14 GULF TERTIARY OF TEXAS. 



composed entirely of glauconite, and between these two extremes are found 

 all gradations in the relative proportions of the two materials. These beds 

 are the equivalent of the Ripley Division of Alabama, and probably are the 

 Southern representative of the "Fox Hills" beds of Nebraska. The "glau- 

 conitic " deposit becomes more argillaceous towards its base, and gradually 

 runs into a great deposit of calcareous clay over twelve hundred feet thick 

 and characterized by large quantities of Exogyra ponderosa. This bed repre- 

 sents the "Exogyra Ponderosa Marls" of Hill's Upper Cretaceous section, 

 and underlies a large part of the great prairie region of Central Texas. 

 These Upper Cretaceous beds dip gently at three to five degrees to the south 

 and southeast, and formed the Texas shore line of the early Tertiary sea. 

 Upon their much eroded surfaces were deposited the Eocene clay and sandy 

 strata which underlie East Texas. The "glauconitic " beds have in fact been 

 so much denuded that they now play only an insignificant part in the strati- 

 graphy of that part of the Southwest included in Texas and Arkansas. They 

 are developed in Southwestern Arkansas near Arkadelphia, on the Ouachita, 

 and near the town of Washington,* but to the southwest of the Red River 

 they are, so far as is yet known, almost entirely wanting until we get to Ander- 

 son County. Here, some six miles east of the Trinity River and over two 

 hundred miles south of the Arkansas outcrops, is found a small calcareous 

 hill, which is the equivalent of the "glauconitic " beds of the Ouachita. This 

 outcrop covers less than two hundred acres of territory and is surrounded 

 and partly covered by clays of Lower Tertiary epoch. It lies over fifty miles 

 east of the main Cretaceous area of Central Texas, and is in fact a Creta- 

 ceous inlier, which doubtless formed an island in the old Tertiary sea. Sim- 

 ilar areas have been described by Hilgardf as occurring in Louisiana, where 

 they crop through the Tertiary strata near Chicotville, Winfield, and Salines, 

 and form a range of outcrops running north northwest and south south- 

 east. Going southwest along the old Cretaceous shore line we again find the 

 " glauconitic " strata well developed on the Rio Grande, both above and be- 

 low Eagle Pass. How far these can be traced to the northeast is as yet un- 

 certain, as the stratigraphy of that region is but very little known. But they 

 doubtless end somewhere between the Rio Grande and the San Antonio 

 River, as they are not found in the latter locality. Consequently it will be 

 seen that the three hundred feet of "glauconitic" strata have, in East Texas 

 at least, been almost entirely eroded, and the underlying "Ponderosa Marls" 

 now form almost everywhere in this portion of the State the uppermost Creta- 

 ceous division. Even this bed has been much eroded in many places, and while 



*R. T. HiU, G-eological Survey of Arkansas, Yol. II, 1889. 



f E. W. Hilgard, " Supplementary- and Final Report of a Geological Reconnoisance of the 

 State of Louisiana," 1873, p. 43. 



