Calkins.] Petrography of the John Day Basin. 



161 



the albite striation is always visible; Carlsbad twins are extremely 

 common; pericline striation is somewhat rare. The zonal band- 

 ing is rarely discernible in the smaller laths, but is always 

 developed to a slight extent in the large crystals. By measure- 

 ment of concurrent extinction angles in Carlsbad twins cut 

 normal to the brachypiuacoid, it is found that the plagioclases 

 are all labradorites, ranging from one extreme to the other of 

 that group. The average composition seems to correspond very 

 nearly to the formula Ab3 An4; the main portion of the large 

 crystals is usually more basic, and the smaller may be as acid 

 as Abi Aiii. 



. The augite is most frequently of the common pale brown 

 variety. A faint tinge of green is occasionally noticeable. In 

 the more highly crystalline phases of the basalt, the pyroxene is 

 not uncommonly a pale violet brown, titaniferous augite. The 

 fact that this variety is confined to the more crystalline and 

 therefore in general more deeply seated rocks is perhaps signifi- 

 cant of some differentiation within the heavy flows . In the hypo- 

 crystalline basalts the augite occurs in two generations. The 

 phenocrysts vary in quality of form; a sharp development is 

 commoner in the faces of the prismatic zone than in the terminal 

 faces. Twinning on the brachypiuacoid is not infrequent. The 

 common inclusions, besides feldspar and iron ore, are irregular 

 inclusions of glass. The groundmass of the hypocrystalline 

 contains augite generally in the form of minute grains or rods. 

 A peculiar manner of growth for the augite is revealed in certain 

 hypocrystalline basalts, and it will be described in the sequel. 



Olivine occurs in somewhat variable quantity, but is always 

 distinctly subordinate to augite. The crystals are generally 

 infei'ior in size to the larger feldspars, and never large enough 

 to be macroscopically prominent. In form they may be either 

 sharply idiomorphic or somewhat rounded. Inclusions appear to 

 be generally absent. The olivine is never entirely fresh, and is 

 often entirely replaced by secondary material which will be 

 described a short distance below. As a rule, the olivine of the 

 hypocrystalline specimens tends to associate itself with augite. 



Iron is always abundant, and may sometimes exceed the 

 olivine in amount. In the glassy basalts it occurs as dust, 



