29

ECONOMIC GEOLOGY

mountain folding. The material of the ore-zone and barysphere
 may be regarded as igneous in the sense that they are
holocrystalline and are formed of minerals that consolidated
under high temperatures and pressures ; but metallic ores
do not appear to be primary constituents of the ordinary
igneous rocks of the earth's crust. Even the deepest seated
plutonic rocks have received the metals in their lodes—except
such as iron, manganese, and aluminium— rom an underlying
layer.
PLuTtonic, JUVENILE, AND MAGMATIC WaTERS—The normal
influence of igneous rocks in ore formation is indirect. Their
intrusion produces fractures and fissures along which solutions
 can pass, and they raise the temperature so that the
gas pressure forces the water upward. The main introduction
 of metals into the upper layers in the lithosphere,
in addition to its normal constituents, is by the ascent of
deep-seated water. This water is known as plutonic from
its deep origin, or as magmatic after its derivation from
igneous magmas, or as Juvenile (Suess) since it is making
its first appearance on the earth's surface.
A large quantity of this water is constantly arising from the
interior (cf. Chap. XIX, Pp. 220-2). It must begin its ascent
through minute spaces. The liquid cavities in quartz often
occur in lines passing from one crystal to the next, showing
that the entrance of the liquid was after the consolidation of
the quartz. This movement must be very slow. At depths
of 6 to 12 miles the temperature is probably above 687° F,,
the critical point of water, which at it can exist only as
steam. Below 30 or 40 miles, the temperature would be
above 3600°F., and water would be dissociated into its
constituent gases, which could combine only in a zone cooler
than their dissociation point. Water after its formation
would gradually work its way upward into a zone where
fissures and cracks would enable it to ascend more freely,
That fractures happen at great depths in the crust is shown
by the evidence of earthquakes which result from deepseated
 disruptions. Ruptured rock surfaces at great depths
would remain in close contact, but not too close for the passage
of films of superheated water. Ruptures must occur below
the level where plutonic water begins its ascent in innumerable
tiny trickles through thin spaces or scattered pores. This