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University of California. 



[Vol. 3. 



ally produced by viscous flow. It is for the most part rendered, 

 nearly opaque by a finely divided ochreous pigment. Small 

 portions, however, are transparent with a rich brownish red color. 

 These transparent portions, between crossed nicols, exhibit weak 

 double refraction; dark brushes parallel to the cross-hairs of the 

 microscope radiate from numerous points on the boundaries of 

 these areas. This phenomenon will be discussed later, in con- 

 nection with some rocks which exemplify it more clearly. 



No chemical investigation of this rock was attempted. Its 

 tnineralogical character, however, combined with the evidently 

 rather high percentage of iron, leads the writer to the belief that 

 this is a glassy acid andesite, corresponding approximately in 

 chemical character to the tuffs composing most of the John Day 

 beds. 



Another thin flow, with brecciated and vesicular surface, 

 was seen in the valley north of Antelope. The color is deep red, 

 owing doubtless to secondary oxidation of the iron content. 

 The rock contains only a few small grains of feldspar, which 

 could not be satisfactorily determined, and of quartz, which is 

 pi-obably all secondary. It is impossible to classify this rock 

 with precision, but it appears to be of andesitic character. 



Immediately overlying this lava in Antelope Valley, and seen 

 by Dr. Merriam in the hills to the west, are two or three flows of 

 rhyolite. The petrographical character of this rhyolite, as will 

 be shown in the description, is well marked and peculiar. In 

 Turtle Cove, about sixty miles from Antelope, a thin lava flow 

 of considerable extent occurs near the horizon that divides the 

 Upper from the Middle John Day. It is a remarkable fact that 

 the petrographical character of this rock, including even the 

 microscopical structure, is practically identical with that of the 

 Antelope rhyolite. The resemblance is so striking that the rocks 

 from the two occurrences can best be described together as a 

 single type. 



Maeroseopieally, the rocks are seen to be of a drab, or of light 

 gray or buff color, and to consist mainly of fine-grained material 

 with lithoidal and compact texture. Sparingly distributed in 

 this groundmass lie phenocrysts of glassy feldspar rarely more 

 than two millimetres in diameter. There are generally no other 



