426 D. R. Semmes — Tertiary Intrusives of 



Capitan Mountain and the other andesitic sills occurring 

 in the Ye so series in that locality. 



The Dunlap sill is approximately eighteen feet in thick- 

 ness and shows two distinct phases ; an upper differentia- 

 tion phase about a foot or less in thickness, which is decid- 

 edly porphyritic, the feldspar occurring as phenocrysts, 

 and of a composition approximating that of a monzonite. 

 Beneath the upper phase lies the main body of the sill ; a 

 medium-grained dark resistant igneous rock, which from 

 examination by a hand glass appears to have the composi- 

 tion of a diorite. 



The contact effects are insignificant. Beneath the 

 intrusive the baked shales are not over three inches thick 

 though in places some of the effects of the intrusion can be 

 seen in the shales a foot or two below the contact. 



The sill is apparently conformable with the bedding and 

 is gently inclined to the south ; the normal dip in this area, 

 however, is almost due east. 



Sills in the San Andreas in Ruidoso Valley. — In the 

 Ruidoso Valley about fifteen miles southwest of the junc- 

 ture of Eio Ruidoso and Rio Bonito at Hondo the San 

 Andreas limestone and the red shales and sandstones 

 immediately underlying are intruded by numerous sills of 

 general grano-diorite or quartz-monzonite composition. 

 Some of these sills are of considerable extent, traceable 

 for a mile or so in the steep walls of the valley, and vary- 

 ing in thickness up to fifty feet. These sills are of 

 slightly more acidic character than the intrusives that 

 have been described. Quartz is an abundant constituent 

 and they are as a rule of coarse granitoid texture. 



Sills in the Yeso near Arabela. — At the eastern foot of 

 the Capitan Mountains near the Mexican town of Arabela 

 the Yeso series occurs highly folded against the flanks of 

 the Capitan intrusion (fig. 7). For a mile east of the con- 

 tact these formations can be traced, frequently repeated 

 by sharp and sometimes overturned folds. Farther to the 

 east the series disappears under the San Andreas lime- 

 stone, dipping 6° to the east. Between the folded Yeso 

 and the San Andreas there occurs a series of red shales 

 and sandstones about 100 to 200 feet in thickness, presum- 

 ably upper Yeso, which shows comparatively slight fold- 

 ing; a transitional phase from the folded formations to 

 those slightly tilted overlying ones. 



Into this folded series numerous intrusions have taken 



