Editorial Comment. 119 
Pierson*, viz: the rocks of the "Castle Mountain Mining Dis- 
trict" in Montana, which present, in a high degree, the stamp 
of "consanguinity." After the discussion of this series of 
rocks, which is further illustrated by a series of triangular dia- 
grams constructed according to the foregoing principles, he 
remarks that "this all appears as if, to a pure ferromagnesian 
magma but slightly variable, had been added, in succession, 
variable amounts of the alkalies, at first rapidly increasing, of 
alumina and silica; then, after the point of saturation of the 
potash, soda and alumina, silica alone continues to increase 
and to take the place even of the ferromagnesian magma. 
The diagram of Iddings, put under the author's form of 
expression by contrasting triangles, also shows perfectly the 
diverse types of magmas formed successively by such mix- 
tures. 
In conclusion the author calls attention to sundry theo- 
retical considerations bearing upon the variation of magmas, 
and discusses briefly certain hypotheses, viz: 
1. There are but two magmas that are susceptible of 
precise definition — the ferromagnesian and the alkaline. These 
have a constant individuality. But it is to be admitted that 
neither of these contains their elements in fixed, or even .in 
stoechiometric proportions, but are quite variable, especially 
the alkaline magma in which the per cent of silica can vary 
from 51 to I GO. The mutual independence of these two funda- 
mental magmas is confirmed by a study of a natural series ac^ 
cording to the method of Iddings. They do not act in the 
same manner in their evolution from their eruptive source. In 
general the alkaline magma remains stable while the ferro- 
magnesian is replaced in whole or in part, by pure silica. 
There is, therefore, a profound contrast between these mag- 
mas. The ferromagnesian seems to play the role of an 
igneous scoria, while the alkaline, being capable of insideous 
injection, seems to be carried, by means of dissolvents and 
mineralizing agents, and disseminated sometimes after the 
fashion of liquid solution or even of volatile gases. 
2. A natural method of differentiation of magmas 
ought to fill certain conditions. These conditions must be 
found in the deep-seated reservoirs at the points of union of 
^Bulletin 139 U. S. Geol. Survey, 1896. 
