FOYAITE-IJOLITE SERIES OF MAGNET COVE 



66 1 



differentiation and admixture (of two members of a series) may 

 to a certain extent be regarded as inverse processes, so that the 

 occurrence of a magma of this anomalous composition need 

 not occasion surprise. Being rich in both of the generally 

 antagonistic groups of oxides, it would be especially liable to 

 differentiation. The general lability of the monzonitic magmas 

 as regards the conditions controlling crystallization has been 

 pointed out elsewhere.^ 



The general chemical composition can also be calculated by 

 the relative volumes of the various phases, which has been the 

 only method heretofore available. This would seem to be far 

 more uncertain than the new method, which is based on the 

 mathematical course of differentiation, since the ignorance 

 of certain data may affect the result very seriously. Thus we 

 cannot tell where the boundaries between two zones really fall, 

 and (beneath the hornstone ridge especially) whether there 

 may not be a zone of transitional material. 



Assuming that the limits come half way between zones, and 

 that they are of uniform thickness in all directions, we can easily 



SiOa 



AI0O3 



(FeO) 



MgO 



CaO 



NaaO 



K,0 



50.02 

 20.89 

 5.87 

 1.36 

 6.89 

 6.86 

 8.10 



^F. L. Ransome, Am. Jour. Sci., Vol. V, p. 370, iS 

 Jour. Geol., Vol. V, p. 376, 1897. 



H. S. Washington, 



