232 A. WIXSLOW — GEOLOGY OF WESTERN ARKANSAS. 



is made here to harmonize the phenomena with the cause, advocated by 

 Reade,* found in expansion of the lower layers by rise of the isogeotherms. 



Immediately preceding the date of elevation there Avas, over what is now 

 southeastern Missouri and northern Arkansas, a land area of pre-Carbonifer- 

 ous rocks, southward from which extended the seas in which the Carbonifer- 

 ous rocks were laid down. In Missouri the total thickness of the Paleozoic 

 strata is estimated by Broadhead to be in the vicinity of 5,000 feet. In Ar- 

 kansas the thickness must be many times this, that of the Carboniferous 

 strata of the Arkansas river valley alone being about 10,000 feet. The 

 rocks of these Arkansas strata, sandstones, and shales bear evidence in the 

 form of ripple marks and mud cracks of having been chiefly shallow-water 

 deposits ; hence the subsidence was both profound and gradual. An eleva- 

 tion of great magnitude succeeded this subsidence. The Arkansas area 

 thus yields another illustration of a frequently observed sequence of a great 

 orographic movement following a period of long-continued and abundant 

 deposition. The thickness of the strata which accumulated during this 

 period must have been great enough to allow a decided increase of tempera- 

 ture of the low-er members, and the explanation of the subsequent movements 

 by expansion through this increase of temperature thus finds undoubted 

 support in this particular. 



The sections represented on preceding pages yield evidence that a move- 

 ment from the south accompanied the plicating action. The explanation to 

 be offered for this is, that the region of most energetic plicating action was 

 south of the area here especially treated. Excessive dips and an incipient 

 metamorphosis of the rocks characterize a belt of country running in an 

 east-and-west direction some twenty-five miles south of this area. It was in 

 this same belt that the post-Cretaceous disturbance already referred to was 

 most active. This last disturbance was accompanied by the formation and 

 intrusion of igneous rocks ; but none such have been recognized as accom- 

 panying the general upheaval of post-Carboniferous date. 



The facts point thus to the conclusion that this belt was characterized by 

 disturbances from post-Carboniferous to post-Cretaceous times. According 

 to the theory of expansion, this belt was the one along which great tension 

 was first developed in the upper layers from the great elevatory movements 

 caused by the expansion of the lower layers. Through continued action of 

 these causes the upper layers were fractured or stretched and the highly 

 plicated lower layers, which were subjected to pressure, were protruded. The 

 intrusion of molten rock in post-Cretaceous times was but the culmination of 

 this action. On this interpretation we can understand how lateral move- 

 ments of the rocks have been produced northward from this axis of dis- 

 turbance ; how the development of strong plications here gave relief to the 



*"The Origin of Mountain Ranges;" by T. Mellard Reade,1886. 



