R. H. Rastall — Differentiation and Ore-deposits. 293 



sense. The basic magma is of higher density and therefore sinks 

 to the lower level. 



With regard to the metallic constituents, sulphides and oxides, 

 it is evident that they will tend to distribute themselves also between 

 the two partial magmas in the ratios of their solubilities. From the 

 association of particular metals with definite rock-types it may be 

 concluded that some metals are more soluble in the acid fraction, 

 some in the basic, while others appear to occur in both. Thus, copper 

 and iron, for example, are found in igneous rocks of almost all 

 types, tin, tungsten, and molybdenum are definitely associated with 

 granites, while nickel, chromium, and platinum belong to basic and 

 ultrabasic types. When the temperature falls still further a similar 

 process will take place with regard to the metallic constituents 

 dissolved in each partial magma, and the oxidic and sulphidic ores 

 will separate out in each case as immiscible layers, as before outlined. 



Now it is clear that this principle is capable of almost indefinite 

 extension ; when the temperature is high enough we may conclude 

 that all substances, whether silicates, sulphides, oxides, fluorides, 

 borides, tungstates, or anything else, may become mutually soluble, 

 i.e. miscible in all proportions, therefore the deeper we descend into 

 the earth the more generalized should we expect the rock-types to 

 become, and the more evenly distributed the metallic compounds. 

 It is obvious, however, that the lowest levels attainable by artificial 

 means or the depth of formation of most deep-seated rocks brought 

 up by geological disturbances of mountain-building and faulting, 

 are but trifling in comparison with the radius of the earth. 



It may be objected that the known varieties of igneous rocks are 

 too numerous and complicated to have been formed in this simple 

 way, but it has been effectively shown by Daly ^ that by far the 

 greater number of rock-types are rare and local, forming but a 

 minute fraction of the total. For example, he shows that in North 

 America the much-talked-of alkaline group (syenites, etc.) constitute 

 only about O'l per cent of the total, and in other parts of the world 

 they are not much more abundant.^ In fact, it can be maintained, 

 probably with truth, that the vast majority of the rock-types to 

 which special names have been applied are due to local circumstances, 

 such as assimilation of limestone and similar fluxing material, 

 giving rise to special and peculiar chemical and physical processes 

 within the magmas. Daly considers that there are but two funda- 

 mental magmas, the granitic and the basaltic, and of these he is 

 inclined to regard the basic or basaltic magma as the most primitive. 

 This is in conformity with the high density of the earth as a whole. 



We therefore picture the evolution of the primitive homogeneous 

 earth-magma as a process of difierential splitting into successive 

 fractions owing to a progressive decrease of mutual solubility with 

 fall of temperature. The exact nature of the process giving rise to 



' R. A, D&lj , Igneous Bocks and their Origin, New York, 1914, p. 47, 



