ORE-FORMATIONS 267 

formations there seems no reason to doubt that a molten magma is not 
infrequently rich in metallic materials, and that solutions of metal have 
proceeded from many batholiths while these were solidifying and cooling. 
We seem, therefore, justified in concluding that igneous rocks are the 
chief, if not the only, sources from which the metals of most epigenetic 
ore-formations have ovz¢ginally been derived. 
The ore-formations due to pneumatolytic action may be looked upon 
as connecting links between the truly syngenetic ores of eruptive rocks 
on the one hand, and the typical epigenetic ore-formations of lodes on 
the other. It is obvious that the process described as magmatic extrac- 
tion is closely related to that of magmatic segregation. In the case of 
the latter the ore has separated out at the time of the eruption and 
consolidation of the molten magma. They are, in short, original 
constituents of the rocks in which they occur. Contact ore-formations 
are, no doubt, also of igneous derivation, for they consist of materials 
which have been extracted from a molten magma and carried into the 
country-rock by superheated vapours. They are thus epigenetic—z.e. of 
later date than the rocks which contain them. Even in cases where 
such magmatic-extraction ores penetrate the igneous rocks themselves as 
- veins and impregnations, they may be yet described as epigenetic. For, 
in such cases, their formation must have been somewhat later than the 
consolidation of the rock they penetrate. It is quite possible, indeed, 
that they may have been derived from some still molten or imperfectly 
solidified portion of the igneous mass. While, therefore, such ore- 
formations probably owe their origin to solfataric or after-action, and 
may thus be said to belong to the same period of plutonic action as the 
igneous rock in which they appear, yet it is obvious that they must be of 
somewhat later formation, and are, therefore, properly included in the 
epigenetic class. o 
Concerning the origin of other epigenetic ore-formations, many 
different and often conflicting views have been held. They are all doubt- 
less secondary formations, derived in the first place either from igneous 
rocks or from veins, etc., of pneumatolytic origin. The process whereby 
epigenetic ores in general have come into existence might be shortly 
defined as a process of concentration. By mechanical and chemical 
operations, igneous rocks have been broken up and the resultant products 
have gone to form sedimentary or aqueous deposits of one kind and 
another. These last have in their turn been subjected to similar changes 
—and new accumulations have been built up out of their ruins. At the 
surface of the earth it is the mechanical deposits—gravel, sand, and 
mud—which are most conspicuous, but immense quantities of materials 
are also carried in solution, some portion of which, under favourable 
conditions, may be observed forming here and there as chemical pre- 
cipitates ; but the great body of dissolved mineral matter finds its way 
out to sea. The less soluble metals and metallic compounds derived 
from the disintegration of pre-existing rocks become concentrated in the 
mechanical and chemical sediments now in process of formation at the 
surface. This may be illustrated by the disintegration of a basic rock, 

