1912] 



Baker: Western El Paso Range 



127 



and slopes in all directions from the summit of the mountain in 

 a domical structure. It forms northwardly sloping mesas on the 

 north flank of the mountain and southwardly sloping mesas on 

 the south flank. These mesas have been cut into by deep and 

 narrow V-shaped gullies. Just southwest of the highest peak of 

 Black Mountain is a depression in the basalt some thirty or forty 

 feet deep and approximately three hundred feet in diameter, 

 which may represent the crater from which the basalt was poured 

 out. Both Fairbanks and Hess have expressed the opinion that 

 the basalt had a local origin in Black Mountain, and the present 

 writer's observations would induce him to regard it as only a 

 local flow. The surface slope of the basalt on the northwest side 

 of the mountain has its even contour interrupted by long narrow 

 ridges with their steeper slopes on the downhill side. These may 

 be pressure ridges. Hyalite was present with other and more 

 common minerals of the amygdules. 



Mr. John R. Suman gives the following petrographic descrip- 

 tion of this olivine basalt : 



This lava under the microscope is seen to be remarkably fresh and 

 well preserved. It has the intersertal structure characteristic of basic 

 igneous jocks. 



A feldspar twinned on the albite law and developed in long rec- 

 tangular idiomorphie crystals makes up by far the greater part of the 

 rock. This feldspar has a high refraction, a positive sign, and a 

 maximum extinction angle on the trace of the twinning plane of about 

 35° showing that it is rather basic labradorite. In one instance twinning 

 on the Baveno law was observed. A suggestion of the glomero-porphyr- 

 itic structure of Judd was seen in the development in one place of quite 

 an aggregation of feldspar crystals in a granular mass. The feldspars 

 were spotted with a black opaque mineral that may have been either 

 magnetite or ilmenite, but owing to its indeterminate nature it will be 

 referred to as "opazite. " 



Surrounding the feldspar laths and filling in the interstitial spaces 

 were irregular grains of augite with high refraction, high bi refringence, 

 positive sign, and appearing light green in ordinary plane polarized 

 light. This enclosed the labradorite, in many eases giving the typical 

 diabasic structure. 



Olivine in rounded grains was also abundant in this rock and could 

 be distinguished by its yellowish-brown color in ordinary light, high 

 refraction and double refraction, irregular conchoidal fracture, large 

 optic axial angle when viewed in convergent light, and positive sign. 

 It was found included in both the augite and labradorite and evidently 

 was one of the first minerals to form. It showed a slight alteration in 



