Ore Formation, by Surface Dccouiposition. — Keyes. 357 
The fact that in many locaHties enormous denudation has taken 
place in the past, has been for a long time widely recognized. 
But that profound erosion, thousands of feet in depth, may 
easily take place in a single geological epoch or period, instead 
of during one or several long geological eras is a proposition 
which has not been so universally appreciated. 
If we apply to any particular locality either phase of the 
hypothesis that generally diffused ore materials are concentrat- 
ed through surface decomposition of the country rocks, we 
should expect to readily find abundant facts in substantiation. 
Either the ore materials of a region should give evidence of 
unusual original accumulation through the phenomena of local 
sedimentation ; or the concentration should be about propor- 
tional to the amount of erosion. 
With regard to the first of these propositions, students of 
ores are now pretty well in agreement that such factors as 
oceanic currents can no longer be considered as apprecialile fac- 
tors directly affecting the localization of ore deposition through 
solution. Besides, there are so many secondarv factors enter- 
ing into the problem, that even if the truth of the proposition 
be assumed, it would be utterly impossible to adduce proofs, 
so completely would they be obscured by attendant features. In 
the light of recent geographic and geologic research, all theo- 
retical deductions appear to militate against the possibility of 
such conditions. 
The second proposition presents some objections not met 
with in the first. The theoretical deductions are conclusive. The 
practical demonstration? present no very great difficulties. In 
the case of the Ozark region, of which a special example has 
been made, the facts are singularly against the hypothesis ad- 
vanced. 
The analyses made by Robertson* of ?iIissouri rocks, show 
the presence of lead, zinc and copper in minute (|uantities. 
While the number of determinations made is far too small to 
enalile any broad generalizations to be stated, it is significant 
that those rocks taken from the mining localities contain a very 
much higher percentage of metallic contents than those collect- 
ed from localities remote from ore bodies. The inference would 
naturally be that the present difference in the amounts of the 
* .Missouri Geol. Stir., vol. vii, p. 479, 189+. 
