Mancos 



DEsctitPTlvE details: tijehas coal field 631 



Feet Inches 



C Shale, dark-colored, sandy at the top 1,345 



Sandstone (Tres Hermanos), hard, quartzose ; 

 contains worm borings and indefinite markings 



of various kinds 145 



Shale, dark-colored, with limestone concretions 



( Gastropod zone) 60 



Sandstone ( Dakota ) 65 



Sandstone and shale, variegated (Morrison) 



2,812 6 



The rocks of the Morrison formation are not well exposed, but where 

 seen near San Antonio do not differ in character from those referred to 

 the Morrison at other localities in central New Mexico. They rest on the 

 gypsiferous red beds of the Manzano group of the Pennsylvanian series 

 (112). 



The Dakota sandstone is hard and quartzose, and, together with the 

 Tres Hermanos sandstone, forms a prominent double ridge along the 

 western side of the field where the rocks are upturned to a nearly vertical 

 position. The dark-colored shale of the Gastropod zone is present at this 

 locality, but it is thinner here than it is farther to the north in the Hagan 

 and Cerrillos coal fields. The concretions of limestone found in many 

 places in this shale, and which contain numerous fossils on the Eio 

 Puerco and elsewhere, were observed at San Antonio, but no fossils were 

 collected from them. 



A sandstone 145 feet thick occurs above the shale containing the Gas- 

 tropod zone. No fossils were found in it, but it is stratigraphically and 

 lithologically the same as the Tres Hermanos sandstone of the other 

 fields described in this paper. 



The principal mass of the Mancos shale is thinner in the Tijeras field 

 than it is in the Hagan and Cerrillos fields, but this may be due in part 

 to mechanical thinning which may have taken place during the disturb- 

 ance which upturned the rocks. On the other hand, it may be that the 

 shale originally thinned toward the south as it does toward the west, but 

 the distance of 10 miles between the fields is scarcely enough to render 

 this hypothesis tenable. 



There are no fossiliferous lenses of yellow sandstone below the coal 

 beds in the Tijeras field as there are in the Hagan field. It has not been 

 determined whether this absence is to be explained on the assumption 

 that the top of the Mancos shale of the Tijeras field is equivalent to the 

 sandstone lenses of the Hagan field or whether the coal-bearing rocks of 

 the Tijeras field are equivalent in age to the lenses of the Hagan field 



