the Lower Pecos Valley, New Mexico. 429 



now occupied by the Kocky Mountains and these move- 

 ments were accompanied by extensive intrusions of batho- 

 lithic character outlining many of the present ranges. 

 Similar movements continued and a great series of minor 

 intrusives and some lava flows were injected into the 

 Paleozoic and Mesozoic sediments surrounding the area 

 involved. This period of igneous activity extended 

 throughout the middle of the Tertiary with unabated 

 intensity, and was represented still later (in late Tertiary. 

 Quaternary and Recent times) by the outpouring of the 

 extensive basaltic flows of northern and western New 

 Mexico. 



That the larger intrusives of the Sierra Blanca region 

 are of post-Cretaceous age is evident from the fact that 

 they intrude Cretaceous sediments. Lindgren (et al.) 

 considers 8 all of the monzonitic and dioritic intrusives of 

 similar character to these to be early Tertiary age. If 

 the upper member of the Sierra Blanca is actually of post- 

 Laramie age as intimated by Wegemann 9 some of the 

 dioritic intrusions are undoubtedly late Eocene or even 

 younger in age. From field observations the writer feels 

 that the more basic and less extensive intrusives (diorites 

 and gabbros) are younger than the more acidic intrusives 

 (monzonites, quartz-monzonites, and grano-diorites) 

 forming the mountain masses. This is borne out by the 

 fact that diorite dikes intrude the monzonite near Nogal 

 Peak. Moreover, the fact that these two classes of intru- 

 sives are distinct in composition and mode of occurrence 

 suggests that they represent earlier and later stages in 

 igneous activity. 



Incompetency near Lincoln. — As has been noted the 

 Yeso formation, intruded by the large monzonite intru- 

 sion that forms the Capitan Mountains, is sharply folded 

 though the overlying San Andreas limestone is only 

 slightly tilted. Moreover the Yeso is extensively in- 

 truded by andesite sills that are folded with the adjacent 

 strata. This shows quite conclusively that the folding 

 must have taken place subsequently to the intrusion, that 

 is in early Tertiary time. The overlying limestones, how- 

 ever, deposited in the Permian have not been folded. 

 The only adequate explanation of such structural condi- 

 tions that suggests itself to the writer is that the rela- 

 tively incompetent Yeso shales, gypsum beds, and lime- 



8 Waldemar Lindgren, L. 0. Graton, and C. H. Gordon : The Ore Deposits 

 of New Mexico, U. S. Geol. Survey, Prof. Paper 68, pp. 35-40, 1910. 



9 Op. cit. p. 428. 



