u^•coNF()H^^Tv at kasi<: of coal mkasuuks 191 



appear to gradualh' merge. Nowhere in this region lias tliere been 

 noted any evidence of unconfonnable relationships, nor do any of tlie 

 Arkansas geologists mention any facts indicating that a stratigrai)hic 

 break might exist. 



The zone of uncertain age is, however, thin, and the basal line of the 

 Arkansas Coal Measures may be regarded as determined within very 

 narrow limits. 



All evidence at hand goes to show clearly that in Arkansas sedimen- 

 tation was continuous during the Carboniferous ; that enormous deposits 

 were laid down during the period, and that while th^ beds were being 

 formed there was in the region no marked orogenic movement. 



SUPERIOR DELIMITATION OF BASAL SERIES 



From the north down to the Arkansas line the Des Moines series of 

 the Coal Measures is well demarcated below — by the unconformity 

 separating it from all older rocks. Its lowest horizon at this point ap- 

 pears to coincide with the horizon taken as the base of the Cavaniol 

 grou}) of Indian territor}^ as traced in detail by Drake. The Cavaniol. 

 in turn, is correlated in the main with the upper or western coal-bearing 

 division or Poteau of Arkansas, which also includes part of the product- 

 ive Coal Measures. 



The base of the Cavaniol group is now taken to be the Grady coal. 

 This horizon may be considered as limiting above the great Arkansan 

 series of the Coal Measures. The latter is therefore entirely below the 

 horizon of au}^ part of the Des Moines series as represented in Missouri 

 and farther north. 



HOMOGENEITY OF THE ARKANSAN SERIES 



Notwithstanding its tremendous thickness in central Arkansas, the 

 unusual development may be considered as comparatively local in 

 nature. From bottom to top it appears to represent practically the 

 same uninterrupted deposition. 



Although divisible into a number of subordinate formations, it is 

 throughout essentially a compact, homogenous geological unit. Hence 

 from every standpoint it is thus best considered. 



SUBDIVISIONS 



The Arkansas geologists have not yet had opportunity to publish in 

 detail their latest opinions regarding the formations or terranes which 



