THE ORIGIN OF IGNEOUS ROCKS. 
161 
fact that there are at least six essential oxides and several less 
abundant oxides involved in the composition of the magma, 
and from the undoubted influence of mass action in determin¬ 
ing the behavior of the elementary molecules, it is evident 
that the results of the process of differentiation may be 
highly complex. 
A study of the diagrams of molecular proportions in the 
groups of rocks from the Yellowstone Park region shows the 
absence of any persistent law of variation. 
In the Electric peak group, with some of the rhyolitic 
rocks attached, the soda and potash vary but slightly from 
the middle of the group to the extremes, or from one end of 
the series to the other, flfliey remained very nearly constant 
throughout the differentiation. In many cases they vary in 
opposite directions; so that the total amount of alkalies is less 
variable than each separately, and there is a tendency for 
them to replace one another; but in some instances they vary 
independently, and in others they vary in the same direction. 
No distinct relationship can be made out between the alka¬ 
lies and alumina, although they all maintain considerable 
constancy and are in this sense alike. In the group of rocks 
from Crandall basin the same general statements may be 
made concerning the alkalies and alumina. In this group 
the variation of the alkalies is even less than in the group 
from Electric peak; but in the group of exceptional dike 
rocks and surface flows, which are connected with the Cran¬ 
dall basin rocks and are represented in diagram 3, the varia¬ 
tion of the alkalies and alumina are greater, and in general 
are in like directions, except at the less silicious end of the 
series, where the alkalies vary in opposite directions. 
In the same manner an examination of the magnesia, 
lime, and ferrous oxide shows no persistent relation between 
them, though in general they vary in like directions and 
decrease rapidly with an increase of silica. It is also evident 
that in these groups of rocks the ferrous oxide is the least 
variable constituent and the magnesia the most variable, lime 
being intermediate. Moreover, these oxides are much more 
24—Bull. Phil. Soc., Wash., Vol. 12. 
