Diopside — Forsterite — Silica. 257 



tions are unnecessary. Resorption may have occurred durin g 

 simple cooling under uniform conditions, the only change o f 

 composition being that in the still fluid portion of the magma 

 as a necessary consequence of the separation of crystals. 



Recurrent Crystallization. 



It has been shown in the discussion of crystallization that, 

 in certain of the artificial mixtures, forsterite may crystallize 

 out, become completely redissolvecl (resorbed) and again crys- 

 tallize out at a lower temperature. This behavior, in a com- 

 paratively simple system, illustrates how complicated the 

 so-called l order of crystallization ' may be in a system so com- 

 plex as an igneous magma. In natural rocks a great number 

 of solid solution series are known to occur and there are a 

 great many possibilities, or better necessities, of equilibrium in 

 the liquid between orthosilicate and metasilicate molecules as 

 exhibited, in the artificial system and in natural rocks, by 

 pyroxene and olivines, between feldspar and feldspathoid 

 molecules and of other equilibria which readily suggest them- 

 selves. The theoretical discussion of crystallization in such 

 systems in terms of simple eutectics cannot lead to any useful 

 conclusion. 



We have, then, in the artificial system studied, a concrete 

 example of the possibility of crystallization in ' two genera- 

 tions.' It is not necessary, however, that the two generations 

 of any mineral, often observed by the petrographer, should 

 have come about in a similar manner. Perfectly continuous 

 crystallization may give the appearance of crystallization in 

 two generations. The earliest formed crystals may continue 

 to grow in size up to a certain point, when the increasing vis- 

 cosity or the increasing ratio of crystals to liquid may necessi- 

 tate the formation of new centers of crystallization. In such a 

 case there may be larger phenocrysts and smaller groundmass 

 crystals of a certain mineral though crystallization was per- 

 fectly continuous. 



The Significance of Zoned Crystals. 



The distinction between the two types of crystallization 

 described for the artificial, pyroxene-bearing melts should be 

 carefully considered in any discussion of the crystallization of 

 an igneous rock magma. In the first type of crystallization, 

 the liquid is at equilibrium at any temperature with all the 

 crystals, and with all parts of the crystals, existing in it. In 

 the second type the liquid is at equilibrium at any temperature 

 only with the crystals separating at that temperature. To 

 realize the first type it is often necessary that crystals already 



