THE GENESIS OF ORE DEPOSITS 407 
pictured by Harker,’ this cause may undoubtedly be invoked to 
explain the separation of pegmatitic material from cooling granites 
under earth pressure. This process has given rise to areas of 
pegmatitic permeation, as described by Barrow,? in the south- 
eastern Highlands of Scotland. It is hardly necessary to point 
out again that pegmatitic mother-liquors often carry notable 
amounts of ore minerals, and in many instances this seems the 
most reasonable explanation of ore-bearing pegmatites. 
From the foregoing brief and imperfect summary it appears 
that ore deposits may arise from all or any of the physicochemical 
processes that have been invoked to explain the origin of hetero- 
. geneity in igneous rocks. In some cases one is applicable, in some 
cases another, but none are apparently excluded on a priori grounds. 
This variety of origins is only what we should expect from the great 
differences observable in the characters of the deposits themselves. 
Here again a correlation can be traced between the nature of the 
effective process and the physical properties of the molten magmas 
of different composition and especially the degree of viscosity. 
Generally speaking, it is admitted that basic magmas are more 
liquid than normal acid magmas in spite of the higher proportion 
of water in the latter. Hence differentiation by diffusion, gravity, 
and immiscible separation are more marked in the basic class, owing 
to lower viscosity. It is only in the differentiated and concentrated 
last residues of the acid class that the solution becomes readily 
mobile and lends itself to formation of veins and pegmatitic permea- 
tion. A similar effect is produced by assimilated fluxes in certain 
cases. These considerations lead again to the same conclusion as 
before, namely, that the ore deposits of the basic rocks are marginal 
and basal, or included segregations of a streaky nature if convection 
currents have been active, while from the acid fluxed magmas are 
formed veins and lodes in all their varieties, either in fissures in 
the rock itself, often congregated near its roof, especially in sub- 
sidiary domes and cupolas,’ or actually external to the intrusion, 
filling fault planes and other fissures and zones of weakness in the 
t Harker, Natural History of Igneous Rocks (1909), p. 323. 
2 Barrow, Quart. Jour. Geol. Soc., Vol. XLIX (1893), p. 330. 
3 Butler, Econ. Geol., Vol. X (1915), pp. 101-22. 
