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University of California. 



[Vol. 3. 



tion pari passu with their accumulation. The climate was 

 presumably hot and arid, for the iron seems to be in a condition 

 of low hydration. The rarity of similar red beds in the higher 

 strata may be accounted for by the supposition that they 

 accumulated more rapidly. 



The writer of course excepts from the foregoing discussion 

 those laminated beds in the Clarno, Upper John Day, Mascall, 

 and Rattlesnake. These are believed to represent local or 

 temporary variations from the ordinary condition of accumulation. 

 The contrast of their structure with that of the predominant 

 pyroclastic material is an exemplification of the proverbial saying 

 that the exception proves the rule. 



EXPLANATION OF PLATE 17. 



Fig. 1. — Hornblende-hypersthene Andesite, Clarno's Ferry, showing 

 altered hornblende and feldspar, iddingsite pseudomorphs, and 

 cavities filled with zeolite. 



Fig. 2. — Pyroxene Andesite, showing replacement of hypersthene by idding- 

 site. The groundmass in the field shown is composed of a 

 dozen interlocking crystal grains of alkali feldspar, thickly 

 interspersed with plagioclase microlites and magnetite grains. 



Fig. 3. — Green Tuff, Middle John Day, Turtle Cove, showing altered pumice, 

 a fragment of lava, feldspar, and augite. 



Fig. 4. — Detail of groundmass of John Day Ehyolite, Antelope. 



Fig. 5. — Semi.-oph.itic Basalt, with plagioclase, olivine (replaced by idding- 

 site), iron ore and augite in order of crystallization, cloudy 

 glass base, and microlitic cavities filled with iddingsite. 



Fig. 6. — Glassy Basalt, The Dalles. Large feldspars partly inclosing augite, 

 which incloses small feldspars. The groundmass is glass 

 charged with magnetite dust and augite-particles, and contains 

 slender prisms of apatite. 



University of California, 



June, 1902. 



