Calkins.] 



Petrography of the John Day Basin . 



165 



feldspar laths without regular orientation, between which lie 

 more or less rounded grains of augite and olivine, iron ore which 

 is in this type characterized by isometric rather than rod-like 

 forms, and a small amount of glass base containing needles of 

 apatite. 



From the description given above, we may gather that the 

 general order of crystallization is as follows: Plagioclase, olivine, 

 iron ore, augite, apatite, but an exception seems to occur in the 

 case of some of the feldspar, which is noted in the last paragraph 

 but one. The large crystals of feldspar also have certainly not 

 been formed entirely in a distinct "intra telluric" period; on the 

 contrary, their frequent inclosure of augite shows that they have 

 continued their growth after many of the smaller laths had com- 

 pleted theirs. Evidence in the same direction is the fact that 

 the largest and most distinctly phenoerystic feldspars occur in 

 the most crystalline facies of the basalt. 



01 i vine-free Basalts in the Lower Mascall. — This is a rock 

 composed essentially of plagioclase and augite, with magnetite as 

 an abundant accessory. Both feldspar and augite occur in two 

 generations, but the feldspar only is recognized macroscopically, 

 imbedded in a grayish black aphanitic groundmass. 



The sharply idiomorphie feldspar phenocrysts in the thin 

 sections are found to be almost' invariably twinned on the Carls- 

 bad law, and each half of the twin is generally marked by not 

 more than three or four narrow striae on the albite plan. 

 Zonal structure of a peculiar kind is always seen. A large 

 kernel bounded by erystallographie planes, is sharply distin- 

 guishable from a peripheral zone, which has a distinctly lower 

 refractive index. In one crystal cut perpendicular to the brachy- 

 pinacoid, the concurrent angles in the two halves of the Carlsbad 

 twin indicated that the kernel was bytownite and the shell 

 labradorite. Granules of augite are often included in the outer 

 portion of the feldspar phenocrysts. It is possible that the basic 

 kernels of the feldspars are truly intratellnrie, and the shells are 

 additions made during the final period of consolidation. The 

 augite, which is pale brownish green, occurs in a few ill-formed 

 phenocrysts often grouped with those of feldspar when the irreg- 



