Area in Louisiana and Eastern Texas. 77 



feet, respectively. J. C. Branner^^ has estimated that the 

 Carboniferous rocks in the Arkansas Valley in Arkansas 

 are 23,780 feet thick; N. F. Drake"^-^ has estimated that 

 the ''Coal Measures deposits" in Oklahoma are 24,500 

 feet thick; and the thickness of the rock formations of 

 Cambrian to Carboniferous age in the Ouachita Moun- 

 tains in west-central Arkansas, as given by A. H. Pur- 

 due,^^ aggregate 24,000 feet. 



The combined width of the two regions here mentioned 

 is 80 to 100 miles, and their length is about 200 miles. 

 As the rocks were compressed into east-west folds and 

 considerably faulted about the close of the Pennsylvanian 

 epoch, they now occupy a smaller area than they did when 

 they were horizontal or nearly so. The compression, 

 as calculated for a large part of the Ouachita Mountains 

 in Arkansas, has reduced this horizontal extent almost 

 one-half. Furthermore, the rock formations of the Ar- 

 kansas Valley and Ouachita Mountains extend an 

 unknown though probably considerable distance both 

 eastward and southward beneath the Gulf Coastal Plain. 



The Pennsylvanian rocks of north-central Texas extend 

 from the Central Mineral region northward to the State 

 line and aggregate more than 5,000 feet in thickness, but 

 only part of the Pennsylvanian sediments were laid down 

 over all the area in which these rocks are now exposed, as 

 is shown by the thinning of the strata to the west and their 

 overlapping in this direction*upon the Bend series of the 

 Texas Geological Survey.^^ 



Age and thickness of the exposed rocks of the Gidf Coastal Plain. 



The exposed rocks of the Coastal Plain are of Lower 

 Cretaceous, Upper Cretaceous, Eocene, Oligocene, Mio- 

 cene, Pliocene and Quaternary ages. E. W. Shaw^"^ has 



^^ Branner, J. C, Thickness of Paleozoic sediments in Arkansas, this 

 Journal, (4), vol. 2, pp. 229-236, 1896. 



^^ Drake, N. F., A geological reconnaissance of the coal fields of the Indian 

 Territory, Am. Phil. 8oc. Proc, vol. 36, p. 388, 1898. 



^" Purdee, A. H., The slates of Arkansas, Arkansas Geol. Survey, pp. 30, 

 48, 1909. 



^" Drake, N. P., Eeport on the Colorado coal field of Texas, Texas Geol. 

 Survey, Fourth Ann. Kept., pp. 374 et seq., 1893; (Eeprint), University of 

 Texas Bull. No. 1755, pp. 16 et seq., 1917. 



^' Shaw, E. W., Stratigraphy of the Gulf Coastal Plain as related to salt 

 domes, Washington Acad. ScL, Jour., vol. 9, No. 10, p. 289, May 19, 1919. 



Am. Jour. Sci. — Fifth Series, Vol. II, No. 8. — August, 1921. 

 6 



