Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO yjj 



with sulphur which is overlain by a second series, comprising 

 black, lignitic, sandy shales with concretions or boulders of 

 gray carbonate of iron, weathering red, lignitic deposits, etc. 

 Fragments of Cardita, etc. 



Claiborne 



The Claiborne, as a whole, comprises several alternations 

 of deposits of clays and sands, and for purposes of descrip- 

 tion and m.apping, is divided into substages. 



Carrizo Sands 

 Sandstones of varying color, texture and thickness. The 

 prevailing color is a grayish yellow, weathering light brown. 

 Some of the beds are white when freshly broken. In texture 

 they range from fairly hard sandstone, lying in beds of two 

 to four feet or more in thickness, to thin slabby, fairly soft, 

 and almost shaly structure. No fossils except a few plant 

 remains have been found in them. 



Marine Beds 



Greenish clays and lignitic sands with palmetto and other 

 plant remains, and some lignite, overlain by carbonaceous 

 clays and sands with gypsum and particles of lignite, capped 

 by brown or buff sandstone. These beds are not so glauco- 

 nitic on the Rio Grande as they are in eastern Texas. This 

 substage usually carries an abundant and characteristic fauna, 

 including such forms as Venericardia planicosta Lam., Anomia 

 ephippioides Gabb, Osfrea divaricata, Nassa texana Gabb, 

 Distortix septemdentata Gabb. 



Yegua 

 At the base, interbedded brown sands, chocolate clays with 

 green sand, and lenticular masses of red sandstone ; then 

 buff and greenish sands slightly calcareous with occasional 

 bands of limestones, gypsum and cannon-ball concretions 

 abundant. Buff sandstone overlain by blue and green fer- 

 ruginous clays with calcareous concretions followed by yel- 

 low sandy clay form the upper portion of the measures. In 

 places the concretions in these beds carry aragonite and 

 chalcedony. While not fossiliferous throughout, the beds 

 carry a typical Claiborne fauna and are characterized by 

 Tellina mooreana Gabb, var., Tiirritella hoiistonia Har., Nat- 

 ica recurva Aldrich. 



