360 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



rather are similar to those of the upper part of the Hueco formation. It seems 

 probable that this difference in the fauna is due to changed environment, which 

 is indicated by the varied lithology. Whether the Guadalupian fauna should be 

 regarded as late Pennsylvanian or Permian is still an open question, but the 

 weight of evidence is in favor of the Permian and that view has been adopted 

 by the United States Geological Survey. 



In an earlier report Richardson ®*'^ described the formations overlying the 

 Guadalupe group, as follows : 



The Castile gypsum is a massive white granular variety. It is comparatively pure, and a 

 characteristic sample analyzed quahtatively by Mr. W. T. Schaller shows it to be of no unusual 

 composition. Considering its great extent the Castile gypsum is remarkably homogeneous, 

 yet it varies somewhat. On the surface it is generally disintegrated and earthy. In places it 

 is grayish or dark in color owing to the presence of organic matter, and at other places it is 

 stained red by iron oxide. Locally selenite is abundant. Some sections show occasional thin 

 beds of banded gray limestone in the gj^psum. Deposits of native sulphur are also associated 

 with the Castile gypsum. The thickness is not known, but it is considerable. A well at the 

 old sulphur works abovit 6 miles north of Rustler Spring shows a thickness there of a little over 

 300 feet, though the base of the gypsum is not known to have been reached. 



5fC 5p *!* 3jC Sp Jp 5jC ^ ^ • 



The Castile gypsum outcrops in a belt between the Delaware Mountains and Rustler Hills, 

 the width of which averages about 15 miles, though at the New Mexico-Texas boundary it is about 

 30 miles. This gypsum belt begins about 15 miles north of the railroad and extends into New 

 Mexico. Within Texas the gypsum outcrops over 600 square miles. The name of the forma- 

 tion is derived from Castile Spring, which is in the midst of the gypsum about 12 miles south 

 of the State boundary. 



The Castile gypsum along its western outcrop lies on httle knolls and valleys of the under- 

 lying Delaware Mountain formation, indicating an erosional unconformity. Another evidence 

 of unconformity at the base of the gypsum consists in the absence of the Capitan limestone. It 

 appears that either the gypsum was deposited at or near the top of the Delaware Mountain 

 formation as a lens which did not extend westward to intervene between the Delaware Mountain 

 formation and the Capitan limestone in the Guadalupe Mountains, or that erosion removed 

 the former southwestward extension of the limestone (the thickness of which is unknown) before 

 the deposition of the gypsum. The former supposition necessitates the correlation of the 

 Rustler formation, which overhes the gypsum, with the upper part of the Delaware Mountain 

 formation or with the Capitan limestone. But there is little to support this interpretation, and 

 it is tentatively assumed that the Castile gypsum and the Rustler formation were formed after 

 the deposition and erosion of a part of the Capitan limestone. 



The Rustler formation consists of a fine-textured white magnesian limestone and less 

 abundant sandstone. The formation occurs in the Rustler Hills, in the dissected southwestern 

 extension of the Rustler Hills between Cottonwood and Hurd's Pass draws, and in a few isolated 

 areas west of the hills. 



The formation as here exposed averages about 200 feet in thickness and varies in composi- 

 tion. In the southern outcrops there is no sandstone and the hills are capped by about_150 feet 

 of massive gray limestone, which directlj'' overlies the gypsum. Northward sandstone is present 

 below the limestone. * * * 



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. 



