I 





12 THE DEVONIAN AND CARBONIFEROUS. [bull. 80. 



bility of the mean tide level of the ocean. The correlations of this period were de- 

 fective, not so much on account of imperfect observation as on account of incorrect 

 theories. " Red sandstone," "Mountain limestone," " Saliferous rocks," and " Grau- 

 wacke" were truly found in America, but they were not the correlatives of forma- 

 tions so named in Europe, because formations present no regularity in the order of 

 sequence of their mineral characters. The perfecting of the New York system of 

 Paleozoic rocks (published in 1842) marks practically the abandonment of the Wer- 

 nerian school of opinion in America. 



The secqnd^jtaga-of development took definite shape in the New York system. 

 Formations were considered as holding a fixed order of sequence, but differences in 

 thickness or even in composition were to some extent allowed as compatible. Still, 

 a general "parallelism of strata" was believed in, and in order to make the interpre- 

 tation fit the facts, " gaps " and " intercalations " were assumed. The application of 

 this principle of correlation is conspicuous in the various attempts at " parallelism" 

 made in the period 1840-1860, and the method is most minutely carried out in the 

 second Geological Survey of Pennsylvania, where the term " persistent parallelism 

 of strata" is named and defined. Fossils were used in these correlations, but rather 

 as arbitrary labels which were of value only when exact identity was recognized. 

 This being rarely the case, fossils played only a secondary part. This principle did 

 not reach satisfactory results, because stratigraphic order and stratification itself 

 offer no intrinsic evidence of the age of the formation, and stratigraphic structure 

 was found not to be uniformly persistent even for a few miles' extent. 



In the first quarter of the century, an Englishman,* William Smith, or "Strata 

 Smith," as he was called, advanced the idea that strata could be identified by their 

 fossils, and fossils have ever since been used with greater or less success in identify- 

 ing formations ; but when the fossils are not of the same but of kindred species, 

 other considerations have been brought forward to establish the correlation. Within 

 the last 20 years fossils have begun to be used on the principle that they contain in 

 themselves intrinsic evidence of their relative age. 



And this brings us to the third stage in the development of the methods of correla- 

 tion in which fossils assume the chief role. Underlying these correlations are the 

 following considerations : Geologic formations in their mineral and lithologic com- 

 position, their stratigraphic and structural characters, and as to their limitations 

 are recognized as strictly local formations; hence the primary principle is that none 

 of these characters can be relied upon for the correlation of formations of different 

 localities. Secondly, fossils are recognized as remains of organisms which possess 

 genetic relationship ; and the specific and varietal characters of the organisms are 

 believed to be indications of these affinities ; and with evolution in time and modifi- 

 cation coordinate with changed condition of environment, the organisms are believed 

 to be extremely sensitive indicators of time relations. Thus the minute and exhaus- 

 tive comparative study of fossils in their stratigraphic and geographic relations is 

 now proving to be not only the best but the only reliable guide to correlation of 

 geologic formations. 



The conclusions reached from this historical study confirm the belief that the de- 

 scription and nomenclature of structural formations should be quite independent of 

 their correlations, and that precision in correlation must be based upon mature and 

 exhaustive paleontologic study, that the time scale must be made independently of 

 the structure scale, and that the time scale of correlation is based fundamentally 

 upon biologic data. 



The investigation leads to the further conclusions that as nomenclature finds its 

 basis in some intrinsic characters of the things named, uniformity of nomenclature for 

 formations is impracticable, since the intrinsic characters of formations are local and 

 have nothing to do with their geologic position; and that uniformity of classification 

 can be looked for only through an exhaustive biologic study of the fossils, and is 

 inapplicable to geological structure, stratigraphy, or formation. 



