222 FIELD COLUMBIAN MusEUuUM—GEOLOGY, VoL. II. 
the contour map, Plate LXVI., the other farther to the south, at 
the point marked I. The latter (Mus. No. E 8192) is of a light 
gray color, and has the typical rough feel of trachyte. As it 
carries an abundance of quartz, however, it is rather to be considered 
a rhyolite. The gray color as seen megascopically belongs to the 
ground mass, for the phenocrysts are practically colorless. These 
phenocrysts are quartz and feldspar in about equal proportion. The 
quartz individuals have irregular outline with a hardness of 7 and 
show no cleavage, while the feldspar is in lath-shaped forms with 
shining cleavage surfaces. A pleasing feature of the feldspar indi- 
viduals is a pearly opalescent reflection like that of moonstone, which 
they exhibit, especially when wet. In size both quartz and feldspar 
individuals reach a length of about 2 mm. and average perhaps 
1 mm. in length. In texture the rock is somewhat open, small 
pores and cavities being not infrequent. Porosity is further shown 
by the fact that when a piece of the rock is immersed in water, bubbles 
continue to rise for a long time. Under the microscope the ground 
mass shows the characters of a devitrified glass. It has a micro- 
granular structure as a whole and exhibits no elongated microlites. 
Opaque, brownish, amorphous particles presenting no farther discrete 
characters, cause a considerable clouding throughout the ground mass. 
As phenocrysts occur abundant large quartz and feldspar individ- 
uals, but femic minerals seem to be lacking except for occasional grains 
of magnetite. These latter lack regular outline, and show more or 
less alteration to limonite. The outlines of the quartz grains are, 
as a rule, irregular, but angular rather than curved. Their substance 
is fresh, and shows few inclusions. The feldspar is largely in the 
form of lath-shaped individuals. It is lhkewise fresh and without 
inclusions or zonal structure. Twinning, after either the Carlsbad 
or albite law, is almost nowhere observable. The feldspars are, as 
a rule, fresh, simple individuals of sanidin. 
An analysis of the rock by Mr. H. W. Nichols gave the follow- 
ing results: 
Analysis of Rhyolite, Liparose, Cerro Mercado. Mus. No. E 8192. 

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