On American Geological History, 399 



Reptiles, — a space in time zoologically occupied by the overlap- 

 ping- of these two ages.* 



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

 AcROGENs or Coal plants, of Reptiles, 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 reahty 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 Reptilian 

 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 Reptilian, at this time 

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

 est 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 Verneuil ; and was first recognized in its full importance, 

 and formally announced in this country, in the Geological Report 

 of Messrs. Foster and Whitney, although previously admitted in 

 an indefinite way by most Geologists. f 



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

 of the lowest granite cannot be excluded. 



* This'Age would perhaps be more correctly styled the .^ge of Conifers, 

 as Couifers, a higher group than Acrogens, were among the earliest of all 

 land plants, occutiug in the upper Devonian as well as Carboniferous ; and 

 the ages in other cases are named from the superior group of species. Yet 

 as the Acrogens were especially characteristic of the era, and the Conifers 

 have their fullest development in the present age, the name above given 

 seems to be preferable ; unless it prove true that the Sigillariaa and 

 Calamites are actually related to the Coniferte as urged by Brongniart. 

 Zoologically, the age has some title to the name, ^gc of Amphibians. But 

 before it closed, true reptiles had appeared. It is a significant fact that 

 the Amphibians in some cases appear to have approached true reptiles, as 

 much as some of the genera of Acrogens the Conifers. An intei^esting 

 example of this, from tlie coal formation of Ohio, has recently been men- 

 tioned by Dr. J. Wyrnan, (Tenth Meeting Amer, Assoc, at Albany.) 



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 General Geology. Senate Executive Document, JSTo. 4, 

 Special Session, March, 1851. Ordered to be printed, March 13, 1851. 

 406 pp. Svo, with many plates, and a large geological map and section. 



