72 H. B. Miser^Llanoria, the Paleozoic Land 



from some vent or vents on or near the old land area to 

 the south. 



The Stanley shale, near whose base the tnffs occur, 

 is underlain, where the tuffs are found, by the Arkansas 

 novaculite and is overlain by the Jackf ork sandstone and 

 other Carboniferous rocks. The novaculite is of Devo- 

 nian age with the exception of its upper part, which may 

 be of early Mississippian age. The facts that this forma- 

 tion is generally free from coarse-grained sediments and 

 that it consists mainly of novaculite, a fine-grained rock 

 which is generally regarded as being a variety of chert, 

 indicate that while it was being deposited the sea in the 

 Ouachita geosyncline was comparatively clear. If so, any 

 land area that may have existed at this time in Louisiana 

 and eastern Texas had a low relief and was subject 

 to little erosion. On the other hand, in Missis- 

 sippian rocks that overlie the Arkansas novaculite, 

 except the tuffs, are all shales and sandstones, and 

 occur in very thick formations in most parts of 

 the Ouachita Mountains, the thickest being the Stan- 

 ley shale, 5,000 to 6,000 feet thick and the Jackfork 

 sandstone, 5,000 to 6,600 feet thick. Any old land 

 area to the south that could have supplied so much 

 sediment in so short a time must have been exten- 

 sive and must have been rapidly eroded and therefore 

 probably included mountains. As the southern land area 

 was low, and was eroded very little during Devonian and 

 possibly during early Mississippian time, the diastrophic 

 movements that produced the mountains must have taken 

 place in late Devonian or early Mississippian time. The 

 occurrence of the tuffs near the base of the Mississippian 

 beds and the probability that the fragmental materials 

 composing the tuffs had a southern source strongly sug- 

 gest that the mountain-making movements were accom- 

 panied by volcanic activity during which the fragmental 

 materials for the tuffs were ejected. 



Fennsylvanian rocks in Arkansas and Oklahoma. 



The Pennsylvanian rock, mainly shales and sandstones, 

 become thinner to the north, particularly the Atoka for- 

 mation which is 7,800 feet thick in the Arkansas Valley in 

 Arkansas, 6,000 to 7,000 feet thick in this valley in Oklaho- 

 ma, and 6,000 feet thick in Pike County, Ark., on the south 



