Calkins.] Petrography of the John Day Basin. 129 



cles of hematite which are the cause of the pink color seen 

 macroscopically. 



The minerals filling original cavities are granular quartz, 

 spherulitie chalcedony, and a zeolite occurring in a few divergent 

 aggregates imbedded in chalcedony. This zeolite, which was 

 recognized as such by its very low refractive index, has an 

 extinction angle of about 22°, measured between the axis of 

 elongation and the axis of greatest elasticity; in this respect it 

 corresponds to brewsterite. 



Hornblende-h ypersthene Andesite of Clarno's Ferry. The 

 specimens from the boulders in the tuff beds of Clarno's Perry 

 are very similar in macroscopical appearance to the hornblende 

 andesite of Hald's Canon. Microscopical study, however, 

 reveals some characters that entitle these rocks to a separate 

 description. 



One of the specimens had originally the same character 

 as the Hald's Canon rock, and the feldspars and groundmass 

 exhibit a similar alteration. No original hornblende, however, 

 now remains, having been replaced by psendomorphs of mag- 

 netite and opaque red ochre. 



The original constituents of a second specimen differed from 

 these in the presence of a rhombic pyroxene, which is now repre- 

 sented only by psendomorphs. The chief interest of the rock 

 lies in its exemplification of several alteration processes. 



In the microscopical section about half the rock is seen to 

 consist of a turbid hyalopilitic groundmass, in which lie 

 phenocrysts of altered feldspar and brown hornblende, with 

 psendomorphs, of a mineral resembling iddingsite, after a 

 rhombic pyroxene. The accessories are apatite, magnetite, and 

 tridymite, while zeolites occur as a filling of cavities and partially 

 replacing feldspar. 



The apatite is of the brown pleochroic variety already 

 described on page 126. 



The feldspar phenocrysts are found by the method of Michel 

 Levy to be acid labradorite, with the approximate composition 

 AbiAni. Their alteration, which is in general far advanced, is 

 of an interesting and unusual character. The writer has 

 observed nothing similar except in the Clarno rocks, and has seen 



