82 . CHARLES KEYES 



these planation periods are clearly indicated by marked unconform- 

 ities displayed in the general stratigraphic succession of the region. 

 When the great peneplanation of Jurassic or early Cretaceous times 

 took place, as marked by the basal surface of the extraordinarily 

 widespread Dakotan sandstones, it was accompanied by extensive 

 and diverse deformation with some display of volcanic action. So 

 extensive was this main evening that so far south as central New 

 Mexico the Dakotan sandstone is seen to repose upon the upturned 



■Seo.L^ 



Fig. 3. — Unconformable relations of Cretaceous and Pennsylvanian beds on 

 Chupadera Mesa, New Mexico. 



and locally vertical edges of the Pennsylvanian limestones. A most 

 notable section demonstrating these relationships is well displayed 

 on the Chupadera Mesa, at Dios Springs, and in the Arroyo 

 Chupadera, about thirty miles northeast of Socorro (Fig. 3) . 



The fancied complexity of the stratigraphy of the Mexican 

 tableland is therefore more apparent than real. Two features in 

 particular tend to obscure the actual mass relationships of the 

 formations. Of these the wide separation of exposed rock sections 

 assumes an importance out of all proportion to its difficulties or its 

 merits. The phenomena attending diastrophic movements in the 

 region are thus liable to serious misinterpretation. Neither the 



