PALEOZOIC DIASTROPHICS OF THE NORTHERN 

 MEXICAN TABLELAND 



CHARLES KEYES 

 Des Moines, Iowa 



A little below the southern boundary of Colorado the Rocky 

 Mountains, in a triple cluster of canoe-shaped folds, plunge steeply 

 beneath the general plains surface of the northern prolongation of 

 the Mexican tableland, never again to reappear. In marked con- 

 trast to the prevaiHng relief expression of the Cordillera, with its 

 stream-cut profiles, the physiognomy beyond is that of typically 

 enisled landscape of the desert, fashioned mainly by the winds. 

 The chain aspect of the Rockies gives way to solitary ridges. All 

 mountains assume the character of short, lofty ranges which, with 

 startling abruptness, rear themselves like volcanic isles jutting from 

 a summer sea. So thickly do these isolated piles stud the vast 

 smooth plateau plains that Button aptly likens them on the 

 map to an army of caterpillars crawling northward out of Old 

 Mexico. 



Inasmuch as the northern extension of the Mexican tableland 

 is included mainly within the present limits of the state of New 

 Mexico its topographical features, so far as the United States go, 

 are in many respects quite unique. Throughout this region' the 

 areal distribution of the geological formations is probably the least 

 understood of any considerable tract in our country. In only a 

 few circumscribed districts is the geological structure brought out 

 properly and correctly in mapping. Elsewhere, according to 

 published information, the region seems to be a veritable terra 

 incognita. Even the larger relationships of the formations are so 

 little known that they have yet to be exactly determined. 



Over this northern segment of the lofty tableland the general 

 plains surface lies evenly about a mile above the sea. Towering 

 still another mile in the air are the innumerable mountain masses. 

 The intermontane plains being chiefly desert or semi-arid, rock 

 outcrops are few in number; and drifting sands and mobile earths 



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