THE REACTION PRINCIPLE IN PETROGENESIS 193 



fractionation under conditions of slow cooling we may, however, 

 have such concentration of minerahzers and consequent lowering 

 of consoKdation temperatures that conditions correspond to those 

 of other facies. 



EMPIRICAL RULES REGARDING ORDER OF CRYSTALLIZATION 



As the result of a most extensive knowledge of the relations of 

 minerals in rocks Rosenbusch formulated certain rules regarding 

 the order of crystallization. When petrologists began to think of 

 the crystallization of rocks in terms of eutectics these rules seemed 

 to be quite at variance with theory, for this stated that those 

 minerals should separate first that were in excess over eutectic 

 proportions. To reconcile this with Rosenbusch's rules giving a 

 fixed order of crystallization was a very difficult problem, and yet 

 it was conceded that his rules represented the facts in very remark- 

 able degree. In the fight of the reaction principle no such difficulty 

 is encountered. It is characteristic of the reaction series, as we 

 have already seen, that however small an amount of any member 

 may form, it always forms before a lower member of the series. 

 In so far as reaction series control the crystalHzation of rocks they 

 tend to produce a fixed order of crystalHzation. 



Another generafization concerning the order of crystalfization 

 came from the French school of petrology. It was that the minerals 

 separate in the order of their fusibifities, the least fusible first. 

 This generafization, too, in the fight of the doctrine of eutectics 

 seemed to be quite absurd, but here again there is a very consider- 

 able accord with the facts. Such substances as spinel and chro- 

 nfite are among the most refractory materials known. Magnesian 

 ofivine has the highest melting-point of the common rock-forming 

 sificates. The more magnesian pyroxenes stand next in the fist, 

 with the more calcic feldspars about on a par with them. The 

 more complex pyroxenes, the amphiboles, and in particular the more 

 alkafic feldspars are lower still. And this fist corresponds very 

 satisfactorily with the order of separation of the minerals from 

 magmas. Again we are dealing with a tendency that is introduced 

 by the presence of reaction series. It is a famifiar character of a 

 reaction series of the continuous type that the higher melting 



