“THE ORDER OF CRYSTALLIZATION IN IGNEOUS 
ROCKS” 
VICTOR ZIEGLER 
South Dakota State School of Mines 
In a recent number of this Journal appeared an article entitled 
“The Order of Crystallization in Igneous Rocks.’’' A series of con- 
clusions far-reaching in their effect on petrography are arrived at 
by the author, which are certainly worthy of further comment and 
discussion. By building up on certain fundamental assumptions, 
the author is enabled to give a series of diagrams showing for each 
of the important rock groups both the order of beginning and the 
order of cessation of crystallization. A further discussion of the 
assumptions seems, on account of the importance of the conclusions 
arrived at, very desirable. 
In considering such a subject as the crystallization of a magma, 
we must first of all remember that we have to deal with a definite 
chemical system whose behavior is rigidly governed by the laws of 
mass action and the phase rule. The laws of physical chemistry 
should throw some light on this problem, and it is the purpose of 
the author to advance some ideas which may affect the conclusions 
arrived at in the above article. 
Can we regard a granite and a rhyolite as representing the same 
chemical system? That is, may we in the full sense of the term 
regard a rhyolite as the quenched portion of the chemical system, 
which, if undisturbed, would result in a granite ? 
Geologists generally agree that the same magma, depending on 
certain variable conditions, such as temperature, pressure, and 
rate of cooling, may yield either a granite or a rhyolite, but prob- 
ably none will maintain that these variable factors, and especially 
temperature and pressure, will have an effect only on the physical 
condition, but not also on the chemical condition of the magma. 
A granite consists not only of a solution of the various oxides in 
tN. L. Bowen, Journal of Geology, XX, 455. 
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