Calkins.] 



Petrography of the John Day Basin. 



1 33 



quartz. This material extinguished almost uniformly over con- 

 siderable areas, but, especially in the more open spaces, exhibits 

 a somewhat divergent or radial structure that gives rise to a 

 "zeolitic" extinction. The properties of the mineral, so far as 

 determined, are as follows: The refractive index is decidedly 

 lower than for balsam. The double refraction is about .010. 

 The extinction is parallel, or nearly parallel, to the elongation, 

 which is optically positive. The mineral does not gelatinize with 

 acids, but a test on the rock powder showed it to be soluble, and 

 the acid solution gives strong reactions for alumina, lime and 

 soda. Although the mineral is not specifically identified, it is 

 without doubt a lime-soda zeolite. In another specimen of still 

 finer texture, this cement, when the slide is viewed between 

 crossed nicols, simulates rather strikingly the micropoikilitic 

 structure described in the Hald's Canon pyroxene andesite. 



Andesitic Tuff of Cherry Creek. — The matrix of the leaf beds 

 at Cherry Creek, while similar in origin to the Clarno's Ferry 

 tuff's, shows evidence of more thorough working over by water, 

 resulting in more thorough lamination, and more nearly perfect 

 sorting of the constituent grains. Although associated with 

 coarse agglomerate beds, the leaf -bearing strata vary in texture 

 from coarse sandstone to fine-grained material resembling porce- 

 lain. A specimen of medium-grained sandstone may be described 

 as typical. 



Macroseopically examined, the rock is seen to consist of 

 rather angular fragments of pink or brown lava, with some 

 feldspar grains. These are cemented to a firm and compact 

 sandstone, colored pale brown by finely divided limonite. 



Under the microscope, the main part of the rock is found to 

 consist of the rock fragments which prove to be of altered por- 

 phyrinic andesite. Their feldspar phenocrysts, which all exhibit- 

 to a greater or less extent the zeolitic alteration described on 

 page 130, seem to be all plagioclase. Several were determined as 

 acid labradorites or basic andesines. No ferromagnesian minerals 

 were observed, but there are a few pseudomorphs, mainly com- 

 posed of limonite, whose forms indicate their derivation from 

 hornblende. The groundmass, though always decomposed, shows 

 the typical andesitic structure, and the feldspar laths have the 

 extinction angles of andesine. 



