412 T. L. Walker — Causes of Variation in the 



variations. According to Soret's principle, the concentration is 

 proportional to the absolute temperature. Thus, in order to 

 have twice as much of these dissolved bases or their silicates at 

 the border as at the center of the eruptive, we would require 

 to suppose that at the time of solidification the absolute tem- 

 perature at the center was twice that at the border. Suppose 

 the temperature at the border of the magma were 700° C. or 

 973° absolute temperature, then the absolute temperature at the 

 center would have been 1673° C. or 1946°. Are such differ- 

 ences in temperature to be expected ? Scarcely. Besides 

 Soret's principle has been established for only very dilute solu- 

 tions and for only slight variations in temperature. The sepa- 

 ration of masses of nearly pure ilmenite as border facies on 

 basic eruptives would, as pointed out by Harker, require much 

 greater difference of temperature, as the proportion of iron in 

 these masses is often twenty-five times that contained in the 

 central part of the eruptive. Such great differences in tem- 

 perature would give rise to convection currents in the magma 

 which would render differentiation quite impossible. 



It should not be forgotten, however, that in all probability 

 differentiation and solidification were in progress at the same 

 time, so that we should compare the proportion of the " dis- 

 solved" contained in the very narrow strip along the border, 

 not with the proportion contained in the present center, but 

 with that contained in the magma which occupied the center 

 at the time when the first border strip solidified, since the 

 present central rock is more acid than the magma which occu- 

 pied the center during the earliest stages of differentiation. 

 Similarly the next narrow strip should be compared with the 

 magma which occupied the central portions of the eruptive 

 reservoir at the time when the narrow strip in question solidi- 

 fied. In this way the argument of Harker is weakened though 

 not destroyed. 



Vogt* accepts Soret's principle as having been a prominent 

 cause of differentiation and even of the formation of marginal 

 deposits of sulphide ores such as the Canadian and Norwegian 

 nickel deposits, and of deposits of titanic iron ores such as 

 those of Eaie St. Paul on the Lower St. Lawrence. He is of 

 the opinion, however, that magnetic forces may have had an 

 influence in localizing minerals rich in iron, after their crystalliza- 

 tion from the magma. In this connection it is interesting to 

 notice that minerals rich in iron are generally among the first 

 to crystallize and that in differentiated eruptive areas they are 

 almost invariably concentrated as border facies. 



Harkerf maintains that the tendency to homogeneity in 



*Stockh. geol. Foren. Forh., xiii, 1891, pp. 520-683. 

 f Loc. cit. 



