THE ORIGIN OF IGNEOUS ROCKS. 
133 
found in a large part of the rocks of the Auvergne, the Sie- 
hengebirge, or Bohemia. It is equally scarce in the cor¬ 
responding rocks of the alkali belt east of the Rocky moun¬ 
tains. 
Biotite characterizes certain groups of volcanic rocks by 
its different range. There are many groups, including ba¬ 
salt, andesite, dacite, and rhyolite, in which biotite occurs 
in the more silicious andesites and continues through dacite 
into rhyolite. This is true of the rocks of the Eureka dis¬ 
trict and in many other localities in western America, but 
in the region of the Yellowstone Park the rhyolite and its 
dacitic facies are free from biotite—the range of the biotite 
being smaller and limited to silicious andesites. 
In Arizona biotite characterizes more basic andesites and 
constitutes biotite-pyroxene-andesites grading into basalt. 
Biotite-pyroxene-andesites also occur in the Troad in Asia 
Minor and elsewhere. Biotite is an essential constituent of 
basic rocks under special conditions, as demonstrated in the 
case of intrusive rocks at Electric peak and of the very sim¬ 
ilar rocks in Crandall basin. 
Orthoclase feldspar as an essential constituent of rocks low 
in silica—that is, trachyte in the strict sense—is character¬ 
istic of certain groups of rocks and particular localities. It 
does not occur in the groups found in the Great Basin and 
along the Pacific coast, but begins to appear as the belt east 
of the Rocky mountains is approached, and it is limited to 
certain belts of country in other parts of the world, to which 
attention has been called by Judd in the paper already cited. 
The development of phenocrysts of orthoclase in the less 
silicious forms of certain groups of rocks is a special phase 
of crystallization found in regions where trachytes occur, 
and is particularly characteristic of the belt along the eastern 
flank of the Rocky mountains. 
The prevalence of sphene in particular groups of rocks and 
its almost total absence from others may also be cited. 
Sporadic grains of primary quartz are a most distinctive 
feature of certain groups of rocks in special localities. It 
