Chap. XIIL] 
KODURITE SERIES : SOURCE OF CO^. 
279 
depth below the surface, we can put the present depth of this 
former boundary between the belt of weathering and the belt of cement- 
ation as not more than 500 feet. As it is not probable that the alter- 
ation of the kodurite rocks accompanied by the formation of oxidized 
ores continued to any considerable depth below the bottom of the belt 
of weathering, we can take this figure of 500 feet as being a guess at 
the maximum depth to which the manganese-ores of this district may 
be expected to extend. It is obvious that as the assumptions on which 
this figure is based are merely guesses, it may happen that the ores con- 
tinue to a considerably greater depth, or that they do not extend so 
deep. But in any case they probably extend deeper than it will be 
profitable to work them, considering the irregular distribution of the 
ore-bodies through the decomposed masses of lithomargic rock in which 
they occiu". 
There is one other point worth considering, namely, the source of 
The souices of the carbon dioxide that is supposed to have brought 
(;arbL,n dioxide. about the alteration of the kodurite rocks. This 
question of the source of the carbon dioxide that plays siich a promi- 
nent part in the formation of ore-deposits is excellently summed up by 
Van Hise on page 96 of the paper already cited. Van Hise says : — 
'I have already pointed out two sources for the excess of carbon-dioxide held 
in the underground waters. Where vegetation is abundant, carbon -dioxide is 
concentrated in the soil. A large part of this is retained in the belt of weathering 
by the process of carbonation of the silicates, but another part joins the sea of 
underground waters. Another source for the carbon- dioxide is that liberated 
from cavities within rocks. It is well known that in many rocks a large amount 
of carbon-dioxide is included in innumerable microscopic cavities. When such 
rocks are complexly deformed in the zone of fracture, the fractures must intersect 
many of these cavities, and thus liberate the carbon -dioxide. Where there are 
zones of crushing, that is, where there are trunk-channels for percolating waters, the 
amount of carbon-dioxide which may thus be liberated may be considerable. 
Another source for the carbon-dioxide is a process of sOication ', 
which Van Hise has previously explained, in which the silica taken 
into solution in the belt of weathering, as the result of the decomposi- 
tion of silicates by the carbon dioxide in the surface waters, percolates 
downwards into the belt of cementation and decomposes the carbonates 
of limestones and other rocks with the liberation of carbon dioxide. 
' Therefore, deep-seated waters are ever receiving contributions of oarbon- 
dioxide, and thus are continually more capable of taking metals in solution, until 
the Waters reach conditions where silication does not occur. 
In this process of silication alone is believed to be an adequate source of 
carbon-dioxide, so that metals may be carried as bicarbonates and the water also 
bold a further excess of carbon-dioxide,' 
