248 



CHARLES KEYES 



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beneath the general plains' surface of the 

 Mexican table-land the Dakotan sand- 

 stone forms a magnificent inward-facing 

 escarpment. Along the steep interior face 

 the Atchison, Topeka, & Santa Fe Rail- 

 road skirts for a distance of fifty miles. 

 There, as elsewhere on the continent, the 

 Dakotan sandstone is one of the geologi- 

 cal landmarks of the region. With a 

 thickness of 300 feet it is, perhaps, the 

 most extensive basal sandstone of which 

 we know (Fig. 6) . 



Immediately following the Dakotan 

 sandstone is a fine succession of marine 

 shales and Hmestones, which in New 

 Mexico have a thickness of upward of 

 2,000 feet. This is the Coloradan series. 

 A quite unusual feature is that the 

 series preserves its subdivisional integrity 

 through a distance of 1,000 miles, to 

 Iowa and Minnesota, on the far side of 

 the Continental Interior basin. 



A succeeding 2,000 feet of strata com- 

 prising sandstones and shales constitute 

 the Montanan series. It is an impor- 

 tant coal-bearing formation, containing a 

 larger fuel supply than do the coal meas- 

 ures of the entire Mississippi Valley. In 

 early accounts of the region these coals 

 were all regarded as Laramian in age. 

 Locally the coal seams overlying the Ortiz 

 laccolith are changed into high-grade 

 anthracite, the quality of which compares 

 favorably with the best hard coals of the 

 Appalachians. East of the Rockies the 

 Montanan beds merge into the marine 

 Pierre shales. 



