THE GEOLOGICAL TIME- SCALE. 1 95 



lands, Frankreichs und des sildwestlichen Deutschlands, 185 6- 1858). 

 Oppel divided the lower part of the Jurassic system (the Lias) 

 into 14 zones cfr beds; characterized successively from below 

 upwards by their dominant fossil forms, chiefly ammonites. 



Thus the successive zones were those of: i, Ammonites 

 planorbis ; 2 , A. angulatus ; '^i, A. Bucklandi ; 4^ Pentacrinus tuber- 

 cidatus ; 5, A. obtiisus ; 6, A. oxy?iotus ; J, A. raricostatus ; 8, A. 

 annahis ; 9, A.Jamesoni ; \o, A. ibex ; 11, A. Davcei ; 12, A. marg-a- 

 ritatus ; 13, ^. spinatus ; i^, Posidonomya Bronnii. Later classifi- 

 cations, elaborations or revisions 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 continuance 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 evidence 

 in determining the time -scale was clearly enunciated by Mur- 

 chison, 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." 

 (The Silurian System, p. 9). 



The making of the geological time -scale has now progressed 

 to the stage when it is pretty clearly seen that the ordinary 

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

 includes two distinct series of facts : ( i ) geological terranes, 

 arranged stratigraphically and classified by their positions rela- 

 tive to each other and by their lithological characters; and (2) 

 chronological time -periods, which may be locally marked by the 

 stratigraphical division planes, but which depend, fundamentally, 

 upon biological evidence for their interpretation and classifica- 

 tion. Gilbert^ has concisely expressed the important fact of the 

 purely local nature of the division -planes separating the forma- 

 tions stratigraphically into stages, series, systems or groups in 



' Gilbert, G. K. The work of the International Congress of Geologists. Proc. 

 Am. Ass. Adv. Sc, August, 1887. Vol. xxxvi., p. 191. 



