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BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



I have visited, in company with Mr. Alexander Deussen, the fos- 

 siliferous exposures near Wellborn, Texas. I collected fossils and 

 have studied them. I concur with Mr. Veatch in his opinion that 

 they are of Jackson age. Mr. Deussen has traced the formation 

 westward; it is persistent and persistently overlies the Yegua for- 

 mation at least for some miles beyond Nueces River.^ The Frio 

 clay overlies the Fayette sandstone, and it contains Ostrea georgiana, 

 a species that is abundant in the Jackson formation in Alabama 

 and in the Barnwell formation, which is the correlative of the Jackson 

 formation, in eastern Georgia. Tlie Fayette sandstone and the Frio 

 clay of Texas are the correlatives of the Jackson formation of Lou- 

 isiana and Mississippi. The following table shows the stratigraphic 

 equivalence: 



Correlation of the middle and upper Eocene of Texas. 



Southward and westward of a line, the location of which is indi- 

 cated on Deussen's map,^ there is a change in the strike of the forma- 

 tions. The line passes between Cotulla and Tilden and strikes from 

 about N. 52° W. to S. 52° E.; northeast of it, the strike of the for- 

 mations is S. 39° W., with a southeastward dip of 48 feet to 1 mile; 

 southwest of it, the strike is N. 19° E., with a dip S. 19° E. of 36 

 feet to 1 mile. In 1912 Mr. G. C. Matson devoted some months to a 

 field study of the area along Rio Grande seaward of the Eocene- 

 Cretaceous contact, and I accompanied him during a wagon trip 

 from Laredo to Samfordycc. As Mr. Matson has not been able to 

 prepare a report for publication, it is fortunate that I made notes 

 on the exposures we examined, and later the marine fossils collected 

 were studied and identified by Dr. C. W. Cooke and myself. Through 

 out much of its com'se between Laredo and Roma, Rio Grande is a 

 subsequent stream — that is, its course is along the strike of the for- 

 mations — and for miles the road is on very nearly the same geologic 

 formation. However, only a short distance eastward from the river 

 higher geologic formations are encountered. The most important 

 difference of the successive formations, as compared with those far- 

 ther east, consists in the slight development of the lignitiferous 

 Yegua formation, which, apparently, is represented by shoal-water 

 marine sands. The correlative of the Fayette sandstone was not 



I a. S. Geol. Survey Water-Supplr Paper 375, pi. 8, 1916. 



