calkins.] Petrography of the 'John Day Basin. 



115 



Quaternary and Recent Deposits. — True stream terraces of 

 rounded gravel are generally well defined along the John Day 

 River and its main branches, and there are also many old 

 alluvial slopes, covered with angular rabble, declining gently 

 from the steep sides of the valley toward the centre, but toward 

 a level much higher than that occupied by the present streams. 



Beds of pure white ash are found at many localities inter- 

 bedded with alluvium of probably post- Quaternary age. 



PETR0GRAPH1CAL DESCRIPTIONS. 



PRE-EOCENE ROCKS. 



General Classification . — The only granitoid rock in our collec- 

 tion is that from the middle fork of the John Day Basin near 

 Ritter. It is provisionally classified here as granodiorite.* 

 Another acid intrusive, probably of pre-Cretaceous age, was 

 found near Spanish Gulch. Serpentines occur on Beach and 

 Desolation creeks, and also at Spanish Gulch, though no specimens 

 were taken at the last-named locality. At Spanish Gulch and 

 Beach Creek there are coarse-grained pyroxenites which are 

 assigned to the species websterite. 



Granodiorite of the Middle Fork. — This is a medium-grained 

 hypidiomorphic granular rock, whose color in mass is a rather 

 light gray. The minerals seen in the hand-specimen by the 

 unaided eye are plagioclase, biotite, hornblende, orthoclase, 

 quartz, and titanite, with an occasional grain of pyrite. Horn- 

 blende and biotite, both exhibiting good crystal form, occur in 

 about equal abundance. Small brown crystals of titanite are so 

 numerous as to form a notable feature of the rock. Quartz is an 

 essential constituent, but decidedly subordinate in amount to the 

 feldspars. These may be seen to comprise two varieties; the one, 

 always pure white in color, is striated; the other, which is 

 of a delicate flesh-tint, is without striatum. The two differ 

 sharply in their manner of crystallization. The plagioclase is 

 generally in rather small crystals of more or less idiomorphic 



*The same rook was referred to in Merriam's paper (op. cit. p. 279) as quartz 

 diorite, that name being given by the present writer after a preliminary examination. 



