574 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



of the rocks, this area lies to the east of the usual limits of Devonian 

 deposits in the Appalachian Valley troughs. The second area is in the 

 Maryland basin, in which recognizable Cambrian and Ordovician deposits 

 are found to the east of the Blue Eidge Mountains. Being as a rule 

 badly crushed and more or less metamorphosed, the age relations of some 

 of these rocks have always been somewhat doubtful. Fossils are very 

 rare, but depending chiefly on physical criteria most of the sandstones 

 and shales have been determined, no doubt correctly, as Cambrian. Ee- 

 cently Dr. E. S. Bassler found fossil evidence in the vicinity of Frederick, 

 showing that at least a part of the limestone of that valley is referable to 

 the Chambersburg limestone. Other parts suggest the Beekmantown of 

 the Cumberland Valley. Some of the shales, too, are late Ordovician 

 (Martinsburg) in age and not lower Cambrian, as had been supposed. 



PART III— STRATIGRAPHIC TAXONOMY 



Principles of Classification 

 general discussion 



In the stud}' of geology, the first essential, of course, is some compe- 

 tent and consistent method of classifying geologic events. Hitherto 

 the paleontological method — ^that is, the advent and disappearance of 

 fossil faunas and floras — has seemed to serve the purpose. In later years, 

 however, accumulated observations have tended to show that fossil evi- 

 dence is not so uniformly reliable as had been supposed; that not only 

 single species, but associations of species sometimes occurred at lower 

 horizons, at other times in younger beds, than the one to which they were 

 credited in the standard section. At first it was thought that the interval 

 between these varying occurrences expressed the time consumed in the 

 gradual dispersal or migration of the organisms. Later when three or 

 four appearances of the same fauna were found to occur at long intervals 

 above each other in the same section, paleontologists began to speak of 

 ^'^shifting of faunas." This shifting was supposed to indicate involuntary 

 migration from place to place within broad long-enduring continental 

 seas in response to local change in environment. These observations 

 naturally suggested distinct paleontologic and lithologic classifications 

 and dual nomenclatures, which while checking each other in a general 

 way were necessarily independent in detail. 



All such suggestions are believed to be detrimental to the progress of 



\ stratigraphic science. There is only one method of classifying geologic 



, history and that is the chronologic; and in the determination of the se- 



