
268 STRUCTURAL AND FIELD GEOLOGY 
such as basalt. By the various epigene agents of change, the ingredients 
of this rock are converted partly into relatively insoluble and partly into 
soluble materials. The felspar, for example, is broken up, and transformed 
into carbonates of lime, etc., and hydrous silicate of alumina—the former 
being soluble, the latter insoluble. The augite and olivine, in like manner, 
yield soluble and insoluble materials. The magnetite and ilmenite, which 
are often abundant, are not so prone to alteration. Hence when basalt- 
rocks are finally reduced by epigene action, their relatively insoluble 
materials are represented by clay, some siliceous sand, and, it may be, 
iron-ores—the latter often becoming mechanically separated in alluvial 
deposits as black “‘iron-sand.” Or the iron content of the basalt may be 
largely carried away in solution as a bicarbonate, and eventually be 
thrown down as a chemical precipitate. Thus, partly by mechanical and 
partly by chemical processes, the iron distributed through the original 
rock as a primary content tends, as the final result of epigene action, to 
become concentrated. In like manner, other heavy metals and more or 
less insoluble ores, derived from the disintegration of many different 
igneous, schistose, and derivative rocks, and from the breaking-up of 
pre-existing ore-formations, are similarly often concentrated in recent 
mechanical and chemical accumulations. 
The process of rock-disintegration and decomposition, however, is not 
confined to the earth’s surface, but affects the crust at all depths to which 
water can descend. The rock-changes produced below ground are, of 
course, almost exclusively chemical. Water descending from the surface 
often plays a double part. It not only attacks the rocks, leaching out 
their soluble materials, but, when it becomes a saturated soluticn, it may 
redeposit its burden in the pores, capillaries, and more open spaces 
through which it filters. The solvent power of underground water is 
rendered evident by the immense quantities of material which are 
brought up to the surface by springs, and by the cavities which result 
from the removal of all this soluble rock-stuff. It is further seen in the 
phenomena displayed by most epigenetic ore-formations—for we can 
hardly doubt that the contents of lodes, etc., ores and veinstones alike— 
have, to a large extent at least, been dissolved out of the rocks of the 
crust at all depths by the action of water. The only conspicuous 
exceptions among epigenetic ore-formations, are those “ contact- 
deposits” which have been formed directly by the heated vapours 
escaping from eruptive masses. 
From a general point of view, therefore, ore-formations would seem 
to come naturally under the following divisions :— 
1. Magmatic-segregation Ore-formations.— Under this head would be 
classed all native metals and ore-masses occurring as original con- 
stituents of igneous rocks. 
2. Vagmatic-extraction Ore-formations.—This division would embrace 
the various ore-deposits which are genetically connected with eruptive 
rocks, and are the result of pneumatolytic processes. 
3. Secretionary Ore-formations.—In this group would be included the 
great majority of ore-deposits formed underground by the chemical 
