242 GEOLOGY. 



America are reasonably distinct; but it does not follow that there are 

 no species in common. Given a single animal, even an expert might 

 not be able to say that it was from the one continent or from the other, 

 though with certain species even this might be done; but if a large 

 number of animals from either continent were available, it would be 

 possible to determine to which continent, that is, to which geographic 

 fauna, they belonged. So with the several Cambrian faunas. They 

 have some species in common, and the common species do not serve 

 to distinguish them, or the groups of strata which contain them, from 

 one another. But certain species are found only in the Lower, certain 

 other species only in the Middle, and still others only in the upper part 

 of the system, and these species serve to distinguish the principal divi- 

 sions. If but few fossils are present, and these not of decisive species, 

 the several members of the system may not be separable one from 

 another on paleontological grounds. Once the succession of faunas 

 is established — Olenellus below, Paradoxides in the middle, and Dikel- 

 locephalus above — it is customary to refer all rocks which contain the 

 Olenellus fauna to the Lower Cambrian, all which contain the Para- 

 doxides fauna to the Middle Cambrian, and all which contain the Olenus 

 or Dikellocephalus fauna to the Upper Cambrian. 



Sequence of faunas based on stratigraphy. — It is not to be under- 

 stood that rocks which contain such faunas are classed together simply 

 because they contain certain fqssils. This is not the principle, or at 

 least not the only or the fundamental principle, which causes them to 

 be grouped together. The order of sequence of faunas is first determined 

 by the superposition of the strata. The Olenellus fauna could not be 

 known to be older than the Paradoxides fauna, if the beds containing 

 the former were not known to underlie beds containing the latter. It 

 is likewise because of their superposition that the rocks containing the 

 Upper Cambrian fauna are known to be younger than those containing 

 the Middle Cambrian fauna; in other words, the basis for correlation by 

 means of fossils must necessarily be stratigraphic. 



Once the succession of faunas is established, the fossils become a 

 most important aid in correlation, and frequently afford the only basis 

 for it. It should be remembered, however, that the classing of rocks 

 of different regions together under the name of Lower Cambrian means, 

 not primarily that they contain the same fossils, but that they are 

 believed to have been deposited at about the same time, and since the 



