McGee — Southern Extension of Appomattox Formation. 35 



loams, exceptionally sandy over sands, exceptionally argilla- 

 ceous over clays, and exceptionally calcareous over limestones. 



The combined volume of pebbles and gravel, arkose, and 

 the local elements of finely divided material, however, consti- 

 tute but the smaller portion of the entire bulk of the forma- 

 tion ; and the general similarity in composition of the forma- 

 tion with the residuary loams and clays both of the Piedmont 

 crystallines and of the Paleozoics of the Gulf slope suggests 

 that these residua contributed largely to the formation. This 

 suggestion gains strength from the phenomena exhibited at 

 Columbia, where the Appomattox takes on a local aspect cor- 

 responding precisely with the local aspect of the residua de- 

 rived from the neighboring crystalline rocks. 



Interpretation. 



The Appomattox formation illustrates a method of geologic 

 correlation which has grown out of the work in the Coastal 

 Plain of eastern America, and which is deemed worthy of 

 statement. 



The primitive method of geologic correlation depends upon 

 tracing actual stratigraphic continuity across or around inter- 

 vening areas. This method is to-day the simplest and safest 

 within the reach of geologists; but it is practicable only 

 within single geologic provinces, and is limited by many other 

 conditions. 



Another method of geologic correlation is based upon 

 petrography. At certain stages in the development of the 

 science of the earth, the various classes of rocks have been 

 more or less widely correlated upon grounds of similarity in 

 composition, texture, structure and other petrographic char- 

 acters ; but it is now generally recognized that these charac- 

 ters are simply the expression of processes and conditions 

 which have been repeated in many parts of the world and in 

 all periods, and thus that the method can only be applied cau- 

 tiously and within narrow limits. To-day, correlation by 

 petrography is practically confined to the ancient crystalline 

 rocks, and even here it is viewed with distrust by leading 

 American students. 



The disciples of William Smith — who are as numerous to- 

 day as the devotees of geology — correlate groups of rocks by 

 paleontology. It is the strength of this method of correlation 

 that, as practically applied, it embraces the desirable features 

 of the more primitive methods ; that it involves also a broader 

 and more comprehensive grasp of phenomena and principles 

 than the simpler methods out of which it was developed ; that 

 it rests upon a sound philosophic basis ; and that it unites the 



