38 JOURNAL OF GEOLOGY— SUPPLEMENT " 



undercooled. Thus if the liquid F is cooled very quickly to a 

 temperature below 1200°, crystallization, if it takes place at all, will 

 give 50 per cent diopside and 50 per cent plagioclase of the uniform 

 composition AbiAui (unzoned) . 



Crystallization with zoning. — -When the cooling is too rapid to 

 give crystallization of the perfect equihbrium type and yet not 

 rapid enough to give the great degree of undercooling referred to in 

 the foregoing, the formation of zoned crystals of plagioclase will 

 result. According as the one or the other of the above-named rates 

 of cooling is approached the degree of zoning is reduced to a mini- 

 mum. With a certain intermediate rate of cooling maximal zoning 

 results. In this case a crystal once separated suffers thereafter no 

 change of composition, the liquid disregarding crystals which have 

 already formed, so that the crystallization of the liquid may be 

 regarded as beginning anew at each instant. 



The effect of this action may be realized by considering that 

 during the crystallization of the liquid F, as already outlined, the 

 liquid portion is separated from the crystalline portion at a tempera- 

 ture of, say, 1220°. At this temperature the liquid has the com- 

 position K and we shall imagine that this separated liquid is 

 crystallized under perfect equilibrium conditions. Instead of 

 becoming completely crystalline at 1200°, as it would if the crystals 

 had not been removed, it now becomes completely crystalline 

 only at 1178°, and the final liquid, instead of the composition M, 

 has the composition S, i.e., is much richer in albite. If the virtual 

 separation of liquid from crystals is a continuous process accom- 

 plished through the intervention of zoning, it is plain that the 

 offsetting in the composition of the final Hquid is limited only by 

 the eutectic albite-diopside which it actually attains in the case of 

 maximal zoning. This fact is true, not only of the special liquids 

 to which reference has been made, but of any mixture of anorthite, 

 albite, and diopside whatsoever. 



Crystallization with subsidence of crystals. — The sinking of 

 crystals of plagioclase in a mass of liquid which is very slowly cooled 

 will obviously affect the upper layers from which the crystals have 

 settled in the same manner that zoning affects the residual liquid. 

 The upper layer is continually enriched in albite and its tempera^ 



