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



shale and seems to be the time equivalent of the Cephalopod zone of the 

 Eio Puerco field. The fauna of the Cephalopod zone occurs in some of 

 the concretions formerly supposed to indicate the Concretion (Septaria) 

 zone, and there seems to be confusion regarding the latter zone. It is 

 probable that some of the two invertebrates described from the latter 

 belong to the Cephalopod zone. 



The determination of the place at which the top of the Mancos should 

 be drawn in the time scale involves some difficult questions. In its type 

 area in southwestern Colorado this shale includes at its top rocks of 

 Pierre age (73). The shale below the coal-bearing rocks in the Cerrillos 

 field seems to have the same range, but in the Eio Puerco region fossils 

 indicative of Benton age occur near the top of the shale. According to 

 Mr. Schrader and Doctor Gardner, previously quoted, the Mancos shale 

 was traced from the Durango region east and south to Cabezon. But as 

 has formerly been stated, the upper part becomes sandy farther south, 

 and in the Eio Puerco field is not readily separated from the overlying 

 Mesaverde. 



East of the Eio Grande the Mancos shale is much thicker than it is in 

 the Eio Puerco field. .At Hagan it has a measured thickness of 2,116 

 feet. A characteristic Benton fauna occurs in the lower 700 feet or 

 more of this shale, but the upper part contains fossils that range 

 upward through the Mesaverde. The few fossils found at the top of 

 the Mancos near Hagan were close to the basal sandstone of the Mesa- 

 verde, but in the Cerillos field this fauna seems to extend downward 

 200 feet or more into the shale. In this latter field the Mancos has 

 a measured thickness of 2,402 feet. The lower part of it is clearly of 

 Benton age and some of the rocks are probably of Niobrara age. The 

 upper part contains a great number of fossils that belong in the fauna 

 of the Lower Montana. This occurrence of Montana fossils below the 

 Mesaverde necessitates the reference of the rocks containing them to a 

 horizon near the base of the Pierre. In order to bring out more clearly 

 the fact that the Mancos shale on the Eio Puerco is faunally similar to 

 only the lower part of this shale farther east, the fossils from the lower 

 part are listed separately from those of the upper part, although all 

 belong to the same formation. 



The fossil invertebrates of the Mancos are included with those of the 

 Mesaverde and the Lewis in the following table, and their general dis- 

 tribution is indicated therein. Unfortunately, a great many of the spe- 

 cies have never been described, and their names as published in the table 

 are of little value in correlation. However, in a report on the fossils 



