MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 325 
at high pressure), and B, melting at relatively low temperature 
(for example, FeS)—idiomorphic crystals of B never appear in A, 
but, on the contrary, idiomorphic and usually somewhat resorbed 
crystals of A appear in B. Here the sequence of crystallization, 
without regard to the proportion of weight between A and B, is 
first A and later B, and this to be understood thus, that the crystal- 
lization of A was completed before that of B began. (See Fig. 36.) 
In the usual silicate eruptives, most frequently consisting of a 
whole series of components, we may also meet corresponding 
crystallization of a certain mineral, completely solidified at a 
relatively early stage. See Figures 20 and 21, representing crystals 
of hypersthene imbedded in diallage. 
Another case of crystallization completed at an early stage is 
illustrated by Figures 8 and 9 (and a theoretical explanation given 
below) with crystallization at the beginning stage of hypersthene, 
while the Fe-Mg silicate at a later stage entered into biotite. 
In this treatise (Part I) we are only going to consider the 
solidification of the rocks (the transition from liquid to solid 
phase). We, however, also discuss the continued change of the 
minerals, which may be founded on the later crystallization of 
a substance originally in solid solution, and furthermore we are 
going to deal with the reactions which appear in the solid phase 
on the boundary between two minerals, and which are an immediate 
‘result of the cooling of the rocks after completed crystallization. 
We shall, however, not discuss the later changes, which are not a 
direct result of the solidification of the rocks, but are founded on 
exterior incidents, as, for instance, dynamo and contact meta- 
morphosis, chemical actions, etc. 
INTRODUCTORY REMARKS CONCERNING THE APPLICATION OF THE 
PHYSICO-CHEMICAL LAWS TO THE CRYSTALLIZATION OF MAGMAS 
Magmas usually consist of a whole series of components, which 
entail a complication of the equilibrium existing in the magma 
and consequently also of the laws of the crystallization. 
In many cases, however, an essential simplification of these 
complications takes place, as, according to H. E. Boeke,’ the 
™ Grundlagen der physikalisch-chemischen Petrographie (1915), p. 104. 
