



270 STRUCTURAL AND FIELD GEOLOGY 
the matter to the proof by an examination of the ore-formations and 
country-rock of the Black Forest, and found that the phenomena were 
in keeping with his expectations. It appeared to him evident, that the 
nature of the ore-formations was directly affected by changes or variations 
in the composition of certain mineral constituents of the country-rock. 
For example, when the mica of the gneiss contained minute proportions 
of copper, cobalt, arsenic, and bismuth, the lodes yielded smaltite 
(arsenide of cobalt), and various ores of copper. In other places where 
the mica of the country rock contained silver, arsenic, bismuth, cobalt, 
and nickel, and little or no copper, the lodes were found to carry arsenical 
ores of silver, cobalt, and nickel, but no copper-ore. Although the 
primary character of the metallic constituents of the silicates analysed 
by Sandberger has been questioned—the metals being now considered 
by many to be subsequent introductions—this does not quite invalidate 
the theory of lateral secretion. That theory explains so many facts, 
indeed, that it must be to a considerable extent true. Nevertheless, it 
is not a complete explanation, for it fails to account for certain notable 
phenomena. If the contents of lodes had always or even often been 
derived by lateral secretion from the adjacent country-rock, then the 
former would depend on the nature of the latter to a much greater extent 
than is found to be the case. Many examples might be cited to show 
that there is no apparent relation between secretionary ore-formations 
and the rocks they traverse. For example, several systems of lodes 
are met with crossing one and the same country-rock, and nevertheless 
carrying very different assemblages of ores. On the other hand, many 
lodes cut through rock-formations of all kinds, igneous, sedimentary, 
and schistose, without showing any marked change in the nature of 
their contents. Once more, the opposite walls of a lode may consist 
of totally different rocks (schists, it may be, on one side, and greywacké, 
sandstone, or limestone on the other), and yet the ores and veinstones 
may be symmetrically disposed in corresponding layers on the two walls. 
Such phenomena as the foregoing occur so commonly that many 
observers have concluded that the theory of lateral secretion must be 
abandoned. It seems to them more likely that the contents of lodes 
have been deposited from solutions ascending from considerable depths. 
We do not know to what depth water penetrates the earth’s crust, but so 
long as it can find a way for itself there seems no obvious reason why it 
should not descend until it attains a temperature at which it can no 
longer exist as water. At what distance from the surface this “critical 
temperature ” (about 690° F.) is reached, can only be roughly conjectured. 
If the increment-of heat as observed in mines and deep borings—1° F. for 
every 50 or 60 feet of descent—be continued indefinitely downwards, the 
critical temperature for water would be reached at a depth of over six 
miles. It is probable, however, that the rate of increase observed near 
the surface does not continue indefinitely, but is more likely to diminish 
progressively with the increasing density. If such be the case the 
critical point for water may not be reached at a less depth than eight 
miles or more. 
