DESCRIPTION OF FOUE :N^EW TRIASSIC UNIOS FEOM THE 

 STAKED PLAINS OF TEXAS. 



By Charles Torrey Simpson, 



Aid, Department of MoUmka. 



The material on wbicli this paper is based ^vas sent to the writer 

 for examination by Prof. E. T. Dumble, State geologist of Texas.^ It 

 was obtained irom the Dockum beds, an extensive formation which 

 nnderlies all or nearly all the Staked Plains of Texas, and southeast- 

 ern New Mexico, reaching farther back into that Territory northwest 

 of the Plains, and having some extension under the Cretaceous area 

 south of them in Texas. The limit of the plains on the east, north 

 and west is marked by an escarpment, which is usually from 100 to 

 200, and sometimes 300 or 400 feet high. The basal portion and occa- 

 sionally nearly all of this escarpment is composed of what are believed 

 to be Triassic beds. They usuaHy extend some 6 or 7 miles beyond the 

 base of the great plain. ^ 



These beds are composed of horizontal strata of sandstone, conglom- 

 erate and clay; and are overlaid in some places by Cretaceous, but 

 more generally Tertiary strata, and underlaid by the rocks of the 

 Permian period, whose lithological characters are so different from 

 those believed to be Triassic that the latter can usually be recognized 

 without trouble. The slight difference in dip, and the sudden change in 

 lithological characters from the Triassic to the Permian, point conclu- 

 sively to a break in the sedimentation of the two deposits. According 

 to the evidence of the fossils and the characteristic material forming 

 them, the Dockum beds seem to have been deposited in an inland, 

 fresh-water basin. The vertebrates, as determined by Prof. E.D.Cope, 

 were shallow fresh-water animals. 



A few fragments of bivalve shells were collected by Professor Cope 

 in the valley of Gallinas Creek, New Mexico, associated with vertebrate 

 remains, which latter led their discoverer to believe the formation was 



'The paper and the accompanying figures were prepared for the report of the 

 Texas Geological Survey, but on account of the failure of the legislature of that 

 State to provide funds for carrying on the investigation, the work of the Survey has 

 come to a standstill. Through the kindness of Professor Dumble, I am permitted 

 to publish the paper in the Proceedings of the United States National Museum. 



2 Third Ann. Rept. Geol. Survey of Texas, p. 227, 1891. 



Proceedings of the United States National Museiuu, Vol. XVIII — Xo. 1072. 



381 



