126 
IOWA ACADEMY OF SCIENCES. 
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, sedimentation was continuous during the Car- 
boniferous, that enormous deposits were laid down during 
the period, and that w 7 hile 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 Arkansan 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 w T hich 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 
Spadra stage 
Norristown stage 
Boonville stage 
Appleton stage 
Danville stage 
