Volcano Ösliima, Idzu. 105 



Bowen's'^ discussion on tlie crystallization of melts of any 

 system involving a mix-crystal series under various conditions may 

 be summarized as follows : — 



(I) Crystallization with Perfect Equilibrium. — Let A and B 

 be respectively the higher- and lower- melting components of the 

 mix-crystal series. When the cooling of the melt is extremely 

 slow and perfect equilibrium prevails, the crystals first separated 

 from the melt are rich in A, but as the cooling proceeds, the 

 crystals, not only those in the act of separating but also those 

 which have previously separated, continually change in composi- 

 tion as a result of the interchange of material between liquid and 

 crystals, becoming richer in B. The crystals formed in this 

 manner would be homogeneous throughout. 



(II) Crystallization icith Great Under cooVnig. — When the 

 cooling of the melt is extremely rapid, the liquid may be under- 

 cooled to a certain temperature before crystallization begins, 

 whereupon the crystals of uniform composition (without zonal 

 structure) will separate out. 



(III) Crystallization with Zoning. — When the cooling is 

 moderately rapid, there may be only a limited opportunity or 

 none at all for a change in composition of the crystals already 

 separated, but tlie composition of crystals in the act of separating 

 would change toward the enrichment of B, so that the resultant 

 crystals would show a zonal structure due to diiference in com- 

 position varying from one rich in A t(j one rich in B. There 

 is a certain rate of cooling wdiich gives maximum zoning, in 

 which case, the outermost zone of the crystal has the composi- 

 tion of pure B, whatever the total composition of the original 



1) •' The Later Stages of the Evolution of the Igneous Rocks," Jour. Geol, Vol. XXin., 

 Supplement to No. 8, 1915. 



