164 
IDDINGS. 
is the same in both cases—that is, it is the result of differen¬ 
tiation. 
RESULTS OF THE DIFFERENTIATION OF IGNEOUS MAGMAS. 
The chemical differentiation of igneous magmas, which 
appears to be due for the most part to so simple a cause as 
an inequality of temperature in different parts of the 
magma, leads to an endless series of variations in the com¬ 
position of so complex a mixture as the magma of eruptive 
rocks and accounts for many of the obscurer relationships 
of these rocks. 
The application of this principle to the • explanation of 
some of the phenomena of eruptive rocks will be pointed 
out. It explains the chemical variability of large bodies of 
rocks or the development of chemical facies of rock bodies 
and the changes in composition of lavas which follow one 
another through the same conduit or are associated to¬ 
gether at one center of eruption. And in a long period of 
volcanic activity the continued differentiation of a particular 
body of magma will lead to the production of extreme varie¬ 
ties of composition, differing more and more from one another. 
It explains the existence of common characteristics in the 
varieties of rock belonging to a single center of eruption, 
and the absence of well-marked types of individual rocks. 
It accounts for the extremely local character of groups of 
rocks connected with particular centers of eruption, and at 
the same time their broader relations as members of the 
larger group of igneous rocks of the whole region. In this 
way it explains the local irregularities and repetitions in the 
order of eruption of volcanic rocks, and in its widest appli¬ 
cation it explains the differences between the rocks of various 
geographical areas over the globe. 
MINERAL COMPOSITION OP ROCKS. 
Some of these results may be discussed with advantage at 
this time. The one which exhibits the independence of the 
oxide molecules from any fixed combination, while in a 
