THE ORIGIN OF IGNEOUS ROCKS. 
195 
There have been established certain laws of concentration, 
one set of which express the principles of osmotic pressure, 
while the other takes into account the force of gravity. The 
application of the first is found in the fact that in the cooler 
part of a solution there will be a concentration of material 
in solution, and this action is of a decided and compara¬ 
tively energetic nature. The second principle is much less 
active, and requires marked differences of gravity and a long 
extent of time to take effect. 
The localized character of the phenomena of volcanicity 
and of minor groups of igneous rocks proves the localization 
of differentiation. And the broader relationship of all the 
minor groups of rocks in one region establishes the existence 
of a more general differentiation, which has acted on a more 
extensive magma, from which all of the igneous rocks of the 
region have originated. This leads to the conception of a 
branching system of fissures in which the process of differ¬ 
entiation may take place with more or less localization—an 
idea expressed by Durocher. 
The geographical distribution of chemically different 
groups of igneous rocks proves that in various parts of the 
earth the sources or reservoirs from which these groups of 
rocks were immediately derived contained homogeneous 
magmas, which differed from one another in composition; 
hence they did not constitute a universal zone or couche 
of homogeneous material, but were localized reservoirs of 
magma. 
This is further proven by the recurrence in various parts 
of the world of similar associations of chemically different 
regional groups of rocks, which indicates that the reservoirs 
of general magma, which differed from one another chemi¬ 
cally, were themselves the result of a more uniform magma, 
which must have been universal, and may have belonged to 
a homogeneous couche. 
The period in which this primitive differentiation took 
place appears to have been geologically remote, for the in¬ 
complete evidence at hand seems to show that the igneous 
