Yol. 57.] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxix 



Bunsen explained the varieties of igneous rock revealed by his 

 analyses by assuming the independent existence of two magmas— 

 the ' normal pyroxenic ' and ' normal trachytic ' — and supposing a 

 process of intermixture to account for the intermediate varieties. 

 Yon Waltcrshausen thought that igneous magmas were arranged in 

 a series of concentric shells, according to specific gravity. Durocher, 

 in his celebrated essay on Comparative Petrology, maintained 



' that ail igneous rocks, modern and ancient, were derived from two magmas 

 which co-exist below the solid crust of the globe, and occupy there each a 

 definite position.' 



His two magmas — basic and acid — do not differ materially from 

 those of Bunsen, and his idea of their arrangement in the earth's 

 crust is practically the same as that of Yon Waltcrshausen. He 

 compared the two magmas to baths of fused metals, which separate 

 into distinct alloys on cooling. He does not give actual illustra- 

 tions, but we may consider one, in order to give precision to the idea. 

 A mixture of 43-64 per cent, of bismuth and 56*36 per cent, of zinc 

 separates at a temperature between 700*^ and 800° C. into two 

 alloys, which arrange themselves according to specific gravity. On 

 cooling, the heavier is found to contain 84'82 per cent, of bismuth 

 and 15*18 per cent, of zinc ; the lighter 2'47 per cent, of bismuth 

 and 97*53 per cent, of zinc. Tf silver be added to the mixture, 

 there is also a separation into two alloys, so long as the amount of 

 silver is less than about 40 per cent. ; when it exceeds this amount, 

 there is no longer any separation. 



Harocher speaks of eruptions which derive their supply from the 

 primary magmas as belonging to the first order, and those which 

 draw their material from more or less isolated magma-basins as 

 belonging to the second order. The latter furnish rocks which 

 depart from the normal type, and this he explains, in part at least, 

 by assuming a process of separation analogous to that by which tho 

 primary magmas were produced. Thus he says : — 



' It is therefore probable that phonolitic and trachytic porphyry are only the 

 two opposite products of a liquation which took place in the niidst of the 

 fluid mass ; they are, as it were, the two inverse allojs into which we so often 

 see a metallic bath divide itself.' 



The type of magmatic difi'erentiation conceived by Durocher may 

 be illustrated by a very simple experiment. Place some phenol and 

 water in a Florence flask : two immiscible conjugate solutions will 

 be formed — a solution of water in phenol at the bottom, and a 

 solution of phenol in water at the top. Now heat the mixture to 



