310 It. T. Hill — Cretaceous Formations of M exico. 



abundant paleontologic evidence show that it all represents the 

 same great geological formation of Lower Cretaceous age 

 instead of Upper Cretaceous and Paleozoic age, as frequently- 

 asserted, and that it is the southern continuation of the greatly 

 altered, thickened and folded beds of the more chalky lime- 

 stones of the Comanche Series of the Texan region. 



The latter can be continuously traced from the region of 

 their typical occurrence in the sub-coastal plains of central 

 Texas to the mountains west of the Pecos (notably the Quit- 

 man mountains) where they are intensely folded and altered 

 from their white chalky aspect into the firm blue limestones 

 above mentioned. This limestone becomes the characteristic 

 mountain rock from Trans-Pecos Texas southward into Mexico 

 where they have been seen by the writer in the mountains 

 in the States of Coahuila, Chihuahua, Kueva Leon, Tamau- 

 lipas and San Luis. From the writings of many authors the 

 wide extent of the " Hippurite limestone," as it is called by 

 Heilprin, can be traced over the States of Sonora, Coahuila, 

 Nuevo Leon, San Luis Potosi, Hidalgo, Yera Cruz, Puebla, 

 Mexico, Morelos, Guerro, Jalisco, Queretero, Colima, Aguas 

 Calientes, and Mihoacan, which with the northern border 

 States previously mentioned and Trans-Pecos, Texas, makes 

 its distribution very general over the Republic of Mexico. 

 Prom the abundant paleontologic evidence there is no doubt 

 but the outcrops in northern Mexico are identical with similar 

 masses described in the southern end of the plateau region by 

 Barcena, Castillo, Yirdet, Heilprin, and Felix and Lenk. 



Owing to the great alteration the limestone has undergone 

 the fossils, with the exception of a few genera, are usually 

 much obliterated or converted into masses of calcite, but in 

 most cases the form of some familiar Gryphaea, Exogyra, 

 ISTerinea, or Pudistes of the Comanche series can be deter- 

 mined. This alteration of the chalky Cretaceous rocks of 

 Texas into the Paleozoic-looking limestone of northern Mexico 

 was first noted in this Journal by Kimball* who called atten- 

 tion to the fact that the mountain limestones of northern 

 Chihuahua were of Cretaceous instead of Paleozoic age as had 

 been asserted by Wislizenus,f and others. 



Subdivisions. — Notwithstanding the wide distribution of 

 this formation throughout Mexico no detailed section of it has 

 been presented and no attempt made to define its subdivisions 

 with the exception of the recognition by Felix and Lenk of 

 the two horizons: the "Hippurites" limestone and Mono- 

 pleura (Tehuacan) beds above mentioned ; while the latter 



* Notes on the Geology of Western Texas and of Chihuahua, Mexico. This 

 JourDal, Nov., 1869. 

 f Quoted by Kimball. 



