Williams.] JAMES HALL. 153 



pact on going southward. Hall speaks of " the brecciated character of 

 the northern exposures of the St. Louis limestone, 7 ' and of the " more 

 or less diagonally laminated limestoue of gray to white color n in the 

 more southern part. 1 



Below St. Louis, along the valley of the Mississippi, from Prairie du 

 Rocher to Chester, the series dipping gradually southward exposes the 

 " St. Louis limestone," with the " Ferruginous sandstone" resting on it, 

 and above this the " Kaskaskia limestone." Hall observed the fact 

 that the limestones thin out toward the north and upon their inclined 

 edges are followed unconformably by the Coal Measures. He drew 

 from this the inference that a contraction of the borders of this sea at 

 the north began with the deposition of the Carboniferous limestone; that 

 this was consequent upon the uplifting of the older rocks at the north. 2 



The limestone of the UpperCoal Measures in Ohio is traced westward, 

 and is regarded as represented by the Carboniferous limestone of the 

 Eocky Mountains. Previously, in the Report on the Mexican Boundary, 

 Mr. Hall had recognized the fact that the Rocky Mountain region must 

 have been an open sea at the time the Coal Measures were being de- 

 posited in the Mississippi Valley and farther eastward. The oscilla- 

 tion during the time of the Carboniferous limestone was mainly up- 

 ward for the Upper Mississippi basin, and during the Coal Measures 

 Worthen shows 3 that the same regiou was gradually sinking, causing 

 the higher Coal Measures to extend farther northward than those below. 



In the classification of the rocks of Iowa it was quite natural that 

 the New York series should take a prominent part in the nomenclature. 

 Although fossils were considered in the correlation, the erroneous inter- 

 pretations, as well as the reports themselves, show that the lithologic 

 characters of the various rocks were considered of chief importance. 

 As Worthen stated in the First Report on the Geology of Illinois, in 

 regard to the beds in Indiana, Iowa, and Missouri, which had been re- 

 ferred to the "Chemung group" of New York, the identifications were 

 r made purely on lithological grounds." 4 It was this dominant idea, 

 that there should be some similarity in the character of the rocks of the 

 corresponding zones in separate regions, that led to the importance of 

 the " Ferruginous sandstone" of the Missouri and, later, of the Iowa 

 Reports. 



Again, the " Siliceous group" in Tennessee and Alabama was regarded 

 as important because probably representing a corresponding Siliceous 

 group in the English classification. 



The " Carboniferous or Mountain limestone" was distinctly recognized 

 in the upper Mississippi region, and a "Millstone grit" was needed to 

 fill out the system as interpreted in England. It can not be denied that 

 great masses of limestone or of sandstone can be traced satisfactorily 

 for hundreds and may be thousands of miles along the geological out- 

 crops, but this expresses only the fact that, for long geological periods, 



1 Geol. Surv. Iowa, Rep., p. 105. a Geology of Illinois, vol. 1, p. 50. 



*Ibid., p. 117. ■» Ibid., p. 109. 



