314 /. D. Dana on American Geological History. 



are a region in Northern New York, others about and beyond 

 Lake Superior, and a large territory stretching from Labrador 

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

 Hall, and the geologists of Canada.* 



The Silurian or Molluscan Age next opens. The lowest rock 

 is a sandstone, one of the most widely spread rocks of the conti- 

 nent, stretching from New England and Canada south and west, 

 and reaching beyond the Mississippi, — how far is not known. 

 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. Moreover, 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 same basis, and essentially within the 

 same limits. It is true that but little of it was above the sea, 

 but equally true that little of it was at great depths in the 

 ocean. 



3. Again, in the remains of life which appear in the earliest 

 layers of this primal rock, three of the four great branches of 

 the Animal Kingdom are represented,' — Mollusks, Trilobites 

 among Articulates, and Corals and Crinoids among Eadiates, — 

 a sufficient representation of life for a title-page. The New 



* The Azoic lands, above the ocean at this time, recognized by Messrs, Foster and 

 Whitney in the Report referred to, were that of the Azoic region, between Lake 

 Superior and Hudson's Bay, that between Lake Superior and Lake Michigan, the 

 Azoic Island of Northern New York ; and the facts they state would add the Mis- 

 souri iron-mountain region, and the metamorphic region of Arkansas as possibly 

 other islands. Mr. Whitney has more recently shown that the occurrence of great 

 masses of specular or magnetic iron is proof that the metamorphic rocks containing 

 them are of the Azoic age or prse-Silurian. 



On the Geological map of northern North America, published by Mr. Isbister 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 Winnipeg line of small lakes. 



f Other marks of shallow water alluded to are wave lines, and the oblique 

 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 cur- 

 rents, or the occasional action of storm waves. This oblique lamination as well as 

 ripple marks, occurs abundantly in the Potsdam sandstone of northern New York 

 (Emmons's Geol. Rep. p. 104, 130); in Canada (Logan's Reports, 1851-52, p. 12 and 

 elsewhere); south of Lake Superior (Foster and Whitney, loc. cit. p. 118); in the 

 Upper Mississippi (Owen, Survey of Wisconsin, etc., p. 48) ; in Pennsylvania and 

 Virginia (Professors H. D. and W. B. Rogers). 



