Gregory.] TYPES OF KHYOLITE. 157 
by sharp angles. Usually two or more pieces situated near each other 
extinguish together. The groundmass is of quartz and feldspar, with 
the type of micropoikilitic structure described above. 
The light-colored rhyolite at times takes on a peculiar structure, 
which is well brought out in weathered specimens. The vesicular 
cavities take the form of narrow tubes 1 to 3 inches long, all of which 
run parallel, the parallelism presumably having been caused by the 
flow of the rock, whereby the vesicles have been drawn out into long 
canals. When the rock is broken, so as to give a cross section of the 
tubes, the appearance is strikingly like that of a cross section through 
an endogenous plant, which shows the fibrovascular bundles arranged 
without order. The longitudinal section, however, exhibits long, nar- 
row slits or troughs, sometimes lined with secondary quartz crystals, 
which give the appearance of birch bark. The color and the selective 
effects of weathering make the likeness still more evident (see PI. VI). 
PERLITIC TYPE. 
Within the space of an inch the white aporhyolite is replaced by a 
rock of an entirely different tj^pe— one which macroscopically appears 
like perlite. Neither in the hand specimen nor under the microscope 
does the contact zone show any sudden change from one fades to the 
other, and it almost seems as if the whole effect was due to weathering, 
and } T et the rocks are entirely distinct in structure. 
Macroscopic description. — Examination of the hand specimen reveals 
a mass of spheres 1 to 5 millimeters in diameter, composed of a gray- 
ish-green substance with interstices filled with the same material. In 
the fresher parts the spheres remain in the mass and are broken across 
by a fracture of the rock, revealing centers filled with a shining green- 
black substance. Where the rock is more weathered the spheres are 
loosened and readily drop out. W^hen a thin section is viewed with- 
out the aid of a microscope the spherule structure is clearly seen, 
especially when brought out by weathering. The spheres — not always 
round in section — are bounded by a brown glassy substance, and the 
interspaces are seen not to differ in appearance from the material 
forming the spheres except that they are more altered. The centers 
of the spheres often contain quartz grains and an area made up of 
chlorite and delessite (sec Pis. V, A and XIII, A). 
Microscopic description. — Microscopically examined, the thin section 
reveals no smaller spheres than were seen with the naked eye. The 
centers of the spheres are found to be oblong or elliptical cavities 
wholly or in part filled as follows: A few quartz grains showing crys- 
tal outlines and marked by inclusions and a rude rhombohedral cleav- 
age project into the cavity. Surrounding the quartzes as a fringe, 
and extending as a zone all around the cavity, is found a brown ish- 
green mineral, probably delessite. The center of the cavity is filled 
