8 JOURNAL OF GEOLOGY— SUPPLEMENT 



Liquid immiscihility in magmas^ — -It is well known that many 

 liquids which are quite homogeneous at high temperatures may 

 separate at lower temperatures into two or more non-consolute 

 liquid fractions, and the hypothesis that igneous magmas form such 

 immiscible fractions is favored by some investigators. Often it 

 seems not to be realized that the formation of an immiscible liquid 

 fraction is not in itself sufficient for differentiation. There must 

 be some mechanism for the relative concentration of the immiscible 

 liquid in certain parts, and those who deny the efficacy of, say, 

 gravity in the production of a local concentration of certain crystal- 

 line phases must, logically, do the same for immiscible liquid phases. 



Vogt has made a study of the question of liquid immiscihility in 

 silicates, considering both experimental and field evidence, and has 

 arrived at the conclusion that the rock-forming silicates are freely 

 miscible in all proportions. For certain minerals like the sulphides, 

 he concedes partial immiscihility.^ 



Some geologists are inclined to disagree. Daly, for example, in 

 speaking of the complete miscibility of liquid silicates, says: "It 

 may be quite true for high temperatures and yet quite untrue for a 

 temperature just above that of crystallization of a given com- 

 ponent. No one has yet succeeded in holding a molten mixture of 

 silicates within this narrow range of temperature for a length of 

 time sufficient to warrant any conclusion on the matter."'' On the 

 contrary, in the great number of quenching experiments which have 

 been made at the Geophysical Laboratory on a rather compre- 



^ The term "liquation" is now quite commonly used in petrologic literature to 

 designate the act of separation into immiscible liquid fractions. This modern usage 

 has probably been the cause of the insertion of this meaning in the supplement to the 

 Century Dictionary. Schweig appears to apply it, however, to the process of separation 

 of Uquid from crystals which he contrasts with separations of any kind in the liquid, a 

 contrast always to be carefully borne in mind in any discussion of differentiation. 

 Schweig's usage appears to be the more justifiable on the basis of the borrowing of the 

 term from the metal-worker, to whom it indicates a process of separation of a liquid 

 portion from a crystaUine or sohd portion such as that accomplished in the Pattinson 

 process for the purification of silver. However this may be, the writer considers it 

 advisable to avoid the term, since, without its context, it does not convey a definite 

 meaning especially with regard to the very much contrasted processes mentioned 

 above. 



^ Silikatschmelzlosungen, I, 96; II, 228. 



3 Daly, Igneous Rocks and Their Origin, p. 226. 



