EARLIER TERTIARY (EOCENE AND OLIGOCENE). 727 



The Cockfield formation comprises lignitiferous sands and clays wMch occur in central 

 Louisiana between the St. Maurice and Jackson formations and is extremely similar in lithologic 

 character to the Wilcox formation, with which it was at first confused. It contains no marine 

 mollusks and is characterized by tliin, impure lignite beds and clays which in many places con- 

 tain plant remains in an excellent state of preservation. It is identical in appearance with the 

 lignitiferous complex to the north (undifferentiated Eocene), of which it forms a part, and can be 

 definitely differentiated only when fixed between fossiliferous Claiborne and Jackson strata or by 

 paleontologic data. (Modified from A. C. Veatch.) 



Of the Claiborne group in eastern Texas Deussen has the following to say in an unpublished 

 manuscript : 



"In eastern Texas the Claiborne group comprises three formations — the Mount Selman, 

 Cook Mountain, and Yegua — which are lithologically and genetically distinct. 



"The Mount Selman formation consists cliieflyof dark-green and brown sands and sand- 

 stones, with thin seams and concretions of limonite. The fossils are not plentiful and usually 

 occur in the form of casts. This formation has been given the name Mount Selman from its 

 typical exposure at that place in Cherokee County." These highly ferruginous sands with casts 

 of fossils and beds of iron ore of economic value occupy extensive areas in Anderson, Henderson, 

 Cherokee, Rusk, Gregg, Harrison, Marion, Morris, and Cass counties. 



"The formation overlying the Mount Selman has been named Cook Mountain, from Cook 

 Mountain,* Houston County. At the base of this formation occur beds of greensand, greensand 

 marl, and iron ore, all highly fossiliferous and of marine origin; in the medial portion lignites, 

 lignitic clays, iand sands of palustrine origin are found; at the top occurs another series of fossilif- 

 erous greensands, greensand marls, and iron ores of marine origin. The thickness along Brazos 

 River is at least 400 feet. The outcrop appears in Robertson, Brazos, Leon, Houston, Anderson, 

 Cherokee, Nacogdoches, San Augustine, and Sabine counties. 



The lignitiferous unit stratigraphically and conformably overlying the Cook Mountain 

 formation, was recognized by Dumble " in 1892 as a separate formation, and he applied to it 

 the name Yegua. It consists of clays, sands, and lignites of palustrine and marine origin. The 

 clays are characterized by the presence of fragments and concretions of selenite. The thickness 

 of the formation varies from 375 to 750 feet. The outcrop appears in Sabine, San Augustine, 

 Angelina, Trinity, Houston, Madison, Grimes, and Brazos counties. 



"On the Brazos beds belonging to the Yegua formation are exposed between the mouth of 

 the Little Brazos and a point southwest of Wellborn, in Brazos County. The beds as exposed 

 in this section consist entirely of palustrine and lignitiferous deposits, barren of marine fossils. 

 On Sabine River beds belongiag to this formation are exposed from a point 3 miles below 

 Columbus to a point a short distance below Robertson's Ferry, in Sabine County. The beds 

 here are typical palustrine deposits." 



The details of the Claiborne group have not as yet been studied between Guadalupe River 

 and Rio Grande. Dumble's work along the latter stream has shown that representatives of the 

 marine Claiborne outcrop from Santo Tomas, 30 miles above Lando, to the vicinity of Roma. 

 The material consists of lignite beds near the base, soft sandstone, and clays, which are more or 

 less calcareous, ^^''^ and greensands. The thickness of the marine Claiborne in this region is not 

 known. 



Jackson formation. — ^The Jackson formation in Louisiana immediately overlies the Cockfield 

 and represents a return to marine conditions, being composed of calcareous gray fossiliferous 

 sands and clays, which in places contain large calcareous concretions. It extends farther 

 up the Mississippi embayment than any other formation bearing marine fossils except the Mid- 

 way. Its most northern outcrop is in Crowleys Ridge, near Forrest City, St. Francis County, 

 Ark. To the south of this locality the formation underlies an extensive area southward from 



1 Kennedy, Third Ann. Kept. Geol. Survey Texas, 1892, pp. 52-54. 

 6 Idem, pp. 54-57. 



<: Dumble, E. T., Report on the brown coal and lignite of Texas, character, formation, occurrence, and field uses: 

 Kept. Geol. Survey Texas, 1892, pp. 148-154. 



