676 JAMES PERRIN SMITH 



are called "representative species." This is especially true in 

 a time of quiet development, where the fauna is largely endemic, 

 and where there was no chance for outside elements to get into 

 the region. 



Indirect correlation also may be of two kinds ; the first of 

 these is where no fossils of extra-regional distribution are known 

 in a formation, but where the formations above and below can 

 be recognized. An example of this is the classification of the 

 Algonkian system or its equivalents ; the clastic pre-Cambrian, 

 and post-Archean sediments all over the world are placed in 

 this division, although no fauna that is characteristic is known 

 in them as yet. Such correlation can only be tentative or pre- 

 liminary, as is the present classification of the Newark formation 

 of the Atlantic coast. 



The second sort of indirect correlation is where no fossils 

 are common to two separated regions, but elements of both are 

 found together in a third region. A good example of this is 

 the correlation of the Cretaceous strata of the west coast of 

 North America with those of the interior and the Atlantic region. 

 During the greater part of Cretaceous time the two regions were 

 separated by a land mass so that their faunas were totally dis- 

 tinct, not only the species but even the genera being different. 

 And these difficulties are seen in the attempts of the earlier 

 stratigraphers to assign the various formations to their proper 

 places. But when the Indian Cretaceous fauna was described, it 

 was seen at once that there were striking analogies between that 

 and genera and species of California. And since the Indian 

 formation was accurately correlated with the Cretaceous of 

 Europe, it became comparatively easy to assign the Californian 

 strata to their proper place by this indirect comparison with the 

 European standard. The Cretaceous of the Atlantic region had 

 long before this been correlated with European, and thus the 

 formations of the Atlantic region and of the Pacific coast were 

 finally placed in harmony through the medium of comparison 

 with a region thousands of miles from either. 



Pale onto lone zones. — Ever since William Smith demonstrated 



