Calkins. | 



Petrography of the John Day Basin. 



139 



ulites of similar size, while numbers of smaller spherulites often 

 lie imbedded in the surfaces of the larger; second, they are 

 always surrounded by numerous gently rounded annular ridges, 

 which are shown by the microscope to have their planes parallel 

 to the surface of flow. 



A microscopical thin section was made through the centre of 

 a spherulite three-fourths of an inch in diameter. The outer 

 portion was found to consist of slender fibres radiating from a 

 central nucleus. This nucleus, with about a third the diameter of 

 the whole, is composed of an aggregate of radiate fibrous groups, 

 very irregular in form. The fibres, as in the specimen above 

 described, always have a small extinction angle and positive 

 elongation. Between these feldspar fibres, there lie numerous 

 minute shreds of a green pleochroic mineral, with straight 

 extinction and moderate double refraction. 



Flow structure is indicated in two ways. Numerous small 

 brown trichites, scattered through the feldspar substance, show 

 a linear parallel arrangement, while in the outer portion there 

 are alternating clear and cloudy bands, the cloudy bands dotted 

 with numerous clear circular areas. The cause of this latter 

 phenomenon is problematical, but it may possibly be explained 

 as indicating an incipient spherulitic crystallization at an early 

 period, afterward superseded by crystallization from more widely 

 separated centres. The radial arrangement of the fibres is inter- 

 rupted in no way by the structures just described. The lines of 

 trichites and the cloudy bands are parallel to the plane of the 

 external rings. 



A few crystals of acid feldspar and a few clear granophyric 

 intergrowths of feldspar and quartz are scattered through the 

 fibrous mass. 



The spherulites over an inch or so in diameter usually eon- 

 tain irregular central cavities, more or less completely filled with 

 secondary minerals, and afford many specimens of iinusual inter- 

 est and beauty. The mineral species represented are quartz, opal, 

 chalcedony and fiuorite, and two zeolites that appear to be new. 

 The description of a few specimens will serve to illustrate their 

 mode of occurrence. 



One of the largest is a fine miniature cavern. The roof is 



