calkins.] Petrography of the John Day Basin. 



small patches with a radiate fibrous structure. The material which 

 gives the rock its green color, and which occurs in abundance, 

 is of apparently micaceous structure. It mostly forms irregular 

 aggregates of deep green to brownish green color, almost opaque 

 in the thick slides. It is seen to have a double refraction higher 

 than that of quartz and to be pleochroic. A single individual 

 seen of perhaps the same mineral was large enough to exhibit the 

 optical properties more clearly. Its outline is oblong and 

 rectangular. The centre is a confused fine-grained aggregate, 

 but a broad peripheral zone behaves optically as a unit, and 

 shows a distinct cleavage parallel to the longer pair of edges. 

 The pleochroism, from deep grass-green to pale yellowish green, 

 is pronounced; the greater absorption is experienced by rays 

 vibrating parallel to the cleavage, to which the extinction 

 between crossed nicols is parallel. The interference colors are 

 bright, indicating a double refraction of at least .020. The 

 mineral thus seems comparable in its optical properties to idding- 

 site. 



ROCKS OF THE JOHN DAY MIOCENE. 



General Classification. — It has already been stated that the 

 John Day system, except for a comparatively small propor- 

 tion of its thickness, is made up of pyroclastic material. The 

 evidences of this fact generally observable in the field are : ( 1 ) 

 the low specific gravity of the rocks, due to the porosity of the 

 constituent fragments; (2) the angularity of these fragments, 

 which is recognized by the harsh, gritty textm-e of the rocks; (3) 

 the general or at least frequent absence of fine lamination, which 

 testifies to a rapid accumulation of the material rather than slow 

 deposition by wind or water. In the coarser-grained specimens, 

 the angular lapilli and fresh feldspar fragments are easily recog- 

 nized, and the tuffaceous character is clearly evident. 



Dr. Merriam's threefold division of the series may be used 

 advantageously in our petrographic description. The lower or 

 "red beds" are found to be trachytic, while the two upper 

 divisions — the "blue" and "buff beds," respectively — are mainly 

 andesitic in character. The lava found near the middle of the 

 series proves to be a rhyolite of peculiar microscopical characters . 



