METHODS OF CORRELATION 1137 



has different species of Paradoxides in the different provinces, but 

 these species are representative, so that they serve to correlate even 

 the zones. The zones with their representative or identical spe- 

 cies are as follows : 



Eastern North America. Western Europe. 



Paradoxides forchhamtneri P. forchhammeri 



P. davidis P. davidis 



P. abenacus P. tessini 



P. eteminicus P. rugulosus 



P. lamellatus P. oelandicus 



Even between provinces as close as New Brunswick and east- 

 ern Massachusetts the species are representative rather than iden- 

 tical. Thus Paradoxides harlani, the large Middle Cambric trilo- 

 bite of eastern Massachusetts, is representative of P. eteminicus of 

 New Brunswick, while Acrothele gamagii of the Massachusetts 

 Middle Cambric is the representative of A. matthezvi of New Bruns- 

 wick. The IMeekoceras beds of the Lower Trias of the Himalayas, 

 Siberia, California, Idaho and Utah are readily correlated, though 

 no species common to these regions are known. The genera, how- 

 ever, which characterize them are sufficiently short-lived and the 

 species of the different provinces are closely representative. 

 (Smith-45.) 



5. Correlation by unconformities and disconformities. 



Correlation by unconformities has in the past been extensively 

 employed, and some stratigraphers have advocated the use of un- 

 conformities as a primary basis for correlation. A little reflec- 

 tion, however, will show that such a method, when used indis- 

 criminately, is sure to lead to confusion and false correlation, for 

 it is a well-ascertained fact that folding of formations was not 

 simultaneous in different parts of a region, nor in different regions, 

 but may be earlier in some and later in others. Thus, while there 

 was a rather widespread period of folding in later Palseozoic time, 

 both in Europe and North America, this folding began in the De- 

 vonic in some sections, and not until the Permic in others. More- 

 over, the formation next succeeding the unconformity is by no 

 means always the same one, and grave mistakes have been made 

 by assuming this to be the case. Thus in some cases the Triassic 

 beds rest directly upon the folded Palaeozoics, while in other cases 

 beds of much later age, even Cretacic or Tertiary, succeed them. 



