Calkins.] 



Petrography of the John Day Basin. 



These tuft's are provisionally classified, from the acid char- 

 acter of the feldspars, combined with the scarcity of quartz, as 

 trachytie. While this diagnosis may be based on somewhat 

 insufficient grounds, chemical analysis, owing to the decomposed 

 state of our material, would perhaps form an equally uncertain 

 basis of classification. 



Bods of the Middle John Day. — The typical rock of this 

 division, and the one forming most of the lower portion, is a blue 

 or greenish blue tuff, usually heavily bedded, and characterized 

 by the presence of abundant brown calcareous nodules. The 

 top of the Middle John Day in the Turtle Cove exposures may be 

 considered as marked by the thin flow of rhyolitic lava. The 

 upper fifty feet or so, if we adopt this limit, include several 

 strata of white, buff and brownish beds with some intercalated 

 layers of greenish material resembling that of the underlying 

 typical blue beds. 



The typical and predominant rock of the middle division is 

 of rather pale bluish green color, in texture usually fine and 

 rather compact, with a surface harsh and gritty to the touch. 

 When the rock is exposed to the weather, the color becomes 

 lighter, and of a more bluish cast, so that the prevalent tint of 

 the bluffs carved from these beds is a sort of " robin 's-egg-blue." 

 The only mineral to be recognized maeroscopically is the glassy 

 feldspar, and whitish lapilli, often sparkling with minute color- 

 less crystals, are always to be detected in all but the finest 

 material. Decomposition is in general much less advanced than 

 in the red beds, and the preparation of slides is not attended 

 with the difficulties encountered in the case of those rocks. The 

 secondary processes, on the other hand, have effected a recement- 

 ing of certain small portions to form a moderately hard rock, 

 with smooth surfaces of fracture breaking indifferently across the 

 constituent pumice fragments. 



The microscopical characters are best seen in slides from the 

 medium-grained phases. Of original minerals in addition to the 

 feldspar, the microscope reveals a rather small amount of green 

 augite, and accessory iron ore, apatite and zircon. The feldspars 

 usually show both albite and Carlsbad twinning, and are referred 

 by their extinction angles in favorable sections, to the species 



