36 McGee — /Southern Extension of Appomattox Formation. 



physical aspect of geology with the record of biotic develop- 

 ment upon the earth in such manner as to form a ]ogical and 

 consistent basis for the current cosmogony of enlightened men. 

 It is the weakness of the method that many rocks are too poor 

 in fossils to be correlated thereby ; that formations may be 

 homotaxial yet not contemporaneous and vice versa ' that 

 fossil facies represent the product of two principal factors of 

 which one (environment) is so variable under local conditions 

 that the product is inconstant among the minor rock divis- 

 ions ; and that the geologic chronometers afforded by fossil 

 plants, fossil invertebrates and fossil vertebrates respectively 

 give unlike time units and, sometimes, discordant readings. 

 To-day the larger groups are confidently correlated by paleon- 

 tology ; but leading American geologists no longer accept 

 identity of fossil facies as final proof of equivalence among 

 the minor rock divisions. 



The method of correlation devised to systemize the struc- 

 ture of the Coastal Plain combines the desirable features of 

 the older methods, and adds thereto the interpretation of the 

 products of the several physical processes operating upon the 

 earth's exterior. /It is a correlation by community of genesis, 

 or homogeny '. /'The method involves a yet broader conspectus 

 of phenomena and principles than the paleontologic method ; 

 for in its application it is necessary to mentally restore the 

 various ptrysical and biotic conditions of the past, just as pale- 

 ontology vivifies the fossils of past ages. / 



Correlation by homogeny is a simple application of the well 

 known principles (1) that geologic processes may be inferred 

 from their products, and (2) that geologic processes are uni- 

 versally interrelated. 



Since the birth of the science it has been the proximate 

 end of geology to ascertain the'genesis of terrestrial phenomena, 

 with a view to the ultimate end of developing a rational and 

 valid cosmogony ; and great progress has been made in this 

 direction, Now, " scientific progress may be measured by 

 advance in the classification of phenomena. The primitive 

 classification is based on external appearances, and is a classifi- 

 cation by analogies ; a higher classification is based on internal 

 as well as external characters, and is a classification by homol- 

 ogies ; but the ultimate classification expresses the relations of 

 the phenomena classified to all other known phenomena, and is 

 commonly a classification by genesis." So, the primary geo- 

 logic classification was based directly upon the objective phe- 

 nomena of the external earth, and early geologic literature was 

 pervaded and the science shaped by this fundamental idea. 

 As time went on this classification was found too narrow to 



