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



in it, but because of its stratigraphic position and its lithologic character 

 it is referred to the Dakota. 



The so-called Dakota sandstone east of the Rocky Mountains in Colo- 

 rado consists of two plates of sandstone separated by a thin shale. This 

 shale, together with the underlying plate of sandstone, has been proved 

 to be of Lower Cretaceous age, leaving only the upper plate in the Da- 

 kota (92 and 121). The writer has observed similar relations as far 

 south as Las Vegas, New Mexico. However, no rocks of Lower Creta- 

 ceous age are known to exist west of the mountains unless the Morrison 

 be of Lower Cretaceous age and the sandstone between the Morrison and 

 the lowest shale of the Mancos constitutes the Dakota of this paper. This 

 sandstone was found in all of the coal fields here described, and the few 

 observations made on it are presented in the section of this paper de- 

 voted to the presentation of details of the areas examined. 



MANGOS SHALE 



The Mancos shale of central New Mexico includes the rocks, mainly 

 shale, above the Dakota sandstone and below the basal sandstone of the 

 Mesaverde. According to Schrader (100), Gardner (110 and 116), and 

 others who have traced the Cretaceous formations from the Durango 

 region eastward and southward through the San Juan River region into 

 the area described in this paper, this formation is essentially equivalent 

 to the Mancos of southwest Colorado (75). The present writer exam- 

 ined it at three localities in the San Juan Basin, namely, at Durango, in 

 southwest Colorado; at Monero, and at Cabezou, in New Mexico. It is 

 continuously exposed between Cabezon and the Rio Puerco field, but east 

 of the Rio Puerco it disappears under a cover of Tertiary and Quater- 

 nary sand and gravel in the Rio Grande Valley, and nothing is known 

 there of its occurrence and extent. East of the Rio Grande the surface 

 is occupied by Paleozoic and older rocks of the Sandia Mountain block, 

 on the eastern slope of which the Mancos and younger rock formations 

 reappear, so little changed from their appearance on the Rio Puerco that 

 even without the aid of fossils it would be difficult to believe that they 

 were not once continuous between the two fields. But this similarity in 

 the lithology and stratigraphic succession is confirmed by the fossils con- 

 tained in them and leaves little room for doubt that the sea in which the 

 Mancos shale was deposited extended from the San Juan Basin eastward 

 over the Hagan-Cerrillos region. 



When detailed observations are made on the Mancos of central and 

 western New Mexico it will probably be subdivided into at least three 

 formations, but for the purposes of this paper it will be preferable to 



