THE FLEXURES FORM A WARPED SURFACE. 231 



the flexures here involved, even accordiug to the most conservative inter- 

 pretations, would produce a surface on which the features of relief would be 

 elongated ellipsoidal domes — it would be a typical warped surface, and, like 

 other warped surfaces, could not be developed into a plane. 



Dates of Elevation and Folding. — From the age assigned these rocks, it fol- 

 lows that the date of their elevation must have been post-Carboniferous. 

 Further, the evidence is strong that this elevation was pre-Mesozoic ; for, 

 firstly, no Triassic or Jurassic rocks are found overlying these Carboniferous 

 beds conformably ; and, secondly, Cretaceous beds rest unconformably upon 

 and in contact with the upturned edges of the Paleozoic beds along their south- 

 ern border, the Cretaceous beds showing little or no evidence of disturbance ; 

 which latter fact demonstrates also that the folding, or at least such portion 

 of the folding as was wide-spread in its efl^ects, antedated that period. But 

 the question whether the elevation and the folding were exactly simultaneous 

 or whether the latter succeeded the former is a detail concerning which we 

 have not complete evidence to present here. The existence o^ a disturbance 

 in post-Cretaceous times is well established through the study of the eruptive 

 rocks of the state, which are of post-Cretaceous age ; but its effect in flexing 

 the Coal Measure rocks seems to have been insignificant. According to 

 Branner : ^ 



"The Cretaceous rocks have not been much disturbed or altered, even where the 

 eruptives come up through them. Neither can it be positively stated that the eruptives 

 greatl}"- disturbed the Paleozoic rocks through which they pass, for the folding, crush- 

 ing, and metamorphosis seem to be just as marked away from the outbursts as in their 

 immediate vicinity." 



General considerations of the magnitude of the movements involved, and 

 of the character and comparative values of the different mountain-making 

 epochs in the history of this continent, incline one to the belief, however, 

 that the folding as well as the elevation of these Carboniferous rocks of Ar- 

 kansas was synchronous with the movement which gave birth to the Appa- 

 lachians, and that the similarity betw^een the structure of this area and that 

 of the Carboniferous area in Pennsylvania is not a mere accident but is due 

 to a trans-Mississippian extension of the same cause. 



Cause of Elevation and Folding. — As above expressed, the flexing of the 

 strata in the coal region of western Arkansas is essentially Appalachian ; and, 

 reasoning from resulting forms, any explanation good when applied to the 

 Appalachians will be good w4ien applied here. A study of the various 

 flexures reveals many features which call for compression and lateral move- 

 ment. The influence of secular contraction in this movement cannot be dis- 

 cussed exhaustively until the results of studies elsewhere in the state are 

 available ; but, in view of the questioned adequacy of this cause, an attempt 



* Private communication. 



