GEOLOGY OF THE INNER EARTH—GREGORY. one 
porphyrite, which the ore in part replaces. An examination of the 
field evidence supports the conclusions of De Launay and Bickstrém 
as to the ore being a bedded deposit overlying a lava flow, but en- 
larged by secondary deposition. 
FUTURE SUPPLY OF IRON ORES. 
This conclusion is perhaps economically disappointing. The pos- 
sible existence of such vast segregations of iron in the acid 
igneous rocks has an important economic bearing. There is only 
too good reason to fear that the chief iron ores are compara- 
tively limited in depth; for most of them have been formed by 
water containing oxygen and carbonic acid in solution, which has 
percolated downward from the surface. Ores thus formed are there- 
fore restricted to the comparatively limited depths to which water 
can carry down these gases. On the theory, however, that these ores 
are primary segregations from deep-seated igneous rocks there need 
be no limit to their depth. They would rather tend to increase in 
size downward, while maintaining, or even improving, in the richness 
of their metallic contents. For these bodies may be regarded as frag- 
ments of the metallic barysphere which have broken away from it and 
revolve around it like satellites floating in the rocky crust. On this 
conception these ore bodies would be of as great interest to the student 
of the earth’s structure as their existence would be reassuring to the 
ironmaster, haunted as he is by constant predictions of an iron famine 
at no distant date. It is no doubt true that many of the richest, most 
accessible, most cheaply mined, and most easily smelted iron ores have 
been exhausted. The black-band ironstone and the clay iron ores of 
the coal fields, which gave the British iron industry its early su- 
premacy, now yield but a smalt proportion of the ores smelted in our 
furnaces. The Mesozoic beds of the English Midlands and of York- 
shire still supply large quantities of ore. Nevertheless the British 
iron industry is becoming increasingly dependent on foreign ores. 
So it would be pleasant to find that the Scandinavian iron mines are 
not subject to the usual limits in depth. I fear the typical iron de- 
posits of middle Sweden and of Gellivara will follow the general rule; 
but Kiruna may be an exception, and its ores may continue far down- 
ward along the surface of its sheet of porphyrite. The uncertainty 
in this case hes in the extent of the subsequent enrichment and en- 
largement of the bed; if most of the ore is due to secondary deposi- 
tion, then it may be restricted to the comparatively shallow depths at 
which this process can act; and though that limit will be of no prac- 
tical effect for a century or more to come, the ore deposit may be 
shallow as compared with gold mines. 
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