566 N. L. BOWEN 



DEDUCTIONS TO BE COMPARED WITH OBSERVED RESULTS 



Throughout the foregoing study of the -reactions between inclu- 

 sions and magma, attention has been directed mainly to its theo- 

 retical aspects, that is, to deducing from equilibrium considerations 

 what reactions should occur, together with the effects of these upon 

 the further crystallization of the magma. All of these deductions 

 can be put to the test by observation of what has actually occurred, 

 in particular by a study of the reaction rims formed about inclusions. 

 It should not be expected that each inclusion will tell the whole 

 story, but a general study of inclusions should do so. Not all the 

 differentiates that might later form from the hybrid mass need be 

 shown by the reaction rims, but certainly there should be formed 

 some whose relationship to these possible later differentiates is 

 established by their frequent association in many areas. 



In some instances examples have been cited which appear to 

 show that the expected reactions do occur. Such are the formation 

 of granitic reaction rims by the action of basaltic magma on acidic 

 rocks, the making of basic inclusions into biotite-rich masses by 

 granitic magma, and others. The formation of alkaline rocks by 

 the action of ordinary magmas on limestones is, at present, incapable 

 of support on the above grounds. No example is known where 

 inclusions of limestone, contained in an ordinary rock, are sur- 

 rounded by reaction rims of feldspathoid-bearing rock. It is true 

 that limestones and alkaline rocks are often intimately associated, 

 but there is no assurance that the magma was not already an 

 alkahne magma before it acquired this association. As we have 

 already pointed out, this appears to be the conclusion that Shand 

 reaches concerning the Sekukuniland occurrence, though he favors 

 also the conception that the limestone emphasized its alkaline 

 nature. 



In the Fen area of Norway, one of the newer areas to which the 

 limestone-syntectic hypothesis has been applied, there is a very strik- 

 ing association of alkaline rocks and carbonate rocks.' However, 

 nothing there displayed demonstrates a change of subalkaline 

 magma to alkaline magma through the influence of the carbonate 



' Cf. W. C. Brogger, "Die Eruptivgesteine des KristianiagebietesIV," Vid. Selsk. 

 Skr. I. Mat. Naturv. Klassc (1920). 



