818 /. D. Dana on American Geological History. 



. Before continuing the review, I will mention some conclusions 

 which are here suggested. 



I. In the first place, through the periods of the Silurian and 

 Devonian, at twelve distinct epochs at least, the seas over this 

 American continent were swept of all or nearly all existing life, 

 and as many times they were repeopled : and this is independent 

 of many partial exterminations and renewals of life that at other 

 times occurred. 



If Omnipotent Power had been limited to making monads for 

 after development into higher forms, many a time would the 

 whole process have been utterly frustrated by hot water, or by 

 mere changes of level in the earth's crust, and creation would 

 have been at the mercy of dead forces. The surface would have 

 required again and again the sowing of monads, and there would 

 have been a total failure of crops after all ; for these extermina- 

 tions continue to occur through all geological time into the Mam- 

 malian Age. 



II. Again: I have observed that the continent of North 

 America has never been the deep ocean's bed, but a region of 



Periods and Epochs thus made out are as follows — excluding minor subdivisions 

 which may make Sub-epochs, and not attempting to give the parallel subdivisions 

 for the West. On this subject, the volumes and papers by Prof. Hall especially 

 should be consulted. 



I. SILURIAN AGE. 



1. Lower Silurian. 



1. Potsdam Period. — 1st Epoch. Potsdam sandstone : 2nd. Calciferous sandrock. 



2. Trenton Period. — 1st Epoch. Chazy limestone; 2nd. Birdseye ; 3d. Black 

 River; 4th. Trenton. 



8. Hudson Period. — 1st Epoch. Utica Shale ; 2nd. Hudson River Shale. (Hud- 

 son River Shale and Blue limestone of Ohio in parts of the west.) 



2. Upper Silurian. 



1. Niagara Period. — 1st Epoch. Oneida Conglomerate ; 2nd. Medina Sandstone ; 

 3d. Clinton Group ; 4th. Niagara Group. 



2. Onondaga Period. — 1st Epoch. Gait limestone ; 2nd. Onondaga Salt Group. 



3. Lower Helderberg Period. — Limestones. (Statement of epochs here omitted). 



II. DEVONIAN AGE. 



1. Oriskant Period. — 1st Epoch. Oriskany Sandstone ; 2nd. Cauda-galli Grit. 



2. Upper Helderberg Period.— 1st Epoch. Schoharie Grit; 2nd. Upper Helder- 

 berg group. 



3. Hamilton Period. — 1st Epoch. Marcellus Shales ; 2nd. Hamilton group ; 3d. 

 Genesee Slate. 



4. Chemung Period. — 1st Epoch. Portage ; 2nd. Chemung group. 



5. Catskill Period.— Catskill Red Sandstones and Shales. (No. IX. of Rogers.) 



III. CARBONIFEROUS AGE. 



1. Suboarboniferous Period. — 1st Epoch, Conglomerates, Sandstones and Shales 

 (with some coal seams) ; 2nd. Sandstones, Shales and Carboniferous limestone. 

 Nos. X. and XI. of Rogers. 



2. Carboniferous Period. — 1st Epoch, Millstone Grit ; 2nd. Lower Coal Meas- 

 ures ; 3d. Upper Coal Measures. Nos. XII. and XIII. of Rogers. 



3. Permian Period. — Probably unrepresented in Eastern North America, except 

 by the events of the Appalachian revolution. 



