CRYSTALLIZATION OF IGNEOUS ROCKS. 
91 
lime-soda feldspar, augite, hypersthene, and iron ores. They 
carry a variable amount of hornblende in many cases, whose 
connection with the chemical composition is quite variable, 
as it is so closely allied to augite chemically. The pyroxene- 
andesites grade into basalt chemically and mineralogically 
as they carry more and more olivine and less hypersthene. 
On the other hand, they grade into more acid andesites 
by increasing percentages of biotite, which characterizes the 
hornblende-mica-andesites . These rocks are composed of more 
alkaline lime-soda feldspar, biotite, hornblende, and a vari¬ 
able percentage of pyroxene and less iron ores. 
As the chemical composition of the rocks varies toward 
the more siliceous end of the series, the last-named andesites 
pass into dacites with the development of quartz and some¬ 
times sanidine, besides the oligoclase and biotite, with less 
and less hornblende and pyroxene. At the more acid end of 
the series are the rhyolites, characterized by quartz, sanidine, 
and a variable amount of lime-soda feldspar, with little or 
no ferro-magnesian silicates. 
These are actual observations for large areas of country. 
When the mineral character of the rocks varies from this 
rule it can be traced in the great majority of cases to varia¬ 
tions in the chemical composition. Thus the development 
of leucite at Leucite Hills, Wyo. Ter., can be traced to the 
large percentage of potash in the rock; the occurrence of 
nepheline in the Black Hills, Dak., to an increase in the 
soda. The mineral composition of the trachytes, phonolites, 
leucite basalts, and other volcanic rocks can be correlated 
with their chemical composition. 
(2.) There are mineralogical variations, however, which 
do not appear to be connected with variations in the chem¬ 
ical character of the rocks; such as the occurrence of primary 
quartz in numerous basalts and andesites; of porphyritical 
crystals of hornblende in the basaltic rocks, or those of olivine 
in acid andesites and in some dacites, and of fayalite in rhy¬ 
olitic obsidian. Such occurrences are widespread, but are, 
nevertheless, exceptional in the sense that they form a very 
