THE FORMATION OF DEPOSITS 23

water being superheated would be a powerful solvent, and
would thus obtain alkalis, that increase its power of dissolving
quartz and metallic sulphides. The metallic barysphere,
like the iron meteorites, doubtless contains sulphides, such as
the iron sulphide, troilite, and phosphides (e.g. schreibersite),
and carbon as graphite. The solution of troilite would give
rise to ferrous sulphate, which is a solvent for gold. The
water would also include chlorides, and fluoric and boric
acids. This complex solvent would work its way upwards
through the rocks, dissolving from them silica and metallic
sulphides. The small spaces would unite into large channels
along major fissures and fault planes, and through them solutions
 would rise more quickly. From the lower zone of
solution the water would reach conditions under which it
would begin deposition. It would be constantly passing
under less pressure and lower temperature, and both influences
 would throw materials out of solution. Chemical
reactions would aid, as contact with lime would neutralize
acid solutions, while carbon by reducing the ferrous sulphate
solution would precipitate metallic sulphides; and as gold
would no longer be soluble it would be precipitated at the
same time. Hence solutions rising up fissures, and especially
up the great faults which mountain structures show to extend
 over 100 miles deep, would bring to the surface metals
from the ore-zone.
Macwmatic OREs—The nature of these solutions is the main
current problem in ore genesis. As the solvent must be
mostly water and the temperature high, they are justly
called hydrothermal ; but as many authorities held that all
the water on or in the earth’s crust falls upon it as rain, the
ore-forming solutions were attributed to surface water which
had sunk underground, had there dissolved scattered metallic
particles and deposited them in lodes as the water was forced
to the surface by the gas pressure due to heat. Ores formed
by this process of lateral secretion appear to be more abundant
 than has been generally admitted in recent years; they
include not only ores of iron and manganese, but many
others, such as the copper ores of Mansfeld, of Cheshire, and
some of those of Katanga, some of the lead and zinc ores of
Mississippi and the rich gold ores of Mount Morgan.
For ordinary lodes the lateral secretion theory has been