156 Udden — Castile Gypsum and Rustler Formation. 



which outcrop in the valley of Pecos River in southeast New Mexico and 

 west Texas, which have been referred to as the "red beds of Pecos Valley." 

 These red beds form part of an east-dipping series and are underlain on the 

 west, on the flanks of the Guadalupe and Delaware Mountains, by the Gua- 

 dalupe Group, and on the east, in the escarpment of the Staked Plains, are 

 overlain by the Dockum formation. The Dockum formation is assigned to 

 the Triassic on the evidence of vertebrate bones. But the precise position in 

 the upper Carboniferous of the Guadalupe Group, which contains a unique 

 fauna, remains to be determined. It is assigned to the Permian by Girty. 



The "red beds of Pecos Valley" consist of vari-colored, chiefly red, sand- 

 stone and shale and interbedded lenses of magnesian limestone and gypsum 

 at least 1600 feet thick. The base of the red beds, considered as the lowest 

 occurrence either of red strata or of gypsum, is not a constant horizon 

 because the stratigraphic position of the red-colored rocks varies along the 

 strike. Very few fossils, shells, and determinable plant remains have been 

 found in these rocks. Concerning the shells which I collected from lime- 

 stone in the Rustler Hills, Texas, from a horizon presumably above the mouth 

 of the well from which you obtained the foraminifera, G. H. Girty reports: 

 "Two forms are included in this collection, one of them suggesting by its 

 shape a small Myalina, the other being perhaps a Schizodus and having the 

 general shape of Schizodus harei." T. W. Stanton reports that in his opinion 

 these shells are Paleozoic. The best collection known to me from the "red 

 beds of Pecos Valley" was made by Beede, who found shells in a limestone 

 lens near Lakewood north of Carlsbad, New Mexico, which he correlates 

 with the fauna of the Quartermaster and Woodward formations, parts of the 

 well-known Permian red beds of Oklahoma and north Texas. 



I hope you will succeed in collecting more fossils from these rocks. 



Yours very truly, 



G. B. Richabdson. 

 Washington, March 30, 1915. 



