Udden — Castile Gypsum, and Rustler Formation. 155 



is to the east. It is evident that the greater part of the ex- 

 ploration is through the Castile Gypsum, which is interbedded 

 with red shale, yellow dolomite, and some gray marl. In this 

 o-ray marl the foraminifera were found, recurring in at least 

 three such beds, at 141-145, 180-195, and at 209-213 feet 

 below the surface. The nature of the ground shown by these 

 samples corresponds with the descriptions in the driller's log, 

 which is not available for publication in full. The upper 300 

 feet were described mostly as gypsum, red clay with some gray 

 marl, and yellow limestone (dolomite), variously interbedded. 

 The accompanying figure shows most of the foraminifera 

 found in these samples. I have seen what appears to me to be 

 identical forms in the basal Comanchean at other points in 

 this part of this state. On the other hand, I have examined 

 many foraminifera from the Carboniferous, including the 

 Permian, without ever finding any assemblage of forms at 

 all like the lot found in these gray marls.* It seems to me 

 they are undoubtedly Mesozoic, demonstrating the post- 

 Permian age of both the Rustler Spring formation and of the 

 Castile Gypsum. Gypsum beds associated with red and gray 

 marls are known to lie near the Jurassic east of the Finlay 

 Mountains in El Paso County, of this state, and I believe that 

 the Castile Gypsum and the Rustler Springs formation will be 

 found to be equivalents of these beds, whether Comanchean or 

 of earlier age.f 



University of Texas, 

 Austin, Tex. 



* Bulletin University of Texas, No. 363, pp. 72-81. 



f Not wishing to rely solely on my own identification of these fossils, 

 some material was submitted to Dr. Joseph A. Cushman, who kindly 

 examined the same. He says that it contains scattered foraminifera which 

 are more likely to be Mesozoic than Paleozoic ; that they are almost entirely 

 Globigerinidae, a family which is rare or wanting before the Mesozoic. " In 

 this lot they are very clearly shown and I certainly should say, Cretaceous." 



I have also submitted a small sample of the foraminifer-bearing cuttings 

 from the Troxel boring to Mr. G. B. Richardson, of the U. S. Geological 

 Survey, and he has kindly furnished for publication with this article a state- 

 ment anenl the questions involved in the correct stratigraphic determination 

 of the Bustler and Castile formations. A desire to throw whatever little 

 new light there may be on an obscure subject, is responsible for this paper. 

 Mr. Richardson's communication is as follows : 



Dear Doctor Udden : 



Replying to your request for a statement from me concerning the age of 

 the Rustler formation and Castile gypsum : 



Clearly more direct evidence of the age of these formations is much to be 

 desired, and I am interested in your discovery of foraminifera. But it seems 

 to me the question may be raised whether present knowledge of minute 

 Mesozoic and Paleozoic foraminifera is sufficient to warrant their use in crit- 

 ical age determinations. 



I have thought such evidence as we have indicates the Permian age of these 

 formations. They occur in the lower part of a distinctive group of rocks 



