Calkins.] 



Petrography of the John Day Basin. 



163 



of the olivine basalts. It is found that the greater number of 

 specimens may be ranged, on the basis of structure, in a graded 

 series. It would perhaps be possible to obtain in the field 

 specimens of almost undifferentiated glass, but none of those 

 represented in our collection have more than 40 per cent, of glass. 

 The highest term of this series, on the other hand, is represented 

 by a number of holocrystalline, ophitic specimens. In the more 

 crystalline facies, the tendency to develop that type of structure 

 generally is well declared, but in exceptional cases, probably 

 owing to movement in the magma near its period of final con- 

 solidation, a typical intersertal structure has been produced. 



• It will be the endeavor of the writer in the following para- 

 graphs to trace the gradual development of holocrystalline 

 structure. This may perhaps be done most clearly by describ- 

 ing four arbitrarily chosen stages in the development of the 

 ophitic structure, and separately noticing in brief the intersertal 

 variety. The logical order of description is that of increasing 

 crystallization. 



A specimen from near the wall of the Davis dyke is perhaps 

 the most glassy of our collection. The characters to be observed 

 in the hand-specimen are the high specific gravity, grayish black 

 color, and fine, compact texture. Abundant minute laths of 

 feldspar may be recognized by the lustre of their basal cleavage- 

 faces; owing to their transparency, they are not distinguished 

 by any difference of color. 



Under the microscope the feldspar and augite appear to 

 belong to two generations. The larger crystals of feldspar, 

 generally well formed, are often grouped with large grains of 

 augite, in which case either the boundaries between the minerals 

 are irregular, or the augite appears to have asserted its crystal 

 form against the feldspar. Augite grains are often included in 

 the outer portion of these large crystals. The smaller feldspars 

 exhibit a great variety of splintery, indented and rum-like forms. 

 The elongated sections often are composed of a number of narrow 

 strips either parallel or sometimes only approximately so, con- 

 nected by narrow bridges. Cross-sections often show a lining 

 banded parallel to the walls, of a deep green, translucent sub- 

 stance which is apparently isotropic. 



