ApRIL 16, 1914| 
exhaustion of this high-grate, free-milling ore early 
in the history of gold mining on the Rand, the mines 
have been worked in low-grade unoxidised pyritic ore; 
and this has shown a gradual but steady impoverish- 
ment with increasing depth—a fact which supports 
the view that the gold of this deposit was precipitated 
by ascending thermal waters in proportion to their 
loss of temperature and pressure. 
One of the most remarkable advances in the science 
of ore-genesis during the period under review is the 
Classification of Ovre-Depostts. 
NATURE 
179 
recognition of the important réle played by meta- 
somatism in the formation of ore-bodies. 
that the rocks adjacent to vein-fillings often contain 
small quantities of metallic ores similar to those com- 
posing the veins themselves, or are altered for some 
distance away from them, was observed at an early 
date; but its significance was very differently inter- 
The fact 
preted. The lateral secretionists pointed on one hand 
Nature of Deposit 
a. Molten Magmas 
Vehicle or Agent of Ore-Deposition 
to ore-disseminations in the wall-rocks as indicating 
the source of the vein-filling, and on the other, to the 
&. Gases and 
Vapours above 
their critical 
c. Deep-seated 
waters, whether 
of magmatic or 
d. Vadose Waters 
e. Mechanical 
Agents such as 
moving water 
and wind 
J. Chemical and 
Bacterial Agents 
in seas, lakes and 
swamps 
Superficiaé 
Fracture-fillings, 
such as_ gash- 
veins in lime- 
stonesandcavity- 
fillings (e.g. the 
heematite-ores of 
Cumberland). 
Zinc Ores: im 
limestones. Tron- 
ores replacing 
limestones (é.g. 
Cleveland), 
Some = ateritic 
trom and man- 
ganese deposits. 
Secondary — en- 
richments of cop- 
per ores. 
Some Lead and 
temperatures meteoric origin 
1. IGNEOUS Certain Massive 
DIFFEREN. | /v07 and Nickel 
TIATES. | Oves associated 
wth basic igne- 
ous  intrustoms 
(e.g. those of 
Sudbury in On- 
tario). ! 
|——_—___—— 
2. CAVITY- Lnjected Tin-\ Pneumatolytic _Hydato- genetic 
FILLINGS. | Oves (e.g. tin-| Cavzty-fillings Cavity - fillings 
pegmatites and| (e.g. tin quartz | (many fissure 
tin-elvans on the | veins). veins). 
margin of gran- 
ite intrusions. 
3. META- Preumatolytic Hydato- genetic 
SOMATIC Replacements Replacements. 
REPLACE- (e.g. tin-greisens (Many veins and 
MENTS. and many con- massive deposits, 
tact-deposits). also the Rand 
Banket). 
4. STRATIFIED Possibly some 
DEPOSITS. Sedimentary De- 
postts in which 
the. cementing 
matertals are 
ores of the 
metals, 
5. RESIDUAL 
DEPOSITS. | 
NO. 2320, VOL. 93] 
Some Lead and| Mechanical Con- 
Copper Ores 
tnterstitialin 
sandstones and 
shales. 
centrates in bed- 
ded depostts (e.g. 
gold and_plati- 
num placers, 
stream-tin, iron- 
sands, detrital 
laterites and 
other metalli- 
ferous gravels 
and sands). 
Chemical and 
Bacterial Sedt- 
ments (e.g. lake- 
and bog - iron 
ores 3 clay-iron- 
stone and other 
sedimentary sid- 
erites ; bog-man- 
ganese-ore and 
other sediment- 
tary manganese 
ores). 
Mantle-deposits 
e.g. pisolitic and 
nodular ores of 
iron (Bilbao and 
Appalachian 
hematites and 
limonites_ of 
manganese (psil- 
omelane) and of 
aluminium(baux- 
ite). 
Eluvial Gravels 
formed near the 
outcrop of veins 
(eg. those of 
gold, cassiterite 
and wolfram, 
galena and zinc- 
ores). 
