274 KEY TO TERRESTRIAL HISTORY 



immediate successors, it was only large groups of strata 

 that were thus identified, but with the courage born of 

 success the method began to be applied in greater detail, 

 and much smaller subdivisions were made amenable to 

 the test. It was the gifted Oppel, too early lost to 

 Science, who, so long ago as 1856, first led the w r ay. 

 Confining his studies to the Jurassic system, he subdivided 

 it into a number of smaller groups of beds known as 

 zones, each of which he found to be distinguished by a 

 group of fossils peculiar to itself. Taking Ammonites as 

 his most trustworthy guides, he associated with each zone 

 the name of some species of Ammonite, which occurs in 

 it, but not in the beds above or below, and thus succeeded 

 in establishing in the Jurassic system alone no fewer 

 than thirty-three of such zones. 



Oppel occupied himself in tracing his zones all over 

 Europe, and was at length able to prove that everywhere 

 within this continent they succeed each other in the same 

 order. 



This was in itself a sufficiently noteworthy fact, but 

 further revelations were in store, for Waagen, working on 

 the Geological Survey of India, was able in 1875 to prove 

 that even in the remote region of Cutch similar zones 

 distinguished by similar Ammonites are present, and 

 succeed each other in the same order as in Europe. Thus 

 a considerable number of Oppel's zones were found to 

 continue in unchanged sequence some four thousand miles 

 to the east of our area ; but, again, still later investiga- 

 tions by Steinmann, Tornquist, and Burckhardt have 

 brought to light a similar succession between lat. 32 and 

 39 S. in the Argentine and Chilian Andes of South 

 America, seven or eight thousand miles to the south and 

 west, and thus many of the thin zones of the Jurassic 

 system have been traced in undeviating order over nearly 

 half the circumference of the globe. 



