350 
MINING GEOLOGY 
materials liberated by weathering and passed into solution is 
merely slowly down the course of the vein. 
The circulation of ore-matter seems to be very much more com¬ 
plex than this and the immediate sources of the ore-materials are 
widely different from a mere settling down along a vein. The 
ordinary circulation of metalliferous groundwaters is graphically 
indicated by the subjoined cut (Fig. 19). 
In the diagram the courses of the various circulations are rep¬ 
resented as merging at a single point or along a given line. The 
latter may be an old mineral vein or it may be a recently formed 
fault-plane. In either case the currents reach ground-water-level 
where their burdens of oxidic ores are mainly reduced to sulphidic 
form, and dropped, forming the bonanza zone or layer of sec¬ 
ondary sulphide enrichment. A minor part goes on downward 
into the profound zone. The proportions of metallic substances 
derived from each source are not easy at this time to accurately 
evaluate. Meteoric sources perhaps supply much larger amounts 
than it has been customary to suppose. The part liberated by 
secular decay of rock-masses is probably the largest. By the 
oxidation of small masses of sulphide ores there is an appreciable 
contribution. Through the constant working over of the bonanza 
layer the ores are kept localized and concentrated. 
At any rate, deposition of ores derived from the vadose is 
very much more important than is usually premised. 
KtYts. 
World's Oil Reserves. From a strictly commercial angle the 
future of petroleum is a theme of transcendent attractiveness. It 
is the most momentous single problem in the industrial activities 
of our century. In the United States, which, since rock-oil came 
into general use, furnishes two-thirds of all the oil yet won from 
the ground, the possible early limitation of supply finds universal 
solicitude. 
Notwithstanding the fact that American fields may now be re¬ 
garded as producing over sixty per cent of the world's annual 
yield, yet the United States alone is consuming more than seventy- 
five per cent of this total output. At present the yearly require¬ 
ments of the United States are not far from 600 millions of 
