GEOLOGICAL SETTING OF NEW MEXICO 247 



stones separated by an erosion interval. The earlier of the two 

 series is confined to the east side of the Rockies, while the later one 

 is represented chiefly on the west side of the Cordillera. Their 

 total thickness is nearly 2,000 feet. 



Like the Triassic sediments the Jurassic succession comprises 

 an earUer and a later series, set off from each other by a great 

 erosional unconformity. Together they represent a column 

 1,200 feet thick. The Morrisonian series is chiefly composed of 

 argillaceous deposits. Of these the Chaquaqua (Chicago) shales 

 occupy one-half ©f the entire section. They are underlaid by a 

 basal sandstone. A notable erosion unconformity separates them 

 from the succeeding Comanchan deposits. 



When in the early fifties of the last century Jules Alarcou, 

 colleague and exiled compatriot of Louis Agassiz, fresh from the 

 Jura Mountains, introduced into this country the European title 

 "Jurassic" it was the Tucumcari section in eastern New Mexico 

 that was involved. Around this Cerro Tucumcari raged a storm 

 center for a full generation. After being overborne by sheer 

 weight of numbers and after being submerged for seventy-five years 

 Marcou finally comes into his own. Cerro Tucumcari proves to 

 contain the full Jurassic section of the region. Marcou erred only 

 in including some of the Dakotan sandstone layers. 



The important Comanchan succession of Texas and the Gulf 

 region finds but feeble representation in New Mexico, and then 

 only as an attenuated border which soon vanishes completely 

 against the Cordilleran uplift. The basal Hmestones of Texas are 

 traceable around the Llano Estacado. The shales extend farther. 

 Erosional unconformity separates its two series, and like sedi- 

 mental breaks mark both top and bottom of the sequence. Because 

 of the fact that the thin shale beds are immediately under the mas- 

 sive Dakotan sandstone they usually escape notice. 



The basal Dakotan sandstone of the Cretaceous succession is 

 especially noteworthy by reason of its very wide geographical dis- 

 tribution. In the Rocky Mountain region it is further remark- 

 able because of its disposition on an old peneplain surface, which, 

 although much deformed, now coincides with the tops of the 

 highest peaks. Where the southern Rockies plunge in a triple fold 



