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



making sandstone rests with uneven base on shale, and in the lower part 

 of this sandstone was found a large bone, apparently a shoulder-blade. 

 Only a small part of the bone was secured, and on examination it proved 

 to be a Dinosaur bone, but no more definite identification was possible. 

 This sandstone was not observed over a wide enough area to assert that 

 it rests unconformably on the "Laramie," but the abrupt change in 

 lithology and the uneven base of the sandstone suggest that it may be 

 the lowest of the Tertiary formations. The presence in it of a Dinosaur 

 and its position below the Animas formation, which has usually been 

 regarded as equivalent in age to the Denver formation, suggests the pos- 

 sibility that the sandstone may be the time equivalent of the Arapahoe 

 formation of the Denver Basin. This sandstone does not seem to be 

 present on Florida Eiver, where Doctor Gardner measured his section, 

 unless the white sandstone at the top of the "Laramie" of that section, 

 which is not always present, represents it. 



The Cretaceous formations of the Durango region and their age rela- 

 tions are shown in tabular form below. The member and zone names 

 used first in the Eio Puerco field have been added to this table, and also 

 the Tertiary formation, so that the table expresses the age relations of 

 all of the fields described in the following pages. With the exception 

 of the Animas beds, there is no doubt of the Tertiary age of the rocks 

 above the "Laramie" in the San Juan Basin. In the other fields de- 

 scribed rocks of similar appearance and composition hold the same strati- 

 graphic position, but their Tertiary age has not been proved. A massive 

 sandstone, probably equivalent to the Pictured Cliffs sandstone member, 

 was observed near Dulce and at the southern outcrop of the "Laramie" 

 of the San Juan Basin northwest of Cabezon. The Punta de la Mesa 

 sandstone member is the base of the Mesaverde in the Eio Puerco field. 

 A similar sandstone occurs at the base of this formation in all the other 

 fields described. The Cephalopod zone was first described in the Eio 

 Puerco field, but was recognized also in the Hagan and Cerrillos fields. 

 The Concretion (Septaria) zone was described first from the Eio Puerco 

 field (77) as occurring "sometimes above and sometimes below^^ the Tres 

 Hermanos sandstone. It seems to be of doubtful value as a horizon 

 marker beyond the limits of the Eio Puerco field. The Tres Her- 

 manos sandstone is typically developed in the Eio Puerco field, but is 

 readily recognized in the Tijeras, Hagan, and Cerrillos fields, and is 

 probably represented throughout the San Juan Basin. The Gastropod 

 zone is best developed in the Eio Puerco field. It is sparingly fossilifer- 

 ous in the Cerrillos and Hagan fields, and in the Tijeras field is repre- 



