GEOLOGIC CONDITIONS 215 



of the United States National Museum, has determined to be a part of 

 the jaw of a Pleistocene horse. It is probable that the fossil was not in 

 place at this depth, for the crater has been filled to some extent by the 

 caving of its sides, as previousl}^ described. The horse may have been 

 entombed after the crater was formed, but a more probable explanation 

 is that the bones, buried near the top of the sands previous to the forma- 

 tion of the crater, reached their present position through the falling of 

 the sides. 



About 200 miles farther north, in deposits similar to those found in 

 Kilbum crater. Cope* found a number of Quaternary fossils, and more 

 recently at Mastodon, a siding on the El Paso and Southwestern railroad 

 between El Paso and ISToria and 15 miles south of the Afton craters, 

 bones have been found of two well known Pleistocene species, Mastodon 

 americanus and Equus compUcatus. These bones were found in a 

 railroad cut by an employe of Mr H. J. Simmons, general manager of 

 the El Paso and Southwestern railroad. Mr Simmons gave them to 

 Dr G. B. Eichardson, at whose request they were identified by Dr J. W. 

 Gidley, of the IT. S. National Museum. At El Paso, in beds which be- 

 long to the same horizon as those at the Afton craters and at Mastodon, 

 and which are similar to them in physical character, bones have recently 

 been found of three Pleistocene species, Equus compUcatus, Elephas 

 columhi (Mammoth), and Tapiris liaysii (?). These bones were found 

 by Mr Walter Kock in a gravel pit at El Paso, Texas, and given by him 

 to Doctor Eichardson, f at whose request they were identified by Doctor 

 Gidley. The occurrence of these fossils in the sands proves that at least 

 the upper part of the valley filling is Pleistocene. 



On the other hand, the Santa Ee marls of the Eio Grande region of 

 central New Mexico, described by CopeJ as late Tertiary, do not differ 

 notably in constitution or physical character from the Pleistocene sands. 

 It would be exceedingly difficult without the aid of fossils to distinguish 

 between these formations. 



PRE-QUATERNART FORMATIONS 



The rock floor of the filled valley consists in part at least of Lower 

 Cretaceous limestone and shale. IIill§ discovered the Comanche near 



* B. D. Cope : Geographical explorations and surveys west of the 100th meridian, by 

 Wheeler, annual report. Annual Report of the Chief of Engineers, U. S. Army, for 

 1875, Appendix LL, Washington, 1875, pp. 61. 



t G. B. Eichardson : Science, new series, vol. 25, 1893, pp. 31-33. 



t E. D. Cope : Proc. Am. Phil. Soc, vol. 21, 1884, p. 308. See also Proc. Phila. 

 Acad. Sci., vol. 26, 1874, p. 1471. 



§ R. T. Hill : Bull. Geol. Soc. Am., vol. 2, 1891, pp. 517-518 ; also Am. Jour. Sci., vol. 

 45, 1893, p. 313. 



