Calkins.] Petrography of the John Day Basin. 155 



and fairly clear, and have the refractive index and double 

 refraction of feldspar, but a remarkably large extinction angle, 

 attaining a maximum of about 40? If, as there seems no reason 

 to doubt, they are composed of acid feldspar, their elongation 

 can not be parallel either to the vertical or the front-and-back 

 axis. 



As secondary constituents there occur opal and quartz lining 

 numerous minute amygdaloidal cavities, and a mineral with very 

 high double refraction disseminated in particles too minute to 

 permit of determination, but supposed to be calcite. Its amount 

 is insignificant. The cloudy substance that clouds a great portion 

 of .the groundmass is perhaps kaolin. 



The second structural variety of these rhyolites shows no 

 flow structure. About three-fifths of its groundmass is cloudy 

 by transmitted light and dirty white or gray by reflected light. 

 The l-emainder, which is fairly transparent, forms irregular, 

 cuspate or roughly oval areas, joining one another to form a sort 

 of web. The cloudy portion, in polarized light, is seen to be 

 composed mainly of weakly bi-refringent fibres, whose arrange- 

 ment is for the most part confused. On the borders of the 

 clearer areas, however, they often form more or less nearly per- 

 fect spherulites. In these structures, the fibres can be shown 

 to have a low extinction angle and positive elongation; they are 

 thus analogous to those of the spherulites in the Eocene rhyolites, 

 and are believed to be of sanidine and anorthoclase, or the latter 

 alone. The more transparent areas, however, are formed of 

 stout fibres, or, more properly, rods, in parallel or diverging 

 groups, with the characteristically high extinction angle and 

 other properties of the mineral forming the peculiar almond- 

 shaped bodies in the variety of this rock first described. The 

 boundaries between the clear and cloudy spaces are not very 

 definite and the transparent rods often seem to form extensions 

 of, or to be interdigitated with, the fibres of the dusky spheru- 

 lites. In such cases, it is generally noticeable that the two 

 varieties do not extinguish together. A portion of the edge of a 

 more than usually large transparent space is sketched in plate 17, 

 fig. 4. The quartz is remarkably inconspicuous, and apparently 

 much less abundant than in the facies described first. 



