/. D. Dana on American Geological History. 309 



The order then reads, the Age of Molltjsks, of Fishes, of 

 Acrogens or Coal plants, of Eeptiles, of Mammals, of Man. 



The limits of these ages are as distinct as history admits of; 

 their blendings where they join, and the incipient appearance of 

 a type before the age it afterwards characterizes fully opens, are 

 in accordance with principles already explained. 



The reality of progress from lower to higher forms is not 

 more strongly marked in these names, properly applied, than in 

 the rocks. If, hereafter, mammals, reptiles, or fishes, are found a 

 little lower than now known, it will be changing but a sentence 

 in the history,' — not the grand idea which pervades it. 



A theory lately broached by one whose recent death has 

 caused universal grief to science, supposes that the Eeptilian 

 was an age of diminished life, between the two extremes in time, 

 the Palaeozoic and Mammalian Ages. But, in fact, two grand 

 divisions of animals, the Molluscan and Eeptilian, at this time 

 reach their climax and begin their decline, and this is the ear- 

 liest instance of the highest culmination of a grand zoological 

 type. 



Preceding the Silurian or Molluscan Age, there is the Azoic 

 Age, or age without animal life. It was so named by Murchison 

 and De V erneuil ; and was first recognized in its full importance 

 and formally announced in this country, in the Geological Eeport 

 of Messrs. Foster and Whitney, although previously admitted 

 in an indefinite way by most geologists.* 



It embraces all the lowest rocks up to the Silurian, for much 

 of the lowest granite cannot be excluded. 



The actual absence of animal life in the so-called Azoic Age 

 in this country is rendered highly probable, as Foster and Whit- 

 ney show, by the fact that many of the rocks are slates and 

 sandstones, like fossiliferous Silurian rocks, and yet have no 

 fossils ; and moreover, the beds on this continent were uplifted 

 and folded, and to a great extent crystallized on a vast scale, 

 before the first Silurian layers were deposited. A grand revolu- 

 tion is here indicated, apparently the closing event of the early 

 physical history of the globe. f 



* Report on the Geology of the Lake Superior Land District, by J. W. Foster 

 and J. D. Whitney, U. S. Geologists ; Part II, The Iron Regions together with Gen- 

 eral Geology. Senate Executive Document, No. 4, Special Session, March, 1851. 

 Ordered to be printed, March 13, 1851. 406 pp. 8vo, with many plates, and a large 

 geological map and section. 



f Foster and Whitney observe, (loc. cit. pp. 7, 26, 132,) that at Chippewa Island 

 (in the Menomonee river, near 45^° N, 88° W,) the Potsdam sandstone lies on the up- 

 turned Azoic slates. At White Rapids, lower down the stream, the same sandstone 

 rests on the tilted edges of the Azoic quartz rock. Near Presqu' Isle (not far from 

 46° 30'— 46° 35' N, 87° 33° W), a similar contact of the nearly horizontal Potsdam 

 and the vertical quartz rock is seen. 



The Azoic of this continent was well studied and defined at a still earlier date 

 by the distinguished geologist of Canada, Sir William E. Logan. In his Annual 



