194 THE JOURNAL OF GEOLOGY. 



One of the fullest elaborations of this biological classification 

 of the geological series to form a time-scale is found in Dana's 

 Manual of Geology. (Manuel of Geology ; treating of the prin- 

 ciples of the science with special reference to American Geologi- 

 cal History, by James D. Dana, 3d ed., New York, 1880.) Here 

 we find the larger divisions called times : I, Archean ; H, Palae- 

 ozoic ; HI, Mesozoic and IV, Cenozoic times. The Palaeozoic 

 time is classified into ages, viz : The age of Invertebrates, the 

 Cambrian and Silurian ; the age of Fishes, the Devonian ; the 

 age of Coal Plants, the Carboniferous, The Mezozoic is called 

 the age of Reptiles. The Cenozoic time includes the age of 

 mammals and the age of man. 



Each of the ages is subdivided into periods and epochs, in 

 which the stratigraphical groups and formations form the basis, 

 and the particular faunas and floras of each constitute the data 

 of determination for the time divisions. Thus the Devonian 

 age includes the following: 



Periods. 

 Catskill ^ 



Ch^"^""g V^ Devonian Age; 

 Hamilton 



Corniferous ^ 

 and, as an example, the Corniferous Period includes the follow- 

 ing epochs : 



Corniferous \ 



Schoharie V = Corniferous Period. 



Cauda -galli ) 



The distinctions upon which these subdivisions are made are 

 primarily stratigraphical, and we have still to seek a time-classi- 

 fication on a purely biological basis for the whole geological 

 series. 



One of the earliest attempts at systematic classification upon a 

 purely biological basis, was made by Dr. Oppel in classifying the 

 Jurassic formations on the basis of the successive Ammonites 

 characterizing the beds. (A. Oppel, Die Jiiraformation, Eng- 



