BTRATIGRAPHIC classification PALEONTOLOGIC CRITERIA 505 



period. In the American Ordovician its place is rather generally taken 

 by the related genus Isotelus, which, on the other hand, is rare in the 

 Baltic region. Throughout the Eopaleozoic the development of the 

 trilobites in central Europe was distinci from that going on in the Baltic 

 region. In fact, the differences between the trilobites of these two regions 

 is greater than is found in comparing the species in either with those 

 occurring in the eastern American provinces. In later Paleozoic ages 

 the trilobite genera seem to have become more cosmopolitan, and hence 

 less distinct in regional development. 



The ostracoda are met with for the first time in the Canadian lime- 

 stone formations in N'orth America. The oldest types are typical Leperdi- 

 tiidsB. As they occur in Nevada and in the Champlain and Ottawa 

 basins as well as in Tennessee, Arkansas, and Oklahoma, the locus of 

 their origin is somewhat uncertain. However, as the southern occurrences 

 seem to be older than the others, I am inclined to place it in the Gulf of 

 Mexico and Caribbean basins rather than in the Pacific or the Arctic. 

 The smaller ostracods, which began rather late in the Canadian, attained 

 a high state of development in the Ordovician and maintained their im- 

 portance to the present time, are almost world-wide in distribution. Taken 

 as a whole, this group of crustaceans seems to have enjoyed unusual facili- 

 ties for dispersal. Of course, some localization in development occurred 

 during the Paleozoic, especially in the Silurian period. But most of these 

 geographically restricted types constitute comparatively small genera. 

 Among large genera the case of Beyrichia is perhaps the most noteworthy. 

 This genus is extraordinarily developed in the Silurian rocks of Sweden 

 and England, but is rather rare elsewhere. A few species are found in 

 Maine and the Appalachian Valley and one in the Waldron shale in 

 Indiana. Another case is Kloedenella, which is extremely prolific in the 

 Appalachian Valley, but rather rare elsewhere. 



Principles of stratigraphic Correlations 



GENERAL DISCUSSION OF THE PRINCIPLES 



The principles or metliods of stratigraphic correlation may be divided 

 into two unequal groups: (1) those showing positive relations, and (2) 

 those implying deductive reasoning. Under the first head comes 



(a) Superposition. — The order in which the strata were laid down is 

 the very foundation of geologic chronology. As determined in undis- 

 turbed regions, it affords positive, though more or less incomplete, evidence 



XXXIV— Bull. Geol. Soc. Am., Vol. 22, 1910. 



