DEFORMATION OF CRYSTALLIZING MAGMA 



N. L. BOWEN 

 Queen's University, KLingston, Ontario 



In the foregoing paper Professor Grout raises certain objections 

 to processes that I have advocated as significant in petrogenesis, 

 and to some of these objections I wish to take the opportunity, 

 offered by the editors, of making a brief reply. 



With particular reference to the Duluth gabbro, Grout says 

 that "the arguments for circulation are conclusive," but offers no 

 further support for the arguments than that formerly offered. 

 The banding and fluxion structures are conclusive evidence of 

 circulation of a sort, perhaps, but not necessarily of convective 

 circulation. Convection suffers from the disability of requiring 

 the further assumption of rhythmic crystalhzation, i.e., crystalhza- 

 tion which is periodic with respect to the nature of the substance 

 crystallizing. In this manner it is hoped to obtain alternating 

 layers of different composition, but it would seem much more 

 probable that convection would effect a thorough mixing of the 

 successive products. The rhythmical crystallization itself is, 

 moreover, an assumption that has nothing to support it in the 

 whole realm of crystallization phenomena. The Liesegang ring 

 effect is a totally different affair, and any assumption of rhythm in 

 crystallization, such as Grout pictures, should be made only in 

 extremis. 



On the other hand the down-warping of the tabular mass of 

 crystallizing magma would seem to necessitate the development of 

 fracturing along planes sensibly parallel to the tabular extension 

 of the mass. The crystalHne mesh would be subject to an action 

 that is to all intents and purposes thrust-faulting along these 

 planes. This action opens the possibility of the filling of the fault 

 "fissures" with liquid from the interstices of that portion of the 

 adjacent mesh that happens to be weakest and of developing layers 



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