EARLIER TERTIARY (EOCENE AND OLIGOCENE). 725 



no limestone was found, but there is a yellowish or bluish sandstone, usually rather soft, con- 

 taining Midway fossils. The Midway also occurs along the Rio Grande, near the Webb- 

 Maverick county line, above the mouth of San Ambrosia Creek.'^^" The material consists of 

 dark-brownish or yeUow clays and soft sandstones. The thickness was not determined. 



According to Dumble ^°' the main divisions of the Texas Eocene extend into Mexico. 



Wilcox formation. — The following statement regarding this formation in Arkansas and 

 Louisiana is modified from that of A. C. Veatch.^'*"* 



Overlying the limestones and calcareous clays of the Midway formation is the Wilcox 

 formation, a series of dark, finely laminated sands and clays containing much vegetable matter, 

 either scattered through the mass or accumulated in lignite beds, and a few layers containing 

 marine shells. It, commonly differs from the underlying Midway in the presence of lignitic 

 material and fossil leaves and when containing marine fossils is readily distinguished from both 

 the Midway and the overlying Claiborne. Toward the coast, where it is overlain by the very 

 calcareous, argillaceous, fossihferous St. Maurice formation of the Claiborne group, its upper 

 limit can be fixed with exactness, but farther inland, where estuarine and swamp conditions 

 persisted until Jackson time, no separation is possible except on a purely paleontologic basis. 

 On the whole, the formation is predominantly sandy. Veatch closes his description as follows: 



"The Sabine [Wilcox] formation and its equivalent beds in the undifferentiated Eocene 

 underHe the whole of Louisiana, except the Umited areas occupied by the outcrops of the 

 Cretaceous and Midway domes, and aU of Arkansas south and east of the Cretaceous and Midway 

 outcrops. Its thickness, as shown by carefuUy constructed sections in which local irregu- 

 larities are reduced to their proper minor importance, ranges from 300 feet in northern Bossier 

 Parish to from 800 to 900 feet near Nachitoches and on Sabine River." 



From an unpublished manuscript by Deussen the following description of the WHcox 

 formation in eastern Texas is taken: 



"In eastern Texas the Wilcox formation comprises a series of beds of palustrine and littoral 

 origin, carrying deposits of lignite and glauconitic, fossiliferous, nonlignitiferous beds of marine 

 origin. This formation consists chiefly of a great mass of lenticular beds of sand, concretion 

 and leaf bearing sandstones and clays, sandy clays, and lignites, aU of palustrine origin. Cross- 

 bedded sands and sandstones also occur. In places the sand is indurated into a hard sandstone 

 that is locally useful for building purposes. Thick lenses of lignite of good grade, irregularly 

 distributed, give to the formation great economic value. Plant remains indicative of the 

 shallow-water origin of these beds occur at a number of places. The lignitic and palustrine 

 deposits apparently grade seaward into glauconitic and fossiliferous marls of marine origin. 

 The great thickness of these shallow-water sediments — 500 to 600 feet — is indicative of sub- 

 sidence contemporaneous with deposition. It is otherwise impossible to account for this 

 thickness. Along Sabine River close to the southern margin of the outcrop of the formation 

 glauconitic beds of marine origin occur, which carry characteristic Wilcox fossils. 



"The rocks of this formation outcrop over a wide belt of country, occurring in large portions 

 of Robertson, Limestone, Leon, Freestone, Navarro, Anderson, Henderson, Van Zandt, Smith, 

 Gregg, Harrison, Rusk, Shelby, Panola, and Sabine counties, occupying a position to the east 

 and south of the Midway exposures. Where they are not covered by deposits of later age they 

 almost invariably give rise to sandy soils easily eroded. The sandy outcrop constitutes a 

 portion of the east Texas timber belt. 



"In northeastern Texas, in the counties of Cass, Marion, Harrison, Panola, Gregg, Smith, 

 and Upshur, the Wilcox formation is generally seen to underlie a red sandstone which carries 

 casts of Venericardia planicosta and appears to be an altered greensand. This red sandstone is 

 provisionally referred to the Mount Selman formation of the Claiborne group. 



"The uppermost beds of the Wilcox formation in this portion of the State consist of a 

 series of laminated or thinly stratified white and red sands and sandy clays, in many places 

 merging into one another. They do not carry any lignite, so far as we are aware, nor have 

 they yielded thus far any organic remains. These deposits are so distinctive in this portion of 



