126 IOWA ACADEMY OF SCIENCES. 



basal line of the Arkansas coal measures ma}^ be regarded 

 as determined within very narrow limits. 



All evidence at hand goes to show clearly that in 

 Arkansas, sedimentation was continuous during the Car- 

 boniferous, that enormous deposits were laid down during 

 the period, and that while the beds were being formed 

 there was no marked orogenic movements in the region. 



From the north down to the Arkansas line the Des 

 Moines series of the coal measures is well demarked 

 below by the unconformity separating it from all other 

 rocks. Its lowest horizon at this point appears to coincide 

 with the horizon taken as the base of the Cavaniol group 

 of Indian Territory, 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 productive 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 Arkansau series of the coal measures. The 

 latter is therefore entirely below the horizon of any part of 

 the Des Moines series as represented in Missouri and far- 

 ther north. 



Notwithstanding its tremendous thickness in central 

 Arkansas the unusual development may be considered as 

 local in nature. From bottom to top it appears to repre- 

 sent practically the same uninterrupted deposition. 



Although divisible into a number of subordinate forma- 

 tions it is throughout essentially a compact, homogeneous 

 geological unit. Hence from every standpoint it is thus 

 best considered. 



The Arkansas geologists have not yet had opportunity to 

 publish in detail their latest opinions regarding the forma- 

 tions or terranes w4iich they consider as making up the 

 coal measures of the state. Winslow's section, however, is 

 not without interest, and is given below: 



Sebastian stage Boonville stage 



Spadra stage Appleton stage 



Norristown stage Danville stage 



