Calkins.] 



Petrography of the John Day Basin. 



147 



mixed with this interstitial dust. The cloudy musses of this 

 mineral contain no individuals that show its crystallographic, or, 

 with any definiteness, its optical properties. The color is grass- 

 green, and a faint pleochroism may sometimes lie detected. 

 Between crossed nicols the mineral generally appears quite dark; 

 the apparent isotropism is due to compensation, for locally there 

 may be observed a feeble double refraction. This material is 

 presumably of the nature of chlorite or glauconite; probably 

 both substances are present. Calcite occurs in the typical blue 

 rock in but small amount . 



These rocks were not analyzed chemically, hut the silica per- 

 centage was determined in a typical sample, as 60%. Thus the 

 silica content and the in in era-logical character of the rocks would 

 both tend to place them in the family of andesites. 



The brown nodules so characteristic of the Middle John Day 

 vary in diameter from a few inches to a foot or more. Their 

 forms are generally spheroidal or ovoid. The material is com- 

 pact and fairly hard, and its calcareous nature is shown by 

 effervescence on the application of weak acids. 



Under the microscope the structure, so far as its original 

 features are concerned, is found to be identical with that of the 

 blue tuffs just described. But the pumice fragments have 

 become thoroughly impregnated with calcite, which lines the 

 larger and fills the smaller cavities. The scattered grains of 

 feldspar have narrow borders of the same mineral, In polarized 

 light, the calcite generally shows the "zeolitie" extinction that 

 indicates a radiate-fibrous structure. In addition to the abundant 

 calcite, a few patches of some unidentified zeolite are found, and 

 numerous small oval areas of brown iddingsite. The general 

 effect, between crossed nicols, of the intricate patterns traced in 

 brilliantly polarizing calcite, varied by the deeper interference 

 colors of iddingsite, is that of an illuminated arabesque. 



The light-colored layers occurring near the top of the Turtle. 

 Cove section show the same structural characters, and the same 

 secondary alteration of the pumice becomes evident under the 

 microscope. Augite was not found in this material. The 

 feldspars, which were rare and not thoroughly studied, apparently 

 include a little orthoclase. The green chloritic substance is 



