10 



THE UNIVERSITY OF OKLAHOMA 



Microscopically, the Black Mesa basalt shows as follows: (1) 

 about 45 per cent of lath shaped, euhedral or idiomorphic plagioclase 

 feldspars with large extinction angles. The composition of these 

 lie as well as could be ascertained, from chemical analysis, between 

 ab^an^-ab^an^ hence probably a transition between labradorite and 

 bytownite. (2) About 45 per cent of a light greenish pyroxene,* 



Fig. 5. 

 light X 70. 



Photomicrograph of Black mesa basalt, ordinary 

 C. W. Honess. 



allotriom.orphic, very small in size and granular, fills the interstices 

 between all the other minerals. This shows that the pyroxenes crys- 

 tallized last, out of their usual order, and together with the pecu- 

 liar lath shaped feldspars, marks the Black Mesa basalt as diabasic. 

 (3) There, is- -alu:il^i-^-^t<:^^5^^s-^er cent oi re.,0^. magnetite shown 

 cnemically to be titaniferous, together with hematite and limonite 

 as decomposition products of other minerals. (4) About .3 per 



*Tlie pyroxene could not be distinguished because of strain and con- 

 sequent distortion of crystals and the development of wavy extinction in all 

 slides examined. 



