iO,2 LACCOLITES AND BYSMALITES. 



and the basic in the upper part, the diffusion taking place through 

 the narrow part of the dumb-bell, and sometimes the average 

 magma became caught in this part and consolidated as diorite. 



The facts are plain enough, and it is perhaps as well to 

 content oneself with them at present ; but there is an explanation 

 which, if only speculative, may be worth mentioning in order to 

 show that this differentiation of an average magma is not wholly 

 unexpected. 



The Ionic Separation of Substances on a Rock-magma. 



When an iron-bearing rock weathers at the surface of the 

 earth, the iron does not travel outwards to the sea along with 

 the soda, lime and potash, but seeks the centre of the earth.* The 

 fact is easily recognisable in the replacements of limestone by 

 iron ores, which is of common occurrence ; in these cases certain 

 conditions have caused precipitation of the iron from solution, 

 but where the conditions are not favourable, then the iron goes 

 on its journey downwards towards the base of the crust. Weak 

 solutions such as those which carry the iron are ionised, and the 

 metallic ion carries a positive charge of electricity ; the metallic 

 ion possibly is attracted by the magnetic core of the earth, but 

 however that may be, the iron goes down and must come to rest 

 in the base of the crust, beyond which water cannot penetrate. 

 This would lead to an accumulation of a positive charge at the 

 base of the crust. Above this is suspended a solution or a fluid 

 magma which is an electrolyte, I that is, one in which the sub- 

 stances are ionised, and the metallic ions carry a positive charge 

 and the acid ions a negative one. Under such circumstances, the 

 negative ions would be attracted by the positive charge at the 

 base of the crust ; and the metallic positive ions would be repelled, 

 and in that way a magma of everage composition would be 

 differentiated into an acid and basic series, the acid part accu- 

 mulating in the lower half of the dumb-bell and the basic in the 

 upper. Whether this cause is sufficient for the effect I do not 

 know, but, considering the immense time during which geological 

 phenomena take place, a small but persistent electrical attraction 

 and repulsion such as exists under the circumstances would have 

 far-reaching effects ; one is astonished, rather, at the smallness of 

 the differentiation ; one would expect a complete separation into 

 ultra-acid and ultra basic magmas. 



* E. H. L. Schwarz : " Causal Geology," 1910, p. 73 ; " Selective absorb- 

 tion of substances in the earth's crust," Report S.A. Assoc, for Advance- 

 ment of Science. IQT3. n. t8t. 



' C. Barns and J. P. Jddine;s, Amer. Journ. Sci. vol. xliv, 1892, p. 242. 



