R. T. Mill — Cretaceous Formations of Mexico. 319 



or Meek and Hayden's series, entirely different in lithological 

 and paleontological aspects. Dr. C. A. White and the writer 

 have frequently mentioned this formation in the Rio Grande 

 region, but no attempt has been made to clearly define its 

 geographic limitations. 



In distinction from the characteristic mountain limestone of 

 the Lower Cretaceous, the Upper Cretaceous formation in 

 northern Mexico is characterized by its more shallow deposits 

 of ferruginous limestones, clays, sands, and lignite all indica- 

 tive of a more shallow origin than the purer lime beds of the 

 main part of the lower series. The beds occur in the north- 

 eastern border States of Chihuahua, Coahuila, and Tamauli- 

 pas, and are at least 5000 feet thick ; the complicated structure 

 of the region does not permit the entire section to be measured 

 in any one locality. The areal development of this formation 

 is principally along the mountains composing the east front of 

 the Cordilleran region between Presidio del Norte at the great 

 bend of the Rio Grande and Tampico and along the margin 

 of the plains adjoining them on the Atlantic side as shown in 

 fig. 2. They are upturned in the mountain structure with 

 the Lower Cretaceous limestones, but owing to their softer 

 nature they are usually degraded from the summits down to 

 the level of the plain where they occur as foot hills of the 

 Sierra de Santa Rosa, Sierra Candella, Sierra Lampazos and 

 other ranges. It is especially developed in the Rio Grande 

 embayment between the great Balcones fault of Texas and the 

 eastern front of the Coahuila Cordilleras. It continues north- 

 ward in the structure of the Trans-Pecos mountains of Texas 

 and New Mexico and in the Rocky Mountain system as the 

 typical Upper Cretaceous and Laramie series of western United 

 States. Eastward it continues as the Cretaceous and Eocene 

 Lignitic formations of the southern coastal plain. 



The subdivisions of this series, which as a whole is a contin- 

 uous deposition, are not distinctly differentiated in this south- 

 western region ; the deposits apparently represent the south- 

 western littoral of the great interior arm of the Atlantic in Upper 

 Cretaceous time and do not extend across the Mexican peninsula 

 to connect with the Pacific as did the Lower Cretaceous waters. 

 The Dakota horizon has no true representative in the region, 

 south of the 32nd parallel, but the Benton shales and clays with 

 the typical Inocerarnus prolAematicxis and Scaphites occur 

 near Juarez and in El Paso, Texas. The chalky beds of the 

 Niobrara sub-epoch are also missing and the whole of the Nio- 

 brara-Pierre of the series is apparently represented by thinner 

 ferruginous clays and impure limestones marked by a com- 

 mingling of the characteristic fauna and the Exogyra jponde- 

 rosa of the Southern States. 



