144 



University of California. 



IVOL. 



Rocks of the Lower John Day. — The lowest division is the 

 one most readily recognized in the field, being strongly charac- 

 terized by the habit of weathering into softly rounded hills, and 

 even more by the prevailing red colors. Interbedded, however, 

 with the dominant red material are occasional layers of white or 

 yellow. 



A close examination of hand specimens affords evidence of 

 their tuffaceous character. Perfectly angular and glassy crystals 

 of feldspar always may be seen in moderate amount. Certain 

 specimens show abundant decomposed lapilli, distinguished by 

 their greenish white color from a fine, earthy red matrix. In 

 contrast to the freshness of the feldspar is the generally advanced 

 decomposition of the unindividualized material. This is made 

 evident by the greasy or clayey "feel" of finer-grained specimens, 

 and even more strikingly by its behavior toward water. When 

 an attempt was made to grind a thin section on a wet plate, the 

 rock showed a strong tendency to crumble and even to slake 

 somewhat after the manner of lime, while rubbing quickly 

 reduced it to a clayey paste. A few rather poor sections were 

 produced by dry grinding. 



Under the microscope, the ashy structure becomes apparent 

 in even the compact clayey material. The larger portion seems 

 to consist of decomposed glass-fragments, often highly vesicular, 

 and rendered in great part opaque by the formation of kaolin 

 and impregnation with red oxide of iron. In smaller amount 

 occur fragments of a compact microlitic rock composed of 

 feldspar laths with the extinction angles of oligoclase or ande- 

 sine, and a little magnetite. A few grains of green augite are 

 found, and a few of quartz. The larger grains of feldspar are 

 always bounded by crystal planes or irregular surfaces of frac- 

 ture. Rather more than half show albite twinning, and their 

 extinction angles are those of oligoclase. The untwinned crystals 

 seemed to be orthoclase. The determinations in both cases were 

 confirmed by examination of cleavage flakes, and by micro- 

 chemical tests with hydrofiuosilicic acid. Black scales with 

 metallic lustre are rather common, and a number, picked from 

 the hand specimen, seemed unaffected by the magnet. They thus 

 appear to be ilmenite. 



