Udden — Castile Gypsum and Rustler Formation. 151 



Art. XIY. — The Age of the Castile Gypsum and the 

 Rustler Springs Formation •* by J. A. Udden. 



In 1904, Richardson described two formations in Culberson 

 County, Texas, the Castile Gypsum, known to be at least 300 

 feet thick, and the Rustler formation, some 200 feet thick. 

 He found them extending in a continuous broad belt from the 

 New Mexico border southward to within 15 miles of the Texas 

 and Pacific Railway, the Rustler formation overlying the 

 Castile. He found that the Castile formation is separated 

 from the Delaware Mountain formation (Permian) by an un- 

 conformity, f From the few fossil fragments which were 

 secured from these formations, he was unable to draw definite 

 conclusions as to their probable age, and he stated in his report 

 that " the age of the Rustler formation is not known." Neither 

 could he make any definite statement as to the age of the 

 Castile formation. His descriptions are the best we yet have. 

 Although he searched for fossils in all exposures examined, 

 only a few poorly preserved fossils were found. Dr. T. W. 

 Stanton, who examined one fossil, found that it might be a 

 Mytilus or a Myalina. Dr. F. H. Knowlton reported on some 

 poorly preserved plant remains as probably being of Mesozoic 

 age. 



On Willis' geologic map of North America, the area of the 

 outcrop of these formations is represented as Permian, and in 

 his Index to the Stratigraphy of North America, he states that 

 " recent work has shown that the Castile Gypsum and Rustler 

 formation are parts of the group of red beds of Pecos Valley, 

 which are of Permian Age." He cites Richardson's paper : 

 u Stratigraphy of the upper Carboniferous in West Texas and 

 Southeast New Mexico.":}: It appears that Richardson in this 

 paper refers these two formations in question to the red beds, 

 mainly on the basis of their lithologic character, and on field 

 work by other geologists who have traced the red beds around 

 the Staked Plains from New Mexico to Oklahoma and to the 

 Pecos Yalley in Texas. 



The Mesozoic as well as the Paleozoic formations are poorly 

 exposed in this region, and all who have worked here have 

 recognized the difficulties attendant on making correct cor- 

 relations between distant and limited outcrops, in beds that 

 rarely yield any fossils. 



It seems pertinent to place on record the finding of some 

 fossils in the Castile formation, which in the writer's opinion 



* Published by permission of Dr. W. B. Phillips, Director of the Bureau 

 of Economic Geology and Technology, University of Texas, Austin, Texas, 

 t University of Texas Mineral Survey, Bulletin 9, pp. 43-45. 

 j This Journal (4), vol. xxix, p. 325, 1910. 



