calkins. i Petrography of the ,l<>lnt Bay Basin. 149 



mineral, occurring in the same manner as the supposed chlorite or 

 glauconite of the blue tuff, appears to be iddingsite ; it shows, 

 when seen in sufficently large units, the high double retraction, 

 strong pleochroism and micaceous form of that mineral, and is 

 reddish brown to greenish brown in color. 



The silica percentage of a typical specimen was determined 

 to be 58.3%. The tuffs of the Upper, as well as of the Middle 

 John Day, are believed to be mainly andesitic. 



The matrix of the plant-remains collected near Lone Rock is 

 a light green rock of fine, even, and compact texture. Examined 

 microscopically in their section it shows the "ashy" structure in 

 a highly typical manner. The constituent particles are mainly 

 of delicate pumiceous glass, with cuspate or ragged outlines, a 

 fact which indicates that the material was deposited quietly, 

 without being rolled by wind or water currents. The coloring 

 matter is identical with that of the typical, Middle John Day 

 rocks. The vesicles of the pumice grains and the interstices 

 between them are filled with a colorless substance generally 

 isotropic but occasionally showing a feeble double refraction, 

 which may be analcite. 



In the beds immediately overlaid by the basalt flows, the heat 

 of the molten lava has often produced a deep red color. Specimens 

 of the rock thus altered, when examined under the microscope, 

 appear to be strongly impregnated with ferric oxide. This 

 appears to have been derived by the destruction of the iron- 

 bearing silicates that color the green and buff tuffs, for these 

 minerals are not found in the baked red rocks. A considerable 

 quantity of secondary quartz and analcite is sometimes present . 



The slender columns found in the Upper John Day where it 

 is cut by the Davis dikes in Turtle Cove are described by Dr. 

 Merriam* as extending about fifteen feet on either side of the 

 basalt in a direction about normal to the plane of the dyke, while 

 single columns are often at least ten feet in length. The 

 diameters of these long columns average about two inches; in a 

 lenticular mass of tuff included in the dyke the diameters of the 

 columns are about five inches. As Dr. Merriam points out, the 



*Loc. cit. Instances of similar development of columns in sandstone and coal 

 are cited by Rosenbusch, Gesteinslehre, p. 20. 



