356 The American Geologist. June, i9oi. 
Its application, however, has not been confined to this region. 
It has been extended to other mining districts, even tliose in 
which typical fissure veins abound. 
Since the most advanced conceptions regarding- the nature 
of erosion have gained such wide hold among geologists, ore 
students have seized with avidit}^ upon some of these principles 
as furnishing a long sought for solution to the manner of many 
concentrations of ore bodies. The attractive features of the 
hypothesis are many. But objections have not been so obvious,, 
and have not generally presented themselves. These last men- 
tioned phases of the subject have entirely escaped the notice of 
mining engineers and special students of ores from the mining 
side. To geologists who have given geographic development 
special attention, the objections appear formidable, and in 
many instances unsurmountable. 
\\' inslow* formally states the hypothesis when he says that 
it "starts with the proposition that the metalliferous minerals 
originally existed in the Archean rocks, either in a disseminat- 
ed condition or in veins. With the decay of these early rocks, 
the minerals became diffused through later-formed sediments, 
this diffusion being quite uniform over contiguous areas. Suc- 
cessive decaying of successively formed rocks simply resulted 
in a transfer of these minerals." Accordingly, the concentra- 
tion process was entirely subsequent to the period of rock for- 
mation. "It is, primarily, a result of great and long-continued 
surface decay of the rocks ; and, secondarily, the result of the 
presence of local favorable physical and chemical conditions." 
This general statement presents numerous slight modifica- 
tions with different writers. Its advocates agree that the dif- 
fused ore-materials are, as degradation of the land goes on, 
carried downward to lower and lower levels, the metal-accu- 
mulating zone retaining always a constant relation to the 
ground surface. From this proposition two main phases pro- 
ceed. One would retain the ore materials in a diffused condi- 
tion ; the other in a concentrated form, or as an enrichment of 
the lower zones. 
The principles of geologic geography demonstrate beyond 
all doubt that lands elevated are worn down to near sea-level 
with far greater rapidity than has been generally supposed. 
* Missouri Geol. Sur., vol. vii, p. 4.77, 1S94-. 
