THE GENESIS OF ORE DEPOSITS 499 
5. Iron, gold, silver, and copper are of more general distribution, 
without much tendency to characterize any special type of magma. 
Iron, however, shows a difference of combination: in the basic 
rocks it is largely combined with sulphur, titanium, or chromium, 
in the acid rocks it concentrates mainly as magnetite. Primarily 
copper is generally combined with sulphur, less commonly with 
arsenic, wherever it is found. 
6. It appears that in the earliest stages of earth history known 
to us there were two primary magmas, granitic and basaltic: from 
these the other existing rock types were derived by differentiation. 
Fic. 1.—Genealogical tree illustrating conceptions of differentiation of igneous 
rocks and ore deposits from primitive magma. 
It is a fair inference that earlier still there was one primitive earth 
magma of intermediate composition, perhaps a siliceous immiscible 
fraction, separated from a mainly metallic fraction that formed the 
heavy core of the earth. In this primitive silicate magma all the 
valuable metals were at first in solution, afterward separating into 
its various differentiated fractions according to their partition 
coefficients, or their degrees of solubility—no doubt assisted by 
gravity-sinking and fractional crystallization. Some metals seem 
to have been more or less soluble in all magmas, while others had 
very decided preferences for a particular type. 
It is on the basis of these principles that the genealogical tree 
here presented (Fig. 1) has been constructed. In its essential 
