Calkins. ] 



Petrography of the John Pan Basin. 



15] 



There is a considerable amount of deep green augite, of the 

 variety found in the John Day tuffs. Only a few grains of quartz 

 were seen. The rock fragments are mainly of lava, and similar 

 to those found in the tuffs. A few fragments of some cherty 

 material were also found. The cement proves to be a mixture of 

 zeolitic material with a glauconitic (?) substance, apparently 

 identical with that so characteristic of the "Blue Beds." 



It is evident that this sandstone has been produced by the 

 working-over of the underlying tuffs. The fragile lapilli so 

 abundant in those rocks arc entirely absent from the sandstone. 

 They were probably reduced in the process of corrosion and of 

 rolling along the ancient river bottom, to a state of fine division. 

 The feldspathic sandstone is the result of a mechanical concen- 

 tration by running water of the more resistant and denser 

 constituents of the tuffs. A small amount of foreign material 

 has been introduced perhaps, bid it is insignificant in amount. 



Lavas of the John Dai/ Series. — The few thin sheets of lava 

 found in various sections, intercalated with the John Day tuffs, 

 fall into four distinct petrographical types, which will be 

 described separately . 



At Bridge Creek a thin flow of lava occurs, whose horizon 

 was determined by Dr. Merriam to be near the base of the Middle 

 John Day. A single specimen from near the upper surface was 

 examined by the writer. 



Macroscopically, this rock is compact and of a light reddish 

 brown color and dull lustre. A few small crystals of the felds- 

 par are the only minerals distinguished by the unaided eye. The 

 structure is that of a flow-breccia, formed by the breaking up of 

 the first formed surface crust, whose fragments became mixed 

 with the still fluid and moving paste beneath. 



Under the microscope, a few grains of green augite are 

 found, and accessory magnetite and apatite. The feldspars are 

 very scarce and usually of irregular outline, so that their deter- 

 mination was not accomplished in a very satisfactory manner. 

 The grains practically all show polysynthetic twinning, and the 

 highest extinction angle observed in the zone was about perpen- 

 dicular to H»10(. 



The glassy groundmass shows the contorted lamination usu- 



