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



these fossils belong to the fauna occurring in the coal measures in the 

 Eio Puerco field, which in turn are referred by him to the lower part of 

 the Montana. 



Shimer and Blodgett describe the Mancos shale from several localities 

 between Cabezon and the Eio Puerco field. They report a characteristic 

 Benton fauna from low in the shale corresponding to the Concretion 

 (Septaria) zone of Herrick and Johnson, and a separate fauna from 

 higher in tliis shale supposed to correspond in general with the Caphalo- 

 pod zone of Herrick and Johnson. The upper zone of these writers con- 

 tains a large number of Benton species, together with a few which they 

 regard as characteristic of the Pierre (see 109). Their work serves to 

 connect the formations observed near Cabezon with those of the Eio 

 Puerco field, for although the Mesaverde is not continuous between the 

 two fields the Mancos is continuous and the overlying Mesaverde occurs 

 in Prieta Mesa and probably also at Cabezon Butte, so that there are only 

 three short breaks in its continuit}" between the large area occupied by 

 this formation in the San Juan Basin and the smaller area in Eio Puerco 

 field. 



RIO PUERCO FIELD 



Aside from brief references in the accounts of early explorations to the 

 occurrence of coal, the first geologic information of the Eio Puerco coal 

 field was given by Herrick and Johnson (T7). The various zones de- 

 scribed by them and the fossils contained in each zone are named in the 

 notes of the annotated list of publications (77) in the latter part of this 

 paper. These zones were found convenient for use by the present writer 

 and are here used in a quotational way. 



The Cretaceous rocks of this field dip toward the east, the degree of 

 dip varying greatly from place to place, and disappear under a cover of 

 Tertiary and Quaternary rocks in the Eio Grande Valley. A section 

 showing the main features of the Cretaceous rocks below the Mesaverde 

 was measured near San Francisco. The line along which the measure- 

 ments were made extends across the gently dipping rocks from the Da- 

 kota (?) outcrop northwest of San Francisco to Punta de la Mesa farther 

 to the south. The fossils collected from the several horizons are named 

 in the section: 



Section of Rods measured in the Rio Puerco Coal Field near San Francisco, 



New Mexico 



Feet 

 Sandstone, massive, yellow (Punta de la Mesa sandstone or base of 



Mesaverde) 77 



Shale, sandy at top 240± 



Limestone, shaly, containing Ostrea luguhris Conrad and Coilopoceras 



Qolleti Hyatt (U. S. G. S. locality No. 3515) 10± 



