THE OREGONIAN REGION 777 



Columbia Kiver valley are much more recent than those of the Snake 

 Eiver in Idaho. 



I have studied but few specimens of the Oregonian basalts — fewer, 

 indeed, than of the Deccan traps. I am indebted to Dr. A. C. Spencer 

 and Prof. R. W. Chaney for some specimens, and to the United States 

 National Museum for others. This paucity of material is to be regretted, 

 as there seems to be more variety among these basalts than in those of 

 India. It is hoped, however, to make a more complete study of them, 

 supplemented by study of the accompanying rhyolites, at another time. 



Megascopically, the Oregonian basalts vary in texture from rather 

 coarse-grained, doleritic forms to dense aphanitic ones, phenocrysts (of 

 labradorite) being rather rare. The color of the coarser forms is a black- 

 ish brown, while that of those which are fine grained is a dark gray to 

 black. 



Brief descriptions of the specimens analyzed will suffice to indicate the 

 microscopic characters. The Dalles (Oregon) basalt is the most coarse 

 grained and in thin section is seen to be typically ophitic, with numerous 

 tables of labradorite and interstitial slightly brownish augite present in 

 about equal amounts. There is considerable magnetite, part of it in thin 

 plates, which penetrates both the feldspar tables and the augite grains. 

 Well formed small octahedra of magnetite also occur. There is a little 

 residual glass, which was apparently originally colorless, but is now a 

 pale bluish green, through chloritic decomposition. The Idaho Falls 

 specimen is similar, but is not quite so coarse grained and is less typically 

 ophitic. This contains relatively much more feldspar and less (colorless) 

 augite than the Dalles rock, and magnetite is practically absent. On the 

 other hand, there is much more glass, which appears to be black under 

 low powers, but which a high power shows is so crowded with excessively 

 minute "dust" grains as to appear almost opaque. A specimen from 

 Shoshone Falls (Idaho), collected by Spencer, is megascopically very 

 fine grained and dark gray; it shows in thin section a typically basaltic 

 texture, with marked flow fabric. Small, short tables of labradorite are 

 intermingled with anhedral grains of almost colorless augite, the two 

 minerals in about equal amount, and with rather numerous, fair-sized, 

 anhedral grains of magnetite. Each of the three minerals incloses and 

 forms inclusions in the other, and the crystallization of all three must 

 have been in great part contemporaneous, although crystallization began 

 apparently with the feldspar. The Shoshone Falls specimen is holocrys- 

 talline. None of the basalts which I analyzed contains olivine. 



The Teanaway basalt, described by Smith, seems to be much like the 

 Dalles and Idaho Falls specimens, while that of Yakima, although similar 



LI — Bull. Geol. Soc. Am., Vol. 33, 1921 



