It. T. Hill — Cretaceous Formations of Mexico. 321 



ance of continuity until towards its close, and deposited at a 

 marine base level now occupied by the eastern masses of the 

 Rocky Mountains and eastern Sierra Madre. 



None of the geologists — Schott, White, Penrose — or the 

 writer, who have seen the continuous section of this formation 

 exposed along the Rio Grande, from the unmistakable Ammo- 

 nite horizon of the Cretaceous at Eagle Pass to the typical 

 Cardita planicosta Claiborne Eocene horizon at Laredo, 

 Mexico, have defined or recognized any distinct break in 

 the continuity of sedimentation, but on the contrary every 

 evidence of rapid estuarine or littoral deposition. In the midst 

 of this section occur the fossiliferous horizons which White* 

 has determined to be typical Laramie species, identical with 

 those of the Colorado region. Some of these for instance, 

 Ostrea Wyoming ensis Meek and Anornia micronemia Meek 

 begin in the Ammonite-bearing Upper Cretaceous of Santa 

 Rosa and Eagle Pass. 



The Montana-Laramie Eocene portion of the formation 

 occupies the vast synclinal basin of the Rio Grande, east of the 

 great bend, which I have termed the Rio Grande Embayment, 

 outcropping beneath the detrital deposits of late Tertiary and 

 Pleistocene age. This synclinal is the contact of the Great 

 Plaius, Coastal Plains and Rocky Mountain regions as shown 

 in fig. 3. The strata at the southern edge of the valley near 

 Santa Rosa can be seen at the foot of the mountain upturned 

 apparently conformably with the Comanche limestone which 

 composes its mass, thus showing that the beds were involved in 

 the great mountain movement of the east front of the Mexican 

 Cordilleras. Likewise the strata of the Montana Laramie 

 division are found across the front ranges of the Cor- 

 dilleras, as shown upon the map, in the great enclosed intra- 

 mountain basin between the Sierra Mercado of Monclova and 

 the Sierra Candella, west of Lampazos, where three thousand 

 feet of the strata occur in sub-vertical escarpments of hori- 



West. -^'^'^F^fssggg gs?^^-- ^ _ E as t. 



Cross section of the Sierra Candella west of Lampazos, Mexico, showing par- 

 ticipation of Laramie strata in folding of the earlier Cordilleras. A, Intrusive 

 Diorite. B, Comanche limestone. C, Laramie beds. 



zontal or slightly dipping strata (see fig. 2). Around the 

 edges of this synclinal basin the Laramie strata are again up- 

 turned as if they once extended over the mountain mass and 



*This Journal, vol. xxv, 1883, p. 207. 



