

ORE-¥YORMATIONS ~— 271 

_ Ata depth of eight miles or so from the surface it is hardly possible 
that gaping fissures and cavities can exist. Under the enormous pressure 
at that depth, the rocks must be in a state of plasticity, and any open space 
formed during crustal[movements would very soon be obliterated by the 
inflow of its walls. It is only in the upper parts of the earth’s crust that 
water can circulate in open fissures. This region has been aptly termed 
by Mr Van Hise the zone of fracture, and is conjectured by him on 
various grounds to extend from the surface to a depth of about six miles. 
At lower depths than this the rocks are in such a condition that even if 
fractured they would soon be welded together again—open spaces could 
not exist. In the zone of fracture, open fissures may well extend down- 
wards for great distances, but much will depend upon the nature and 
geological structure of the rocks they traverse. As these vary much in 
the resistance they offer to compression, we can readily understand that 
one and the same fissure may remain open in some parts of its course and 
be closed elsewhere. Many fissure-veins, as we have learned, show well- 
defined walls, while the structure of their included ore-formations leaves 
us in no doubt that the mineral matter has been deposited in what were 
at one time empty cavities. In many other lodes only one wall is seen, 
and all the phenomena lead to the conviction that no such continuous 
cavities existed in their case—the fissures being filled up with crushed 
and broken rock, amongst the interstices of which the mineral solutions 
subsequently made their way. Again, in not a few cases, no walls to a 
lode are visible—a mere narrow crack or close fissure passing through 
or bounding on one side the ore-bearing rock. In such a case the ore- 
formation does not occupy a cavity, but impregnates the country-rock on 
one or both sides of a narrow fissure. All the phenomena of impregna- 
tions and disseminations, in short, show us that water makes its way not 
only along the various division-pianes of rocks, but soaks more or less 
readily through the rocks themselves. 
It is not necessary, however, to suppose that the water coming from 
plutonic depths is of meteoric origin. Indeed, such evidence as we have 
would lead us to believe that surface-water, in the paucity or absence of 
open fissures, does not usually penetrate much below 2000 feet. It is the 
experience of miners in all parts of the world, that deep mines are 
generally dry and sometimes even dusty. Yet we know that when open 
fissures in such mines are tapped they not infrequently yield heated 
alkaline water. It is quite possible that this water may originally have 
descended from the surface, but, on the other hand, it may have come 
from plutonic sources. For,as we have seen, all molten rocks contain 
vast volumes of water-vapour and gases—to the action of which the 
pneumatolytic phenomena associated with batholiths are obviously due. 
According to the theory of ascension, therefore, the chief agent in the 
formation of secretionary ore-formations is probably the heated waters 
given off by plutonic masses. Not only would these waters (usually 
alkaline) carry with them mineral solutions derived from the molten 
magma, but as they continued to ascend they would attack the rocks 
through which they passed. Finding their way upwards by open fissures 
