182 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLINES 



depth is about 1,800 feet. In southwestern Texas, about El Paso, the 

 maximum thickness is about 1,700 feet, and a smaller depth continues 

 across southern New Mexico into Arizona. To the south, in Sonora,. 

 Dumble has reported about 3,000 feet of limestones beneath supposedly 

 Upper Ordovician formations that may be of Cambro-Ordovician time. 

 On the other hand, while the two ends of this trough (Ouachita and 

 Sonoran embayments), and particularly the former, continued to accu- 

 mulate strata intermittently throughout the Paleozoic, yet the longer 

 central reach across Arizona, Xew Mexico, and Texas was dry land dur- 

 ing Silurian, Devonian, and Mississippian times. In all of this we see 

 that there was a good beginning for the development of a long transverse 

 syncline, and that it failed to evolve into a geosyncline because the land 

 elements Columbia and Siouia, or the Great Plains country, remained 

 neutral or slightly positive in relation to the oceanic level. In other 

 words, we see that Columbia was not in motion northward ; only its north- 

 eastern portion, Llanoria. The latter was a decidedly positive and peri- 

 odically rising crustal element that eventually moved northward, crowd- 

 ing the strata of the Ouachita embayment against the ancient granitic 

 Ozark dome. Because the Ouachita trough had not tlie usual length of 

 geosynclines, and since it was often dry land, it is preferably regarded as 

 a southwestern embayment of the Appalachian geosyncline. Undoubt- 

 edly the causation for the Ouachita embayment was the decided negative 

 condition of the southernmost portion of the Appalachian geosyncline 

 that was evolving toward a deep Gulf of Mexico. The compensating re- 

 action of this sinking area against Llanoria, on the other hand, caused it 

 to be a decidedly positive element, and especially so after Mississippian 

 time. As we shall see, Llanoria was the highland that furnished the 

 tens of thousands of feet of clastic sediments for the Ouachita trough of 

 Pennsylvanian time. 



The Ouachita embayment, as previously stated, emerges from beneath 

 post-Paleozoic deposits at Little Rock, Arkansas, and the petroleum wells, 

 about the Wichita Mountains of Oklahoma show that the trough did not 

 extend beyond these mountains. Therefore this embayment had a length 

 of about 400 miles. To the east of Little Rock it must have continued 

 unbroken and in open connection with the Appalachian geosyncline. As 

 is well known, Branner^^ long ago pointed out that the Appalachian 

 trough appeared to continue into the Ouachita embayment. It is now 

 certain, however, that the line of strike of the formations of Arkansas 

 can not be connected with the last of those in Alabama, where the Appa- 



=*^ .J. C. Branner : The former extension of the Appalachians across Mississippi, Lou- 

 isiana, and Texas. Amer. .Tour. Sci. (4), vol. 4, 1897, pp. 3.j7-3T1. 



