612 W. T. LEE STRATIGRAPHY OF COAL FIELDS OF NEW MEXICO 



The lower part of the shale overlying the Dakota sandstone is clearly 

 of Benton age, and Niobrara is probably also represented, although these 

 formations are not so clearly separable as they are in southern Colorado. 

 The upper part of the shale contains a Pierre fauna, but it is not certain 

 that this fauna is indicative of latest Pierre time. On the other hand, it 

 contains a fauna very similar to that of the Lewis shale at Dulce on the 

 opposite side of the mountains. If it be assumed that the highest coal- 

 bearing rocks at Dulce are of Laramie age, the correlation would make 

 the top of the Pierre of the Eaton field and the top of the Lewis at Dulce 

 equivalent to highest Montana and tend to place the lower coal group of 

 the Eaton field in the Laramie; but the fossil plants above the marine 

 rocks at both localities indicate Montana rather than Laramie age. It is, 

 therefore, probable that the shale does not at either locality represent 

 latest Montana time. 



Some geologists have referred the Trinidad sandstone to the Fox Hills, 

 but no unquestioned Fox Hills is known south of Colorado Springs. The 

 fossils of the Trinidad, named in the foregoing section, belong to the 

 Montana fauna, but are not of distinctive Fox Hills species. This fact 

 indicates that the Trinidad may be equivalent in age to some part of the 

 Pierre of other localities, and that Fox Hills time may be represented by 

 something younger than Trinidad, perhaps in part by rocks that were 

 entirely removed during the post-Cretaceous erosion. The flora of the 

 lower coal measures, described as Montana in age and similar to that of 

 the Mesaverde of the central New Mexico fields, resembles in a lesser 

 degree the flora of the so-called "Laramie" of southwest Colorado and 

 northwest New Mexico — a flora which Doctor Knowlton maintains is 

 "Montana and not Laramie." The evidence seems to indicate that the 

 coal measures below the unconformity in the Eaton field belong in the 

 Montana somewhere between the Mesaverde of the central New Mexico 

 fields and the so-called "Laramie" of northwestern New Mexico. 



Post-Cretaceous Unconfoemity 



Evidence has previously been presented (111 and 119) to prove that 

 the unconformity in the Eaton field is equivalent in time to the post- 

 Laramie unconformity of the Denver Basin, and that the erosion repre- 

 sented by it was comparable in amount in the two regions. The youngest 

 rocks left in the Eaton field by this erosion are the Montana coal meas- 

 ures. If younger Cretaceous formations ever existed there, they were 

 eroded away. The formation next above the unconformity is correlated 

 by fossils as well as by stratigraphic position with the Arapahoe and 

 Denver formations. 



