May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 2a 
Their outlines are angular and their shapes polygonal to pyramidal. 
Some are scoriaceous and some compact. The variations in color are 
from light to dark lavender and brown. As in the rock previously 
described, minute grains of quartz with irregular outlines are scat- 
tered through the mass. 
A fourth hand specimen (Mus. No. E 8183) illustrates the pre- 
vailing rock at the eastern end of the mountain. It was collected 
at the point marked C in Plate LXVI. The appearance of a polished 
surface is shown in Plate LXX., Fig. 2. This is also a breccia, and 
is of coarser texture than any of those previously mentioned. It 
has a general brick-red color, but many of the included fragments 
are lavender or white. These differences of color mark the fragments 
of which the rock is composed. ‘The fragments are irregular in shape 
and have angular outlines, there being no visible signs of attrition 
previous to consolidation. In size they range from an inch (2.5 cm.) 
square to minute particles. They also vary considerably in texture, 
some, usually the larger ones, being scoriaceous, while others are fine- 
grained and compact. The scoriaceous portions are somewhat fri- 
able, and are the first portions to weather out. When this takes 
place cavities are left. Throughout all portions of the rock, as in 
the specimen previously described, are scattered irregular grains of 
transparent quartz. 
Under the microscope the three breccias above described pre- 
sent nearly the same appearance. The ground mass is a devitrified 
glass varying in coarseness of grain, the variations usually appear- 
ing in ramifying areas. ‘There are few phenocrysts to be seen. Those 
visible are usually quartz, but occasionally feldspar. They lack, as 
a rule, regular outlines, and seem to have little or no genetic rela- 
‘tion to their surroundings. Some of the quartz grains, however, 
have hexagonal outlines, and some of the feldspars are lath-shaped. 
In portions exhibiting flow structure the lines of flow often bend 
around a phenocryst, showing a unity of origin of the two. Sphe- 
rulites, or fragments of them, occur occasionally, especially in por- 
tions of the rock exhibiting flow structure. 
It will be seen from the above descriptions that the rock along 
the northern side of the mountain is fragmental in character, and 
that it increases in coarseness toward the east. In origin it may be 
of the nature of a flow breccia, and of the same age as the other 
rhyolites of the mountain. 
The rocks upon the southern Vide of the mountain show none 
of the clastic character of those on the northern side. On the con- 
trary, they are homogeneous and of primary origin. Two specimens 
were collected to illustrate these, one at the point marked H upon 
