PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 515 



(pages 491-501), the occurrence of a particular genus or species may he 

 relied on as establisldng the presence of sediments of the age of which the 

 fossil is thought to be diagnostic only so far as the diastrophic and faunal 

 history of the trough, basin, province or hemispher-e has been determined. 

 The testimony of the fossil, therefore, is unqualifiedly acceptable only 

 within provincial boundaries in which its range has been established. 

 Beyond these boundaries, in some other basin, the same species, or its 

 slightly different ancestor or descendant, may tell a different but no less 

 consistent story. As a theoretic proposition, this principle is generally 

 recognized today, but in practice most paleontologists seem inclined to 

 forget it until the facts have become indisputable. We have, for example, 

 passed the necessity of proving that LeiorhyncJius, a middle and upper 

 Devonian genus in New York, is found only in much younger rocks iu 

 Arkansas and Nevada. But, if I am right, there are many similar and 

 equally notable instances before us, some of them already suggested, 

 others as yet unsuspected. Hence, a species and sometimes even an 

 association of species may be highly diagnostic of a certain bed or for- 

 mation, and, therefore, of its age, in one basin, and of a different age 

 in another basin or province. This is true more particularly of long- 

 lived species of cosmopolitan habitat which invaded one or another of 

 the continental seas when conditions favored such invasions. NiduUtes 

 favus is a good example. This species invaded the southern Appalachians, 

 middle Tennessee and central Kentucky areas from the south during 

 middle Stones Eiver time and is diagnostic of this age in those areas. 

 In the Massanutten-Chambersburg basin in the middle Appalachian 

 region a scarcely distinguishable descendant of the same species invaded 

 from the 'north middle Atlantic and is there diagnostic of the much later 

 Nidulites bed of the Chambersburg formation. The occurrence, men- 

 tioned in a preceding paragraph, of very slightly modified descendants 

 of Champlain Valley Chazy species in a later, lower Mohawkian, zone 

 at Lexington and other localities in Virginia, affords an illustration of 

 this principle as applied to an association of species in different basins 

 of the same province. Better known examples are the recurrences of 

 the Hamilton fauna in the Portage and Chemung formations in New 

 York, Pennsylvania, and Maryland, and of the Spergen fauna in the 

 Tennessean and early Pennsylvanian in the Mississippi Valley. The 

 occurrence of nearly identical forms of Leio7-hynchus in the middle De- 

 vonian in New York, in basal Waverlyan in Missouri, and in Tennessean 

 deposits in Arkansas, Nevada and California illustrates the principle 

 as applied to distinct provinces. 



