134 
IDDINGS. 
characterizes the rocks of the Tewan mountains in New Mex¬ 
ico,* where it is found in variable amounts in basalt, pyrox¬ 
ene-andesite, hornblende-pyroxene-andesite, hornblende- 
mica-andesite, and rhyolite, the quartzes being colorless. It 
also occurs throughout a group of rocks in the vicinity of 
Crescent peak, Colorado, where the rocks range from basalt 
through andesite to what is possibly trachyte. In the Eureka 
district, Nevada, the whole group of rocks from basalt to 
rhyolite, including pyroxene-andesite, hornblende-mica-an¬ 
desite, and dacite, is characterized by quite sporadic quartzes, 
which are found in all of the rocks though not in all of the 
hand specimens. In most all of these cases it is dark-colored ; 
and dark-colored quartz is particularly characteristic of the 
rhyolite of this district, though in some forms of the rhyolite 
and of the other rocks it is colorless. Thus it is not only the 
presence of quartz grains in all the members of the rock 
group in each of these instances, but the particular quality 
of the quartz that distinguishes them. 
The illustrations that have been briefly given are some of 
the more striking features of different groups of volcanic 
rocks. They could be elaborated until all of the mineralogi- 
cal and structural characteristics of different rock groups 
were depicted ; for it is the combination of particular minerals 
with their special crystal forms and modes of development 
which produces what we term the habit of a rock, and when 
large groups of rocks from different regions are compared it 
is often noticed that the habit of the rocks of one group is 
distinctive. But this is not the place to enter into such de¬ 
tails, nor does the writer consider himself prepared to describe 
such features at length without further study. The investi¬ 
gation of the characteristic features of various rock groups is 
possible only when full collections have been made from the 
localities in question and when they can be compared directly 
with those of other regions. 
* Iddings (J. P.) On a group of volcanic rocks from the Tewan moun¬ 
tains, New Mexico, and on the occurrence of primary quartz in certain 
basalts. Bull. No. 66, U. S. Geol. Surv. 8°. Washington, 1890. 
