428 D. R. Semmes — Tertiary Intrusives of 



Structure. 



The structure of this portion of southeastern New 

 Mexico is that of a gently inclined east-dipping mono- 

 cline rising gradually on the west from the Pecos River 

 and the Eed-bed area beyond to the crests of the Sacra- 

 mento Mountains, and there breaking off forming steep 

 scarps to the west. Toward the lower part of these 

 escarpments, near Alamogordo, the Paleozoic series from 

 the Pennsylvanian down lies conformably beneath the 

 Permian and rests upon the pre-Cambrian granites. 



On the eastern side of the divide the exposed members 

 of the Manzano group show structure in wide contrast to 

 that on the western slope. The pronounced unconform- 

 ity below the San Andreas limestone is very striking. 

 Fan-shaped and overturned folds frequently occur, the 

 whole series being repeated many times within a short 

 distance. The lower portion of the Yeso series is unex- 

 posed but at least 800 feet of these folded sediments can 

 be recognized. Nearer the Pecos River in the eastern 

 portion of the district the general east dip of the Permian 

 strata is broken by occasional sharp and elongated anti- 

 clinal ridges, the best example being the Border Hills, 25 

 miles west of Roswell. These anticlines are similar in 

 strike and in structure, being sharp, fan-shaped folds, 

 expressed in the surface as prominent ridges, sometimes 

 rising several hundred feet above the surrounding coun- 

 try, and showing a general trend of N 10° E to N 40° E. 



Toward the west of this eastward-dipping monocline of 

 Carboniferous strata, and encircling the peaks of the 

 Sierra Blanca, the Cretaceous coal-bearing strata lie in a 

 structural basin some thirty miles long (N-S) and twenty 

 miles wide. A large amount of igneous material showing 

 variations in composition and mode of occurrence has 

 been intruded into these sedimentary series. This 

 igneous material can readily be divided into those larger 

 intrusives of more acidic composition that form the Sierra 

 Blanca, Capitan, Jicarilla and Pajarito Mountains ; and 

 the smaller or more basic intrusives such as have been dis- 

 cussed, which dissect and permeate all of the sedimentary 

 rocks of the area. These larger intrusives are probably 

 older than many of the smaller ones, for near Nogal Peak 

 the larger monzonite intrusive is cut by later diorite ( ?) 

 dikes. 



Age of the Intrusives. 



Toward the end of the Cretaceous mountain-making 

 movements were inaugurated throughout the entire area 



