204 FIELD COLUMBIAN MuseEuM—GEOLOGY, VOL, II. 
square millimeters down. They are thickly scattered through both 
the red and violet layers, but are rather more abundant in the former. 
Magnétite is also present in minute grains, rarely visible in the 
mass of the rock with the naked eye, but readily separating with a 
magnet when the rock is powdered. In thin section the feldspars, 
with occasional grains of magnetite, form the only individualized 
minerals. The feldspars show polysynthetic twinning, often with un- 
dulatory extinction. They are generally undecomposed and free 

‘ Fic. 3. Rhyolite. Ciudad de Rocas. x r. 
from inclusions. The alternate arrangement of compact and pum- 
iceous streaks seen with the naked eye appears in greater detail 
when observed under the microscope. Bands appearing wholly 
compact megascopically, are seen microscopically to be penetrated 
with narrow, elongated, pumiceous streaks. The compact bands are 
heavily impregnated with an opaque red pigment, which is doubt- 
less oxide of iron, that renders them nearly opaque, even in thinnest 
section. The pumiceous streaks are composed of great quan- 
tities of lath-shaped microlites, generally arranged in a confused 
fashion, though sometimes exhibiting a partly radial arrangement. 
There is also much curving and twisting of glass fibers and micro- 
lites, especially about the phenocrysts. The characters described 
