7000 feet.t 
It will thus be observed that the swelling out in this case is eastward towards the 
Atlantic border, and due to the accession of sedimentary materials in that direc- 
tion. In the opposite direction, where the ocean of the period prevailed, calcareous 
rocks are developed, and the sedimentary deposits thin away proportionately. 
During the ‘‘ Trenton period” nearly the whole of the United States was the 
bed of the sea, over which marine limestones were being formed, while land lay over 
the Canadian area, the eastern border lands having probably retreated far into the 
Atlantic area; but in the succeeding ‘‘ Hudson period” we again have the evidence 
of land towards the Atlantic borders, as the sedimentary strata—which in the dis- 
tricts of Lakes Huron and Iowa are of slight thickness (25 to 100, and 180 feet)— 
swell out to 1200 feet in Pennsylvania, and are of still greater thickness in Vir- 
ginia and Eastern Tennessee} (Plate VIIL., fig. 3). 
It is unnecessary that I should pursue this portion of my subject further. 
From what has been said it will, I hope, be clear that the originating lands of the 
* Logan, ‘ Geol. of Canada,” p. 87; Dana, ‘‘ Man. of Geol.,”’ p. 172. 
+ It is scarcely necessary to remark that the source of this sediment could not have been in the 
Appalachian mountains themselves, which only came into existence after the Carboniferous epoch, as 
shown further on. (See p. 316.) 
| Further west still, as shown by Mr. Clarence King, the Silurian rocks are composed in Wahsatch 
and Nevada almost entirely of limestone, from 1000 to 4000 feet, where the clear waters of the ocean 
prevailed.‘ U. 8. Explor.,” vol. i. p. 248. 
