228 Hague and Iddings— Volcanoes of 
purities. The white deposit of silica, owing to its low specific 
vity, was easily removed by water and mechanical methods. 
pyramid, and when rotated about the c axis on a needle point 
between crossed Nicols they showed the extinctions in the 
prism zone always parallel to that axis. Examined in con- 
vergent light the interference figure was that of a biaxial min- 
eral with large angle between the optical axes and with a nega- 
tive bisectrix perpendicular to the plane of the pinacoid, 
( ©), the plane of the optical axes being parallel to the 
other pinacoid. It was therefore proved to be an orthorhombic 
mineral with negative optical character. The green augite 
th 
that from Buffalo Peak in Colorado, as given by Whitman 
Cross,* or with that from Santorin as published by M. Fouqué.t 
To the feldspar, hypersthene and augite of first consolidation 
in these hypersthene andesites must be added the largest grains 
of magnetite scattered through the rock. As remarked before, 
, 
shaped and rectangular feldspars, smaller pyroxene crystals 
and magnetite grains, which vary greatly in size in different 
specimens. The darker colored rocks have a g 
rich in brown or globulitic glass with small microlites of feld- 
spar and pyroxene; the lighter colored ones having colorless 
: * Bulletin of the United States Geological Survey, No. 1, 1883, 
+Santorin et ses 8, Paris, 1879. 
ses Eruption 
