406 On American Geological History, 



as recognized by Messrs. Foster and Whitney and Prof. Hall, and 

 the geologists of Canada.* 



The Silurian or Molluscan AgQ next opens. The lowest rock is 

 a sandstone, one of the most widely spread rocks of the continent, 

 stretching from New England and Canada south and west, and 

 reaching beyond the Mississippi, — how far is not knov/n. And 

 this first leaf in the record of life is like a title page to the whole 

 volume, long afterwards completed ; for the nature of the history 

 is here declared in a few comprehensive enunciations. 



1. The rock, from its thin, even layers, and very great extent, 

 shows the wide action of the ocean in distributing and working^ 

 over the sands of which it was made; and the ocean ever after- 

 ward was the most active agency in rock-making. 



2. Moreovei, ripple marks, such as are made on our present sea- 

 shores or in shallow waters, abound in the rock, both through the 

 east and west, and there are other evidences also of moderate depths, 

 and of emerged land.f They all announce the wonderful fact, that 

 even then, in that early day, when life first began to light up the 

 globe, the continent had its existence, — not in embryo, but of full- 

 grown extent ; and the whole future record is but a working upon 



* The Azoic lauds, above the ocean at this time, recognized by Messrs, 

 Foster and Whitney in the Report referred to, were that of the Azoic re- 

 gion, between Lnke Superior and Hudson's Bay, that between Lake Supe- 

 rior and Lake Michigan, the Azoic Island of Northern N'ew York ; and the 

 facts they slate would add the Missouri iron-mountain region, and the meta- 

 morphic region of Arkansas as possibly other islands. Mr. Whitney has 

 more recently shown that the occurrence of great masses of specular or mag- 

 netic iron is proof that the metamorphic rocks containing them are of the- 

 Azoic age or pree-Silurian. 



On the Geological map of northern N'orth America, published by Mr. Is- 

 bister in the Quarterly Journal of the Geological Society for 1855, xi, 497, 

 the Azoic is shown to extend in a narrow band northwestward from Canada 

 to the Arctic sea between Hudson's Bay and the W^innipeg line of small lakes. 



•j- Other marks of shallow water alluded to are wave lines, and the obli- 

 que lamination characterising many subordinate layers in the rock, — the 

 latter due to changing currents, like the ebb and flow of tides, or variations 

 in tidal or other currents, or the occasional actions of storm waves. This ob- 

 lique lamination as well as ripple marks, occurs abundantly in the Potsdam 

 sandstone of northern IS^ew York (Emmons' Geol. Rep., p. 104, 130) ; in Ca- 

 nada (Logan's Reports, 1851-52, p. 12 and elsewhere) ; south of Lake Supe- 

 ?ior (Foster and Whitney, loc. cit, p. 118); in the Upper Mississippi (Owen, 

 Survey of Wisconsin, Ac, p. 48) ; in Pennsylvania and Virginia (ProfeSvSO's 

 H. D. and W. B. Rogers), 



