D. R. Semm.es — Tertiary Intrusives. 415 



Art. XXVIII. — Notes on the Tertiary Intrusives of the 

 Loiver Pecos Valley, New Mexico; by Douglas R. 

 Semmes. 



Location and Topography 

 Stratigraphy: Areal Geology. 



Stratigraphic Section ; 



Magdalena Group 



Abo Eed Sandstone 



Yeso Formation 



San Andreas Limestone 



Eed-beds 



Cretaceous Series 



Tertiary (?) Conglomerates 



Caliche 

 Intrusives 



Eailroad Mountain Dike 



Devil's Eacetrack Dike 



Dunlap Sill 



Sills in the San Andreas in Euidoso Valley 



Sills in the Yeso near Arabela 

 Structure 

 Age of the Intrusives 



Incompetency near Lincoln 



Age of the Folding in the Eoswell Area 



Introduction. 



In the central part of Chaves County remote from other 

 evidences of igneous activity there occur two large and 

 prominent dikes and an igneous sill intruding the Permian 

 Red-beds that form the surface throughout the area. The 

 two dikes are to be found on the eastern side of the Pecos 

 in that comparatively level area broken only by occasional 

 low sandstone — or caliche-capped mesas, that extends 

 from the Pecos River to the bluffs of the Llano Estacado, 

 some fifty miles to the east. Neither dike can be traced 

 across the Pecos, but to the northwest on the western side 

 of the river, as indicated on the accompanying map (fig. 

 1), there occurs a sill, now exposed by erosion to form a 

 prominent north-facing scarp, which from its lithologic 

 character appears to be related to the dikes, though there 

 can be found no actual connection between them. 



The topography of this part of the Pecos Valley is the 

 characteristic red-bed type of mesas, " breaks' ' and inter- 

 vening mesquite and cactus covered plains. The eleva- 

 tion of the river in this portion of its course is about 3,500 

 feet, from which the country rises rather rapidly to the 

 peaks of the Sierra Blanca on the west, almost 12,000 feet 

 above sea-level, and on the east to the Staked Plains, 



Am. Jour. Sci. — Fourth Series, Vol. L, No. 300. — December, 1920. 

 31 



