Area in Louisiana and Eastern Texas. 71 



the Ouachita uplift, with shale beds occupying the corre- 

 sponding position on the north side, indicates that the 

 land whence these sediments were derived lay to the 

 south, just as in the case of the Appalachians it lay to the 

 east.'' G, H. Ashley ,^^ who studied a large area of 

 Carboniferous rocks underlain by the Stanley, Jackfork, 

 and Atoka formations south of the area examined by 

 Griswold, says that they apparently confirm Griswold's 

 conclusion regarding the southern source of the Carboni- 

 ferous sediments. 



Concerning the source of the clastic sediments of not 

 only the Stanley and Jackfork but of the Caney and 

 Atoka formations, David White^^ says : 



"Toward the northeast [of north-central Texas], somewhere in 

 the region of the Red River Valley, a Mississippian-Pennsylva- 

 nian land barrier existed which is now bridged by later Pennsyl- 

 vanian ''Red Beds" or Cretaceous strata. The existence of such 

 a land mass is predicated by the sediments (elastics) of the Jack- 

 fork, Stanley, Caney, and Atoka formations as well as by the fos- 

 sils. The sediments of these formations could hardly have been 

 derived from the Ozark uplift, nor does it seem probable that 

 they could have originated in the areas now marked by the Ar- 

 buckle- Wichita uplift." 



Tuffs of Mississippian age in the Ouachita Mouniains. 



Tuffs occur near the base of the Stanley shale in Polk 

 County, Ark., and McCurtain County, Okal., in three and 

 possibly four or f[^e beds, which range in thickness from 

 6 to 85 feet, the lowest bed being the thickest and the most 

 widely distributed. According to E. S. Larsen, who has 

 studied them in thin sections, these tuff's are composed in 

 large part of devitrified and silicified volcanic glass and 

 of feldspar and other minerals. A study of the tuffs in 

 Arkansas has been begun by the writer but has not yet 

 been completed. However, as they occur only on the 

 south side of the Ouachita Mountains, and thin out toward 

 the north, and as the size of grains of their component 

 materials apparently decreases toward the north, it would 

 seem that the volcanic materials they contain were ejected 



^^ Ashley, G. H., Geology of the Paleozoic area of Arkansas south of the 

 novaculite region, Am. Philos. Soc, Proe. vol. 36, p. 248, 1897. 



^^ White, David, Discussion of paper by F. B. Plummer on the stratigraphy 

 of the Pennsylvanian formations of north-central Texas, Assoc. Amer. Petro- 

 leum Geologists, Bull., vol. 3, p. 149, 1919. 



