"Vol. 57.] ANNIVEESARY ADDRESS OP THE PRESIDENT. IxXXV 



I am by no means averse to making heavy drafts on the bank of 

 time for geological purposes, but unless some effective answer to 

 Mr. Becker's arguments can be found, I think that we shall have 

 to give up unaided molecular flow as an important factor in the 

 origin of petrographical species. 



Mr. Becker has not, however, simply confined himself to destructive 

 criticism. He has proposed a theory of differentiation dependent on 

 ' fractional crystallization.' During the cooling of a mass of molten 

 matter in a dyke or laccolite, convection-currents will be established ; 

 these will act as stirrers, and, aided by diffusion, will tend rapidly 

 to restore homogeneity in the liquid mass after it has been destroyed 

 by the deposition of the first-formed crystals on the walls of the 

 cooling surfaces. He compares a laccolite in which the marginal 

 parts are different from the centre, to a barrel of cider which has 

 been frozen from the outside. During the earlier stages nearly pure 

 ice is formed on the walls, while the alcohol is concentrated in the 

 central portion ; from this a liquor, gradually increasing in strength, 

 may be drawn off as consolidation progresses. Here we see a further 

 development of the idea originated by Darwin. 



All forms of the differentiation-theory take as their starting- 

 point a homogeneous magma, and then proceed to derive from it the 

 different varieties of igneous rocks as we now see them by magmatic 

 or some other form of differentiation. Are we justified in taking 

 this view ? As applied to certain districts, and especially to the 

 Christiania district which Prof. Brogger has done so much to 

 elucidate, it has proved of great value. But if we look at the general 

 question, there are many facts which should give us pause. The 

 earth's crust is certainly heterogeneous, and if magmas are, in any 

 case, formed by the refusion of solid rocks, it is probable, as 

 Mr. Becker has pointed out, that such magmas would be hetero- 

 geneous at the start. Even the refusion of homogeneous rocks may 

 give rise to a heterogeneous magma, comparable to that produced by 

 Paraday in his experiments on glass. The cause of some of the 

 variations in igneous rocks is therefore probably to be sought for in 

 actions which antedate the formation of the magmas. But even 

 homogeneous magmas may become modified by the absorption or 

 assimilation of the rocks through which they pass. This point has 

 been clearly established and especially emphasized by M. Michel 

 Levy, Prof. Barrois, and Prof. Lacroix in France, and by 

 Dr. Johnston Lavis, Prof. Sollas, Prof. Cole, and Mr. Harker in 

 this country. 



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