426 O. B. Keyes — Triassic System in New Mexico. 



who has examined its details rather carefully, says regarding 

 the character of this unconformity : "The slight difference in 

 dip, and sudden change in lithological character of the Triassic 

 beds from the Permian, point conclusively to a break in the sedi- 

 mentation of the two formations. At some localities the Tri- 

 assic beds are overlain by Cretaceous, but generally by Tertiary 

 material. The Cretaceous escarpments or buttes resting on 

 the Triassic beds are often two hundred feet thick, and mostly 

 limestone. The denuding forces that for an immense length 

 of time were cutting these Cretaceous rocks back towards their 

 present limits must have carried away a great deal of the Tri- 

 assic before it was Govered by Tertiary. The strata thus 

 enclosed between two unconformable beds must of necessity 

 vary in thickness, and so we find it varying from a few feet to 

 nearly four hundred feet. Even in localities close together the 

 beds vary considerably in thickness. The average, however, 

 will probably reach two hundred feet." 



Of the appearance of the two formations a short distance 

 east of the New Mexico line the same writer * observes : 

 " The contact between' the Dockum beds and the underlying 

 Permian is clearly marked. Both the color and lithological char- 

 acteristics of the two formations bear a striking contrast. The 

 Permian is a bright red argillaceous sand, slightly shaly, 

 though sometimes massive, is characteristic for stratification 

 planes, and below the top forty feet is interstratified with 

 massive and fibrous gypsum, the gypsum becoming more abun- 

 dant toward the base of the section exposed. The Dockum 

 beds, arenaceous clays, in contact are a yellowish purple or a 

 yellowish red, sometimes decidedly yellowish. The bedding is 

 usually uniform and lacks the stratification planes so character- 

 istic of the Permian. The contrast between the formations 

 along their contact is so great that the contact may be 

 located as far as the eye can see stratification planes in the 

 freshly eroded outcropping bed, or as far as it can distinguish 

 sharply contrasting colors." 



The upper limiting horizon of the Triassic section is well 

 defined. In the east, around the Llano Estacado in the Cana- 

 dian and Pecos valleys, the superjacent formations are beds of 

 the Comanche series of the Early Cretaceous. A little farther 

 to the westward the massive .Dakota sandstone of the, Mid- 

 Cretaceous age is the capping member. West of the Bio 

 Grande there comes in between the Wingate division of the 

 Triassic Bed Beds and the undoubted Dakota sandstone a 

 series of red and white shaly sandstones having a thickness of 

 1,200 feet, the exact age of which is at present not definitely 

 determined. This formation is thought to belong to the Tri- 



* Ibid., p. 241. 



