Calkins.] 



Petrography of the John Day B<isi>t 



131 



to vary considerably in that mineral. The current figure, as 

 determined by Michel Levy,* was probably not based on the 

 study of a large number of specimens. In view of these consid- 

 erations, the mineral with the higher double refraction is referred 

 to heulandite. 



The hornblende, originally of the same character as that of the 

 Hald's Canon rock, has been largely transformed in a peculiar 

 manner. The change, beginning on the exterior, produces a 

 green hornblende, whose color is in beautiful contrast to that of 

 the original mineral. The cleavage cracks in all directions are 

 parallel in the two minerals, and twinning, when present in the 

 original, continues without interruption into the paramorphic 

 shell. The pleochroism of the secondary hornblende, in shades 

 of a pecnliar grayish green, is about as intense as the pleochroism 

 of ordinary biotite. Between crossed nicols, the interference 

 colors are in contrast to the brown hornblende, only moderately 

 high. The extinction angle is 17°. 



The iddingsite pseudomorphs are without magnetite rims, and 

 have the characteristic outlines of idiomorphic crystals of rhombic 

 pyroxene. Although none of the original mineral was found, 

 their derivation from a rhombic pyroxene, probably hypersthene, 

 cannot be doubted. The iddingsite is here of a greenish brown 

 color, with the usual optical characters, and is generally arranged 

 in divergent or spherulitic groups. 



Lining the filled-in original cavities of the rock, especially 

 visible in polarized light, there is always an abundance of 

 minute tablets, which, by their hexagonal form, their refraction 

 intermediate between that of balsam and that of the inclosing 

 zeolite, and their weak double refraction, are identified as 

 tridymite . 



Andesitic Tuff of Clarno's Ferry.— The material that forms 

 the bluffs of Clarno's Ferry varies greatly in degree of coarse- 

 ness; it includes, on the one hand, coarse agglomerates with 

 huge andesitic boulders, and, on the other, tuffs of such fine 

 texture as to preserve the most delicate leaf-impressions. 

 Although they show evidence in many places of the strong 

 action of water, these rocks are practically all of volcanic rather 



*Les Mineraux des Roches, Paris, 1888, p. ail. 



