28 GEOLOGICAL BIOLOGY. 



The distinctions upon which the above divisions are based 

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

 classification on a purely biological basis for the whole geo- 

 logical series. 



Biological Classification of Oppel. — One of the earliest at- 

 tempts 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 charac- 

 terizing the beds.'^ Oppel divided the lower part of the 

 Jurassic system (the Lias) into 14 zones or beds, characterized 

 successively from below upwards by their dominant fossil 

 forms, chiefly ammonites. 



Thus the successive zones were those of: i, Aiinuonitcs 

 planorbis ; 2, A. angulatus ; ^, A. Bucklandi ; 4, Pcntacrimis 

 tubercnlatus ; $, A. obtusiis ; 6, A. oxynotiis ; "J, A. raricos- 

 tatiis ; 8, y^. annatiis ; 9, A. Javicsoni ; \0^ A. ibex ; \\, A. 

 DavcFi ; 12, A. margaritatiis ; 13, A. spinatus ; 14, Posido- 

 nomya Bronnii. Later classifications, elaborations or re- 

 visions of Oppel's system, have been made by Wright, in 

 i860; Judd, 1875 ; Tate and Blake, 1876, etc. This method 

 of classification recognized the principle of temporary con- 

 tinuance of species and of associated faunas ' and it has been 

 applied with greater or less success all through the geological 

 scale of formations for the definition of the lesser divisions. 



As early as 1838 the importance of the biological evi- 

 dence in determining the time-scale was clearly enunciated 

 by Murchison, who wrote in the introduction to the Silurian 

 System, " that the zoological contents of rocks, when coupled 

 with their order of superposition, are the only safe criteria of 

 their age.'' f 



Geological Terranes and Time-periods Contrasted. — The making 

 of the geological time-scale has now been traced far enough 

 to clearly demonstrate the fact that the ordinary classification 

 of geological formations, as found in our text-books, includes 

 two distinct series of facts: {\) geological terranes, arranged 

 stratigraphically and classified by their positions relative to 



* A. Oppel, " Die Juraformation, Englands, Frankreichs und des siidwest- 

 lichen Deutschlands " (1856-1858). 

 f " The Silurian System." p. g. 



