THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXVIIL — Fractional Crystallization of Rocks ; by 

 George F. Becker. 



Among the phenomena most often appealed to in support 

 of the theory of magmatic segregation or differentiation is the 

 symmetrical arrangement of material in certain dikes and lac- 

 colites. This separation seems to me readily explicable in cer- 

 tain cases without resort to the hypothesis of the division of a 

 homogeneous fluid into two or more distinct fluids. I have 

 already called attention to the process in brief terms ;* though 

 very well known it has not otherwise been invoked, so far as I 

 am aware, to explain rock differences. If the suggestion has 

 previously been made, it seems time that it should be repeated. 

 If we suppose a dike in cold rock tilled with mobile lava 

 which does not overflow, or has ceased to overflow, the mass 

 will be subjected to onvection currents, because the liquid 

 near the walls will be cooler than that near the median plane 

 of the dike. A circulation of lava will then take place, the 

 descending flow at the sides being compensated by ascending 

 flow near the central surface. The conditions are roughly rep- 

 resented in the diagram below. If the lava is a homogeneous 

 mixture of two liquids of different fusibility, then the crusts 

 which first form upon the walls will have nearly the same com- 

 position as the less fusible partial magma. If one follows 

 mentally a small portion of the liquid in its circulation, it will 

 clearly deposit at each of its early contacts with the growing 



*This Journal, vol. iii, 1897, p. 39. 



Am. Jour. Sol— Fourth Series, Vol. IV,' No. 22.— Oct., 1897. 

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