0. i?. Keyes — Triassic System in New Mexico. 427 



assic system. It is not impossible that it is Jurassic or Creta- 

 ceous in age. It is the beginning of a great formation which 

 extends a long distance to the northwestward into Arizona, 

 western Colorado and Utah and which has been regarded as 

 representing the Jurassic period. 



East of the Rio Grande a very marked plane of unconform- 

 ity separates the Dakota sandstone from the Red Beds beneath. 

 This break in sedimentation represents a profound erosion 

 period, to which more detailed reference is made in another 

 place 



The stratigraphic extent of the Triassic strata in eastern 

 New Mexico embraces about 500 feet of the general geological 

 section. In the west the vertical measurement is very much 

 greater. Dutton places it at 3,500 feet. 



As regards correlation of the Triassic formations, that por- 

 tion of the Red Beds which has been regarded as of Triassic 

 age may be compared, on the one hand, with the cognate beds 

 of the Texas section, and on the other with the enormous 

 thicknesses of Triassic strata in Arizona, Utah and Colorado . 



The standard section of the Triassic in New Mexico should 

 be considered typically developed in the northwestern portion 

 of the region, where the section is most complete and most 

 extensive. In comparing this sequence with the Texas, Okla- 

 homa and Kansas sections of the Red Beds there are presented 

 some difficulties of an unusual kind. The planing off of the 

 folded Paleozoics including the Red Beds in great part, during 

 the erosion interval which existed in the eastern New Mexico 

 region just before the deposition of the Dakota sandstone, 

 removed a very large portion of the formation. 



As at present understood, the general relationships of the 

 Carboniferous part of the Red Beds, the Triassic Red Beds, 

 and the associated formations are best indicated by diagram as 

 given below. 



TERTIARY" 



Fig. 1.— Relationships of the Triassic Formations in the southern Rocky 

 Mountains. 



The cross-section traverses New Mexico in a nearly east and 

 west direction, passing through the Cerro Tucumcari and the 

 Zuni mountains. t . 



Owing to mountain-making movements which took place m 

 the region in the latest Carboniferous or in Early Cretaceous 



