Keyes — Dakotan Series of Northern New Mexico. 125 



so far as to discuss in the most positive manner sections which 

 they had never been near. Instead of clearing up the points 

 under discussion, this long drawn out controversy only served 

 to make the entire question more obscure. 



When I was first suddenly made acquainted with the 

 Cretaceous formations of the region, it was in the field, 

 before it was possible to consult carefully very much of 

 the literature on the subject. In mapping and in local de- 

 scriptions in the northern New Mexican province I assumed 

 the Dakota sandstone to be the great massive plate of yellow 

 sandstone about 500 feet in maximum thickness. Above it 

 were the Colorado shales and beneath in many places a pecu- 

 liar succession of sandy shales, shaly sandstones and clay 

 shales. Several papers were even published on JSTew Mexican 

 geology in which this idea of the Dakota formation of the 

 region was expressed. The chief reasons for considering this 

 great plate as a formation by itself and as representing the 

 Dakota sandstone were (1) that it immediately underlay the 

 Colorado shales, which were well identified by numerous fossils, 

 and (2) that the sandstone rested in marked unconformity 

 upon the formations beneath. When, later, the literature was 

 gone over carefully in order to compare the published observa- 

 vations of others with my own, it was with much surprise that 

 I found that prevailing opinions included in the Dakota sec- 

 tion a much greater sequence than I had done. This led 

 immediately to a detailed examination of many of the more 

 critical of the described sections ; and the location of the real 

 difficulties of former interpretations. 



The use of the term Dakotan series for the sequence of 

 massive yellow sandstones which form the bottom of the Cre- 

 taceous section over the greater part of New Mexico is based 

 upon the accepted terminology of the general Mesozoic section 

 of the Rocky Mountain region. As a definite geologic title 

 the name Dakota was first applied by Meek and Hayden,* in 

 1862, to the basal member of the Cretaceous of the Upper 

 Missouri River district. Although included in their "Earlier 

 Cretaceous " division, this is not the Early Cretaceous division 

 as at present understood, but is the base of what has long been 

 known as the " Upper Cretaceous." In the general geological 

 section the formation belongs properly to the Mid Cretaceous 

 period. 



As the entire succession of the Mid Cretaceous and Late 

 Cretaceous formations is upturned along the eastern flank of 

 the Rocky Mountains, the Dakotan division is readily traced 

 from the original locality southward into central New Mexico, 

 and the title given to the series in the north appears to be 

 *Proc. Acad. Nat. Sci., Phila., vol. xiii, pp. 510-520, 1862. 



