THE MAKIXG OF THE GEOLOGICAL TIME-SCALE. 



25 



historical point of view. In 1856* he wrote: "Geology 

 is not simply the science of rocks, for rocks are but incidents 

 in the earth's history, and may or may not have been the 

 same in distant places. It has a more exalted end — even the 

 study of the progress of life from its earliest dawn to the ap- 

 pearance of man ; and instead of saying that fossils are of use 

 to determine rocks, we should rather say that the rocks are 

 of use for the display of the succession of fossils. . . . From 

 the progress of life geological time derives its division into 

 ages, as has been so beautifully exhibited by Agassiz." 



Referring to the nomenclature he used in the classification 

 of American geological history he speaks of having adopted 

 for the subdivisions of the Paleozoic the names given by 

 the New York geologists; but, he adds, "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 nomenclature proposed by Dana in 1856 is given in the 

 following table : 



I, 



Silurian Age. 



1. Lower Silurian. 



I. Potsdam Period. 



2. Trenton Period. - 



3. Hudeon Period. 



2. Upper Silurian. 

 I. Niag-ara Period 



1st epoch. Potsdam sandstone. 

 2d " Calciferous sand- 



rock. 

 Chazy limestone. 

 Birdseye. 

 Black River. 

 Trenton. 

 1st epoch. Utica Shale. 

 2d " Hudson River Shale 

 (Hudson River shale and 

 Blue limestone of Ohio in 

 parts of the West). 



1st epoch 

 2d 



3d 

 1 4th. - 



II. 



( 1st epoch. Oneida conglomer- 

 ( ate, etc. 



2. Onondaga Period . . ist epoch. Gait limestone, etc. 



3. Lower Helderberg Period, etc. 

 Devonian Aee. 



st epoch. Oriskany sand- 

 stone, etc. 



I. Oriskany Period 



■■■{'■■ 



* Americait Jom-nal of Scu-ncc, vol. XXH. pp. 305 and 335. 



