THE FORMATION OF DEPOSITS 19 
rocks are older than the ores. Some of the most extensive 
reas of igneous rocks, such as the Deccan, Equatorial 
Africa, Iceland, the volcanic islands of the Atlantic, are 
Practically free from lodes, Where, however, igneous rocks 
are traversed by deep-seated tension faults, as in the Rocky 
Mountains, they contain important ore-fields. } 
The distribution of ores in igneous rocks is usually indepen- 
dent of the nature of the rock. Different parts of one igneous 
tock may have different ores, as at the Butte copper field, 
Montana. Special rocks have, it is true, been regarded as 
attended by particular ores, such as norite by nickel; but 
various igneous rocks have been called norite apparently 
because nickel occurs with them. Granodiorite was regarded 
as the plutonic rock most intimately connected with gold, 
before it was realized that the granite of the petrographer is 
relatively scarce. Platinum is often found with serpentine ; 
but only a most Optimistic prospector would expect platinum 
wherever serpentine occurs, and as serpentine is an altered 
rock the platinum oe may have been formed during the 
secondary changes. Many ore-fields have no igneous rocks 
yet the gold-quartz of their lodes, as at Warrandyte in Vic- 
toria, may be indistinguishable from that of a lode beside 
a dyke. "As ores of different kinds exist in one igneous 
rock, and ores of the same kind in different igneous rocks, 
the source of most ores is outside the rock in which they occur. 
~ Tur ORE-2ONE—The source of the ores appears to lie 
IN a zone deeper than that of the ordinary igneous rocks 
(Gregory, “Ore Deposits and Distribution in Depth,” 
Ir, R.Inst., 1906, p. 9). The most certain fact about the in- 
terior of the earth is its high specific gravity which is probably 
due to its large proportion of metals. If the specific gravity 
of the earth increased evenly from the surface to the centre, 
the rise in specific gravity would be so slow that rock heavy 
With meta] would not occur sufficiently near the surface 
to feed the lodes, But the innermost core of the earth 
(cf. p. 16) is probably lighter than the nickel-iron around it, 
and the barysphere is doubtless surrounded by an ore-zone 
composed of mixed silicates and metallic minerals (Fig, 8). 
The surface of the barysphere is probably irregular and 
Peaks rise from it into the lithosphere and upraise the over- 
lying ore-zone to a level at which they feed the lodes,