/. D. Dana on American Geological History. 317 



ceedingly profuse, was three or four times destroyed and re- 

 newed : — not renewed by a re-creation of the same species, but 

 by others ; and although mostly like the earlier in genera, yet 

 each having characteristic marks of the period to which it be- 

 longed. And while these Devonian Periods were passing, the 

 first land plants appeared, foretellers of the age of verdure, 

 next to follow. 



Then come vast beds of conglomerate, a natural opening of a 

 new chapter in the record ; and here it is convenient to place 

 the beginning of the Carboniferous Age, or the Age of Acro- 

 gens. Sandstones and shales succeeded, reaching a thickness in 

 Pennsylvania and New Jersey, according to the Professors Kog- 

 ers, of thousands of feet ; while in the basin of the Ohio and Mis- 

 sissippi, in the course of this era, the Subcarboniferous limestone 

 was forming from immense Crinoidal plantations in the seas.* 



Another extermination took place of all the beautiful life of 

 the waters, and a conglomerate or sandstone was spread over the 

 encrinital bed : and this introduced the true coal period of the 

 Carboniferous Age ; — for it ended in leaving the continent, which 

 had been in long-continued oscillations, quite emerged. Over 

 the regions where encrinites were blooming, stretch out vast 

 wet prairies or marshes of the luxuriant coal vegetation. The 

 old system of oscillations of the surface still continues, and 

 many times the continent sinks to rise again, — in the sinking, 

 extinguishing all continental life, and exposing the surface to 

 new depositions of sandstone, clays, or limestone, over the accu- 

 mulated vegetable remains ; in the rise, depopulating the seas by 

 drying them up, and preparing the soil for verdure again ; or at 

 times, convulsive movements of the crust carrying the seas over 

 the land, leaving destruction behind. And thus, by repeated alter- 

 nations, the coal period passes, some six thousand feet of rock 

 and coal-beds being formed in Pennsylvania, and fourteen thou- 

 sand feet in Nova Scotia. 



I have passed on in rapid review, in order to draw attention 

 to the series or succession of changes, instead of details, f So 

 brief an outline may lead a mind not familiar with the subject 

 to regard the elapsed time as short; whereas to one who follows 

 out the various alternations and the whole order of events, the 

 idea of time immeasurable becomes almost oppressive. 



* This Subcarboniferous limestone is sparingly represented in Pennsylvania among 

 the sandstones and shales; but according to Prof. W. B. Rogers it increases to the 

 southward, and in Virginia acquires a thickness of 1500 to 2000 feet. 



f The names given to the subdivisions of the Palaeozoic rocks are the same that 

 have been laid down by the New York Geologists, whose assiduous and successful 

 labors in a territory of so great geological importance, entitle them to pronounce 

 upon the nomenclature of American Rocks. I have varied from the ordinary use of 

 the terms only in applying them to the periods and epochs when the rocks were 

 formed, so as to recognize thereby the historical bearing of geological facts. The 



