174 



University of California Publications. 



[Geology 



The rocks of the middle division are andesitic tuffs of a 

 characteristic green or greenish-blue color. Thin flows of rhyo- 

 litic lava are interbedded with these tuffs at Bridge Creek and 

 in Turtle Cove. 



The upper division is composed of tuffs petrographically 

 similar to those of the Middle John Day but prevailingly light 

 buff in color. At a number of localities sands and gravels are 

 found near the top of the formation, indicating a change in 

 the mode of deposition. 



No sharp stratigraphic line can be drawn between the Middle 

 and Upper John Day, the color of the beds usually serving for 

 their discrimination in the field. Faunally, the dividing line 

 between them may be fixed by the downward range of 

 Promerycoclioerus which is not known to occur in the Middle 

 John Day. This limit is for the present regarded as lying about 

 100 feet above a prominent stratum of coarse gray tuff exposed 

 quite generally near the top of the Middle John Day in Turtle 

 Cove, where, by differential weathering, it usually forms a 

 terrace. The bluish-tinted tuff beds above this stratum contain 

 a rodent fauna practically identical with that occurring lower 

 down in the Middle John Day and are, accordingly, incorporated 

 witb the middle division. 



Before the extravasation of the Columbia Lava, the John Day 

 formation was subjected to erosion. The surface thus produced 

 is known to have supported a growth of timber in at least one 

 locality. Partly charred and partly silicified wood has been 

 found at the contact of the buff beds with the lava, numerous 

 sticks and stems extending some distance upward into the lower 

 portion of the lava flow. Angular unconformity has been ob- 

 served between the lava and the John Day, due to gentle folding 

 of the latter formation previous to the outpouring of the lava. 



The Columbia Lava is built up of numerous heavy sheets of 

 olivine basalt with relatively insignificant amounts of basaltic 

 tuffs interbedded with the flows. 7 In Oregon, the lava series 

 reaches a thickness of one to two thousand feet. 



Resting on the Columbia Lava without observed uncon- 



7 Calkins, F. C. A Contribution to the Petrography of the John Day 

 Basin. Bull. Dept. Geol. Univ. Cal., Vol. 2, p. 159. 



