PROBABLE STRATIGRAPHICAL EQUIVALENTS OF 

 THE COAL MEASURES OF ARKANSAS. 



One of the most striking features connected with the coal 

 field of the western interior basin is the enormous thickness 

 which the productive Coal Measures appear to attain in the 

 southern part of the area as compared with the northern and 

 larger portion. From the Minnesota line, southward through 

 Iowa and Missouri, to northwestern Arkansas — a distance of 

 more than 500 miles — the principal coal-bearing series retains a 

 thickness of not more than 500 to 600 feet. Cretaceous and Terti- 

 ary beveling and planing, as well as later erosion, have of course 

 thinned out the beds to a feather-edge towards the east. Pass- 

 ing into Arkansas the formation rapidly becomes thicker until 

 it has a vertical measurement of more than thrice that to the 

 north. According to Branner's 1 latest estimate the "Coal Meas- 

 ures" of that region are over 2400 feet thick. 



In the various comments upon this great thickness which 

 the Carboniferous rocks above the Mississippian series at once 

 assume on passing to the south side of the Boston mountains, 

 into the Arkansas valley, no hint has been given as to the prob- 

 able conditions that produced such a remarkable phenomenon. 



One thing that has tended to greatly obscure the real facts 

 has been the assumption that the old Algonkian and old 

 Cambrian or Silurian areas of the Ozark region formed, during 

 all Palaeozoic times, a large island in the shallow continental sea. 

 The evidence, as recently set forth, seems indisputable that not 

 only did no "Ozark Isle" exist during late Palaeozoic time, but 

 that the present dome-shaped, island-like character of the region, 

 with the central old rocks, and the concentric belts of newer 

 deposits around, was not acquired until Tertiary times. Further- 

 more, there is every reason to believe that the Carboniferous 



1 Am. Jour. Sci., (4), Vol. II, p. 235, 1896. 



356 



