williams] CONCLUSIONS. 2G1 



that correlation became a problem of dip and thinning of the rocks, or 

 of number and thickness of coal-beds or of sandstone strata. The re- 

 sult was that almost every State having Coal Measures had its own 

 classification of details, with the apparent symmetry of a lower, a mid- 

 dle, and an upper division. As far as a local coal bed could be traced 

 so far there was correlation. This method of correlation led to the 

 theory of "persistent parallelism of strata," which was applied very 

 considerably in the second Pennsylvania survey, and to some extent 

 in all the Coal-Measure areas. In Pennsylvania this theory was ap- 

 plied, and the resulting correlations were unsatisfactory in proportion 

 to the distance the correlations were carried. It was not, strictly speak- 

 ing, correlation. It was rather an actual tracing of the strata from 

 outcrop to outcrop by geometrical processes. The correlations were 

 unsatisfactory because in the clastic rocks which there prevail the 

 details of lithologic characters, as composition, fineness, or coarseness 

 of grain and thickness of strata, are not uniform, but vary considerably 

 even in a short distance. Occasionally there were fossiliferous strata 

 in the Coal Measures which gave a clew to the true position in the 

 standard stratigraphic scale. 



In the Mississippian province the first attempts at correlation were 

 with European standards. In this case there were two fundamental 

 data upon which the correlations were based. These were the "Coal 

 Measures" and the "Mountain Limestone." The presence of coal beds 

 in association with underclays and sands was taken as evidence of the 

 Coal Measures of the English geologists, and the finding of limestones 

 below these Coal Measures containing fossils determined to be identical 

 with those described from the Mountain limestone of Derbyshire, in 

 Martin's "Petrificata Derbieusia," was the reason for calling the lime- 

 stone " Mountain limestone." As far as the general correlation was con- 

 cerned the determination was correct, but when attempt was made to 

 push the correlation to details it was found impracticable to fit either the 

 standard English scale or that already developed in the Appalachian 

 province to these rocks of the Mississippian province. The result was 

 that as the details were accumulated by geological surveys the geologists 

 developed a classification and nomenclature of their own, in the same 

 way that the New York geologists had done for their State. The chief 

 work accomplished in this province was the elaboration of the series be- 

 tween the Devonian and the base of the Coal Measures, called " Subcar. 

 boniferous " and " Lower Carboniferous," which is so characteristic of 

 this region that I propose to give it the name "Mississippian series." 

 The discussion of the facts determining the upper limit of the Coal Meas- 

 ures, as seen in the chapter on the Permian Problem of Kansas and 

 Nebraska, may also be considered as one of the results of the study of 

 this Mississippian province. 



One of the most instructive illustrations of the principles of correla- 

 tion is seen in the determination of the base of the Mississippian series. 



