178 
NATURE 
[APRIL 16, 1914 
presupposes the existence of a continuous sheet of 
water in circulation between the ground-water level 
and the lower limit of the ‘‘zone of fracture,” no cir- 
culation being admittedly possible in the underlying 
“zone of flowage.’”’ The weak point in Van Hise’s 
assumption of a ‘‘sea of underground water ’’ lies in 
the fact that deep mines are usually found to be dry, 
the drainage being confined to the upper levels. This, 
for instance, is the experience in the copper mines of 
Lake Superior, in the gold mines of the Rand, and in 
those of Bendigo. Van Hise, in reply to this criticism, 
attributes this dry zone to the closing of the passages 
by cementation; but the restriction of the ground- 
water circulation is equally fatal to the meteoric de- 
rivation of deep-seated thermal springs and other 
phenomena connected with vulcanicity. 
In recent years there has been a partial reaction to 
igneous views. Thus certain classes of ore-deposits 
are now held to have been formed by a differentiation 
of igneous magmas prior to consolidation. Such, for 
instance, is the origin ascribed to certain titaniferous 
iron-ores in basic eruptives, chromite in peridotites, 
nickeliferous pyrrhotite in norite, and _ primary 
platinum in ultra-basic rocks. Similarly pegmatites, 
and even some quartz-veins, are considered to have 
originated by the consolidation of the aqueo-siliceous 
residuum of a slowly cooling granite magma. 
But more important in its application to ore-deposi- 
tion than magmatic differentiation is the theory which 
Vogt has founded on the metalliferous emanation 
hypothesis, by which Elie de Beaumont and Daubrée 
sought to explain the origin of tin-ore deposits. 
According to the pneumatolytic theory, certain agents 
minéralisateurs, such as fluorine, chlorine, sulphur, 
phosphorus, silicon, and boron, have the property of 
forming with the metals volatile compounds, which 
escape from the granite-magma as gases with low 
critical temperatures (the aura granitica of Elie de 
Beaumont). These compounds ascend through already 
formed fissures in the overlying rocks, or force their 
own passage by attacking the minerals that compose 
them. In this manner, for instance, cassiterite, 
wolfram, tourmaline, fluorspar, topaz, beryl, axinite, 
datolite, apatite, etc., are deposited either in the 
granite itself, or in the sediments comprised within its 
metamorphic aureole. 
Closely connected with pneumatolysis in the rdle 
ascribed in ore-deposition to the so-called magmatic 
waters, a term that has come into use for water not 
of atmospheric origin, but dissolved or occluded in 
some way in molten magmas, from which it separates 
by liquation and distillation on the fall of temperature 
and pressure. In it are concentrated the substances 
that (at the existing temperature) are more soluble in 
water than in the silicate magma. 
Suess, in an address on the Karlsbad springs, de- 
livered in 1902, directed attention to the connection 
existing between thermal springs, vulcanicity, and 
ore-deposition. He applied the term hypogene or 
juvenile to thermal springs (like those of Karlsbad) 
which, originating in the depths of the earth’s crust, 
bring water to the surface for the first time. Such 
hot springs are, in fact, the last survivors of vul- 
canicity, being the relics of a late stage of fumarole 
activity. Their mineral content comprises readily 
soluble compounds of the alkalies and alkaline earths, 
together with, and partly in combination with, sul- 
phur, chlorine, and carbon dioxide, the less soluble 
metallic compounds having already been deposited as 
ores at lower depths in the earth’s crust. According 
to Suess the after-products of eruption vary with the 
temperature; in the earlier (pneumatolytic) phases of 
emanation the gases are dry and their deposits (such 
as tin-ore and its accompanying boron, fluorine, 
tungsten, and uranium minerals) are the products of 
NO. 2320, VOL. 93| 
sublimation, At a later period, strongly alkaline mag- 
matic waters are given off, and to these are attribut- 
able the sulphide and arsenide phases of vein-forma- 
tion, e.g. the deposition of iron pyrites, chalcopyrite, 
primary bornite and chalcocite, enargite, galena, 
blende, etc. 
But although, as we have seen, waters of meteoric 
origin have been displaced from their pride of place 
as agents of deposition for what we must term the 
primary sulphide ores, they are undoubtedly the forma- 
tive agents for a considerable number of ore-deposits, 
including the products of oxidation, chlorination, and 
reduction above the permanent water level, and the 
secondarily enriched ores usually found immediately 
below the junction of the zone of oxidation with the 
zone of primary sulphides. So important are the 
functions of the vadose waters (to use Posepny’s term 
for the shallow water circulation) in dissolving and re- 
depositing at a lower level the ores of copper in a 
concentrated form, that it has been confidently stated 
that the bulk of the copper production of the world, 
not alone in the past, but also at the present time, is 
drawn from the zone of secondary enrichment. This 
view will, perhaps, appear exaggerated in the light of 
the results recently obtained by Sales at Butte; but for 
the majority of the great copper deposits of the world 
it may still pass unchallenged. 
In the same way, vast deposits of high-grade iron- 
ore have been formed as the result of secondary en- 
richment, but under entirely different conditions from 
those that determine copper-ore enrichment. Thus 
the hzmatite ores of Lake Superior are believed by 
Van Hise to have been derived by the oxidising and 
concentrating action of vadose waers, from a low- 
grade cherty iron carbonate originally deposited under 
water as a chemical sediment; and he draws the 
important conclusion that ‘‘the ore-bodies cannot be 
expected to extend beyond the depth to which the 
descending waters may bear oxygen and precipitate 
iron oxide.’”” He has ‘“‘no doubt that vastly more 
high-grade iron-ore will be taken out in the Lake 
Superior region above the r1ooo-foot level than below 
it.” If this be true, the iron-ores of that district, 
with more than 60 per cent. of metallic iron, are not 
inexhaustible. 
The enrichment of gold-ores also takes place in the 
zone of oxidation; but in their case the action of the 
vadose waters results in an abstraction of the more 
soluble and less valuable metals, leaving behind a 
smaller quantity but a relatively richer material; in 
other words a diminution of the specific gravity of the 
whole material raises the gold tenor. Furthermore, 
there is also an increase in the fineness of the gold, 
due to the removal of a portion of the silver with 
which it is alloyed. The Mount Morgan mine in 
Queensland is a good instance of a gold-ore enrich- 
ment brought about by the vadose circulation; here 
the oxidation of a pyritic copper lode with subordinate 
gold has, by the removal of the sulphides of iron and 
copper, led to the formation of an upper zone of 
cellular quartz, in which the increased ratio of gold 
to vein-stuff was the vera causa of the richness of one 
of the premier gold mines of the world. But, as with 
increasing depth the mine-workings are extended 
below the oxidation-zone, the copper production is 
becoming more important than the gold yield. 
The Witwatersrand Banket is another example. In 
this case the primary ore is auriferous iron pyrites 
disseminated in a quartz-conglomerate on which in- 
tense silicification during cementation has impressed 
the character of a quartz vein. The removal of the 
pyrites from the zone of oxidation, which extends to 
200-300 ft. below the surface, left an enriched free- 
milling ore that gave marvellous returns on the amal- 
gamation plates of the first Rand mills. Since the 
