158 
IDDINGS. 
cement for the other constituents of the rock. In these 
instances the solvent must have had the composition of an 
alkali feldspar. In the remarkable leucite rock from the 
Stinking Water canyon, which consists of abundant augites 
and olivines in a groundmass of ortlioclase and leucite, 
the solvent consisted of alkalies, alumina, and silica in 
such proportions that there was too little silica to convert 
all of the alkalies and alumina into alkali feldspar, and it 
was necessary for the mixture to split into alkali feldspars 
and leucite, the alkalies and alumina entering into both 
these minerals in the ratio of 1 to 1, but the ratio of the 
silica molecules being 6 in feldspar and only 4 in leucite. 
In many kinds of gabbro and dolerite, where an ophitic 
structure has been developed, the last mineral to crystallize 
was pyroxene, and the solvent of the other constituents 
must have had the composition of pyroxene. While in horn- 
blende-picrite, with large poikilitic hornblendes, the com¬ 
position of the solvent must have been that of hornblende. 
Thus, on the supposition that the compounds in a molten 
magma exist in the form of definite mineral molecules, it is 
evident that no one of them can be considered as univer¬ 
sally playing the role of solvent for the others. But, from 
the arguments already presented, it appears that the com¬ 
pounds in a molten magma do not exist as definite mineral 
molecules, nor even as fixed silicate molecules, but that 
they behave as simple oxide molecules, capable of arrang¬ 
ing themselves in different associations according to physi¬ 
cal circumstances; hence it is not correct to assume that any 
definite compound is the solvent in a molten magma, but 
that the role of solvent is assumed temporarily by different 
compounds, according to the relative abundance of the ele¬ 
mentary constituents and the physical conditions under 
which the magma exists. 
Soret’s 'principle .—With this conception of the flexibility 
of the solution of molten magmas, it is in order to inquire 
by what means their differentiation into magmas of very 
variable composition can have taken place. One answer 
