260 THE DEVONIAN AND CARBONIFEROUS. [bull. 80. 



the rocks and brought the facts to light the difficulties of exact correla- 

 tion have been as great as the complexity of the facts. 



In the historical development of the geology the northern part or the 

 Appalachian province was first developed; afterward, and by other 

 men, the Mississippiau province was surveyed and interpreted. 



Among the numerous problems which American geologists have had 

 to solve, I have selected a few to show the methods employed in corre- 

 lations and the reasons why one method has led to erroneous and an- 

 other to correct results. The object of correlation is and has been to 

 bring newly discovered formations into their proper places in already 

 established systematic classifications. Hence in studying the princi- 

 ples of correlation it has been necessary to deal mainly with the classi- 

 fications. The original classifications may have been founded on wrong 

 principles, and in such cases, however correct the methods of correla- 

 tion may have been, the results were unsatisfactory. In the first stage 

 of the history this was the case. The Wernerian classification was 

 based on the supposition that the stratigraphic order of deposits and 

 the lithologic composition of the separate members had some natural 

 relation to each other. This is not the fact. It was on this account 

 that all the work of Amos Eaton, in New York State, though based 

 upon careful observation and accurate record of the facts, was a failure 

 so far as the correlations were concerned. After he had perfected the 

 Wernerian system, thoroughly adapted it to our facts, and provided an 

 American translation, so to speak, of the German method, the fallacy 

 of the method was exposed and the whole of his scheme was abandoned. 

 The New York rocks were admirably adapted to the construction of 

 a correct classification of the Paleozoic systems, except for the highest 

 member. For that the adjoining State, Pennsylvania, furnished what 

 New York lacked. For nearly half of the State the dip of the rocks is 

 scarcely greater than 50 feet to the mile, and they are so regular that 

 numerous sections could be easily examined running through the same 

 series of deposits, the local variations noted, and, most important of 

 all, great quantities of fossils were obtained. The result was that the 

 New York rocks for the Silurian and Devonian systems furnished the 

 standard classification for North America, aud after 1843 (the date of 

 the completion of the final reports of the geological survey of the State 

 of New York) whatever imperfections might have been detected were 

 easily corrected by reference to the strata themselves. All mistakes 

 in correlations of these formations thereafter were the fault of the 

 method of correlation, not of the classification used. 



The Carboniferous rocks of Pennsylvania are mainly arenaceous and 

 argillaceous, and marine fossils are rare in them. The classification 

 that was developed was therefore one based chiefly upon stratigraphic 

 and lithologic characters. Heroic attempts were made to trace the 

 various lithologic units of the system beyond the State; but even from 

 county to county in Pennsylvania the modifications were so constant 



