E. Base — Pennian of Coaliuila, N. Mex. 193 



careous cement of the conglomerate, I fear that he must 

 have confounded some other brachiopod with Gypidula; 

 unfortunately he does not give a figure of this specimen. 

 Our faunas are so rich in specimens and so evenly dis- 

 tributed over practically the whole thickness of the 

 Delicias beds, that there does not remain the slightest 

 doubt about the age of the entire series. 



The petrographical character of the Lower Permian of 

 the Sierra del Sobaco is entirely different from anything 

 of the same age which I have seen in the Trans-Pecos 

 region of Texas or in New Mexico. The upper limestone 

 containing RicJithofenia and those of the Pichagua cliff, 

 as well as others in the higher portion of the Delicias 

 beds, resemble to a certain degree the Word limestone of 

 the Glass Mountains, but the former are much more 

 irregular in distribution and far less in thickness. The 

 sandstones and conglomerates composed of igneous 

 detritals are different from anything I have seen in 

 Texas or New Mexico. I may also mention that near the 

 buildings of Las Delicias the Cretaceous rests on an in- 

 trusive mass of Paleozoic age, showing an abraded 

 surface and belonging either to the Permian or to a series 

 immediately below this formation. The intrusive is of 

 syenitic character. It is certainly not a post-Paleozoic 

 intrusion, as it is overlain by a green sandstone composed 

 of its material, about one or two meters thick, and in no 

 place intrudes farther into this sandstone or the Creta- 

 ceous limestone. These conditions are very well exposed 

 at and between the two springs which flow down to the 

 hacienda. 



Notwithstanding these marked petrographical differ- 

 ences, the Permian of Las Delicias is most probably a 

 continuation of the Texas Permian, as is shown by the 

 character of the fossils. 



Our section shows also the reason why the Paleozoic 

 rocks appear here on the surface of the Mesa Central. 

 The whole of the Trias- Jura-Neocomian is missing; we 

 have here probably the southernmost part of the large 

 Trias-Jura continent of Texas and New Mexico. The 

 Aptian, represented by the limestone and gypsum zone 

 with Monopleura and Orhitolina texana, rests uncon- 

 formably on the Lower Permian; the beds of the older 

 formation also show a much stronger dip than those of 

 the Cretaceous, and the strike is somewhat different. 

 The Trias-Jura continent did not extend much farther 



Am. Jour. Sci. — Fourth Series, Vol, I, No. 2. — February, 1921. 

 13 



