328 Richardson — Stratigraphy of the upper Carboniferous 



tains, they being more than 50 miles distant from the nearest 

 known Carboniferous outcrops farther north, in the vicinity 

 of the Texas and Pacific Railway, where what are presumably 

 equivalent beds are all limestone. The section in the Chinati 

 Mountains compared with those described below illustrates 

 the variability of the upper Carboniferous deposits in the 

 area under consideration. 



Section in Rio Grande Yalley, New Mexico (No. 4, figs. 1 

 and 2). — Rocks of Carboniferous age have long been known to 

 occur in the ranges on both sides of the Rio Grande in New 

 Mexico, although the section there exposed is not nearly as com- 

 plete as farther east. Gordon, Lee and Girty* have recently 

 summarized the knowledge of the upper Carboniferous in 

 that region and they divide the rocks into the Magdalena and 

 Manzano groups. 



The Magdalena group, the older of the two, consists chiefly 

 of limestone but it contains minor beds of shale and sandstone. 

 Gordon states that in Socorro, Valencia, and Bernalillo coun- 

 ties the Magdalena group comprises 1,000 to 1,300 feet of 

 sediments, which he divides into the Sandia formation (con- 

 sisting of sandstone, shale, and limestone) and the overlying 

 Madera limestone. In southern New Mexico the Magdalena 

 group is chiefly limestone and its outcrop in the San Andreas 

 Range, save for minor interruptions caused by intrusive igne- 

 ous rocks and by deposits of unconsolidated Quaternary debris, 

 can almost be traced into the Hueco limestone on the western 

 flanks of the Franklin Range in Texas. Pennsylvanian fos- 

 sils from the two limestones confirm this correlation. The 

 Magdalena is separated from the overlying Manzano group by 

 a local unconformity which is not persistent throughout south- 

 east New Mexico and west Texas. 



In the northern part of Rio Grande Yalley the Magdalena 

 group is overlain by a mass of red beds to which Herrick 

 applied the name Manzano. Lee has recently revised the 

 nomenclature and includes in the Manzano group the follow- 

 ing three formations named, in ascending order : the Abo sand- 

 stone, consisting of a maximum thickness of 800 feet of coarse 

 red to purple sandstone with subordinate shale and earthy 

 limestone ; the Yeso formation, consisting of 1,000 to 2,000 

 feet of alternating vari-colored strata of shale, friable sand- 

 stone, gypsum, and earthy limestone ; and the San Andreas, 

 a massive limestone of variable thickness which overlies the 

 red beds in the southern part of the Rio Grande region. 

 Farther north the San Andreas limestone becomes less promi- 



* Gordon, C. H., Notes on the Pennsylvanian Formations in the Rio 

 Grande Valley, New Mex., Jour., of Geology, vol. xv, 1907, pp. 805-816; 

 Lee, Willis T., and Girty, Geo. H., The Manzano Group of the Eio Grande 

 Valley, New Mex., Bull. U. S. Geol. Survey, No. 389, 1909. 



