406 R. H. RASTALL 
as to which we have as yet no absolute evidence, but it seems quite 
clear that this cause may be legitimately invoked to explain cases 
of magmatic sulphide segregations. Vogt has shown by quantita- 
tive determinations that the mutual solubility of norite magmas 
and sulphides is very small indeed; at 1300° C. and one atmosphere, 
less than 0.5 per cent in the case of pyrrhotite, while copper and 
nickel sulphides are practically insoluble in norite melts. More- 
over, it is improbable that these relations are seriously altered by 
high pressures.* 
Up till the present time we have no definite information as to 
the part played by assimilation and stoping in the formation of 
ore deposits. In the nature of things such a process would neces- 
sarily be difficult to detect, since it would itself destroy its own 
traces. It is possible, however, that some masses of ore in acid 
and intermediate rocks may have been introduced in this way, 
such as the deposits of iron, copper, and gold in magmas of the 
monzonitic and dioritic facies. This, however, is pure speculation 
with no tangible evidence to support it. It seems possible that in 
the earliest solid crust of the earth, afterward remelted, there may 
have been segregations of metallic ores, afterward reabsorbed and 
differentiated, as suggested by Morrow Campbell? As to this, 
likewise, there can in the nature of things be no positive information. 
It is known that in some localities, for example, southwestern 
Norway, there is a notable concentration of minerals of the rare 
earths into syenitic pegmatites, as described by Brogger.3 If we © 
accept Daly’s theory of the origin of alkaline rock types by the 
fluxing effects of assimilated limestone in normal magmas, it 
would seem to follow that the concentration of the rare earths 
here must also have resulted directly from the assimilation. On 
this question as a whole further information is needed, as few 
observations are available on the occurrence of ores in connection 
with highly alkaline magmas. 
With regard to the squeezing out of liquid residues from partially 
consolidated rocks, like water from a sponge, as so graphically 
t Vogt, Norsk geol.tidsskr., 1917, Pp. 77- 
2 Bull. Inst. Min. Met., October, 1920, p. 3. 
3 Brégger, Zeitschr. fiir Kryst., Vol. XVI (1890). 
