STANTON: UPPER CRETACEOUS STRATIGRAPHY 61 



San Ygnacio where they have been described in general terms 

 by Herrick*' and Johnson and in greater detail by W. T. Lee.^ 

 The Mancos is here expanded to a thickness of fully 2000 feet 

 and it includes several cliff-making sandstones so that its general 

 aspect is greatly altered (Section No. 3, p. 58.) 



The upper 800 feet or more consist of more or less sandy shale, 

 generally weathering yellowish, with many bands and lenses of 

 soft sandstone. Beneath this is a massive, yellowish, cliff-making 

 sandstone approximately 100 feet thick separated by 350 feet 

 of dark shale from the brownish, shaly, calcareous sandstone 

 which forms the zone of Ostrea lugubris and Scaphites warreni 

 already mentioned. Beneath this zone is another dark shale, 

 about 500 feet thick, followed by Herrick and Johnson's ''Tres 

 Hermanos" sandstone which consists of two distinct massive 

 beds 66 feet and 37 feet thick, respectively, separated by about 

 50 feet of dark shale. Another dark shale, 55 to 60 feet thick, 

 intervenes between the basal bed of the "Tres Hermanos" sand- 

 stone and another somewhat variable sandstone which seems to 

 represent the Dakota, although it is here only from 25 to 40 feet 

 thick. 



The section of the Mancos just described is all well exposed in 

 the west side of Prieta Mesa on the Rio Puerco near the village of 

 Casa Salazar and the sandstones, especially the ''Tres Hermanos," 

 make conspicuous cliffs on frequent exposures for 25 miles 

 along the Rio Puerco. There are minor variations in the thick- 

 ness of individual members but the general character is consistent 

 thruout this distance. According to the faunal evidence the 

 1200 feet of rocks immediately above the Dakota (?) should all 

 be correlated with the Colorado group, and this part of the sec- 

 tion instead of being all shale, as in the typical Mancos, includes 

 at least 200 feet of sandstone in three massive beds. A still 

 further increase in the sandstones of the part of the section cor- 

 responding to the "Tres Hermanos" sandstone and associated 



" Geology of the Albuquerque sheet (New Mexico). Denison Univ. Sci. Lab. 

 Bull. vol. xi, art. ix, pp. 175-239. 



^ Stratigraphy of the coal fields of northern central New Mexico. Bull. Geol. 

 Soc. Am. 23: 571-685. 



