the Lower Pecos Valley, New Mexico. 



423 



ture. The rock should be classified as an olivine gabbro. 

 The interlocking feldspar aggregate is quite common in 

 dike rocks (diabases), but the composition is of a more 

 basic character than most of the other intrusives of the 

 area. 



Devil's Racetrack Dike. — This dike extends from Sec. 

 15, T-10-S, R-26-E to Sec. 7, T-10-S, R-29-E, Chaves 

 County, a distance of approximately 15 miles. Like Rail- 

 road Mountain it can be traced eastward until lost in the 



* # «> , r? \ 



'&h> ^ 



i 



Fig. 4. — Photomicrograph of Railroad Mountain dike. 50 X. Showing 

 diallage (D), olivine (O), and magnetite (opaque) in a matrix of inter- 

 locking plagioclase crystals (P). 



Mescalero sands, and to the west it disappears beneath the 

 caliche-capped plains east of the Pecos River, some 15 

 miles northeast of Roswell. No traces of it can be found 

 in the gypsum bluffs that form the east bank of the Pecos 

 about six miles west of the point where it disappears. 

 Devil's Racetrack in contrast to Railroad Mountain forms 

 no topographic feature but on the contrary is sometimes 

 expressed on the surface by a slight depression, the dike 

 rock being more readily eroded than the country rock. 



