422 



D. R. Semmes — Tertiary Intrusives of 



Mountain. From this point eastward it trends more to 

 the north running approximately N 86° E for a few miles 

 and then N 78° E until lost in the sands. 



The trend of the dike is approximately normal to the 

 general strike of the formations through which it cuts, and 

 in consequence the side walls vary in character with the 

 formation. The contact zones are comparatively narrow, 

 in some cases the contact effects being only recognizable a 

 few feet from the contact. In other places typical contact 

 zones are to be seen, the walls being thoroughly recrystal- 

 lized and many of the more common contact minerals 

 being developed. In most part, however, the effect of 



Fig. 3. — Eailroad Mountain dike as exposed along the eastern banks of 

 the Pecos Kiver. Looking east. 



intrusion seems to have been merely a slight baking of the 

 country rock. Numerous prospect holes have been dug 

 along the entire length of the dike but little mineralization 

 seems to have occurred. A few samples showed very low 

 gold values. 



The dike rock is a massive,, dense, dark blue, medium- 

 grained, granitoid rock composed of pyroxene (diallage) 

 and olivine in a felt-like mass of interlocking lath-shaped 

 crystals of plagioclase (fig. 4). Considerable magnetite 

 is scattered throughout, which is undoubtedly a primary 

 constituent. Almost no secondary alteration other than a 

 slight amount of sericitization has occurred, a fact that 

 explains the resistance of the dike rock to erosion and its 

 consequent occurrence as a prominent topographic fea- 



