Editorial Comment. 117 
The best ores and the largest bodies will be found near 
the surface, and calculations for deep mining should not be 
based on surface ores and shallow workings. 
The pendulum of theories regarding the origin of sulphide 
ores seems in this paper to have swung to its extreme limit as 
to the work of descending waters. 
For some five years past, geologists, and in particular those 
who have been students of the Lake Superior region, fresh 
from fields where descending waters have been the chief 
agents of deposition of some of the largest ore bodies in the 
world, have been applying the lessons and examples of that 
region and the theories of modern chemistry to the elucidation 
of the problems which have for centuries engaged the investi- 
gators of sulphide ores and vein mineralization. 
Until very recently the suggestion of the deposition of 
metallic sulphides from surface waters carrying oxygen and 
oxidized salts would have been considered a vagary or idle 
theory born of unsound chemical knowledge. When, how- 
ever, natural sulphides of copper and silver were found depos- 
ited on bronze coins in France and elsewhere, an explanation 
was necessary, and the one first advanced and generally adopt- 
ed was the reducing action of carbon. 
These instances, however, were believed to be entirely ex- 
ceptional, and all metallic sulphides in veins were still ascribed 
to non-oxygenated ascending ground waters. 
Physical facts noticed in the mines of ^Montana, Colorado 
and elsewhere, however, indicated very strongly the second- 
ary origin of many sulphide ores, and their deposition by des- 
cending oxygenated waters of ordinary temperature. 
Convinced finally of the possibility of producing the richer 
,metallic sulphides by reaction between solutions of sulphates 
and undecompo§ed primary sulphides, the writer conducted 
some laboratory experiments in Butte during 1899 and 1900. 
These experiments resulted in the artificial production of chal- 
cocite in the wet way. It was deposited upon pyrite from so- 
lutions of copper sulphate in the presence of sulphurous an- 
hydride, one of the natural products of the oxidation of pyrite. 
This discovery was soon followed by the important papers of 
Van Hise, Weed and Emmons on the genesis of ore deposits, 
in which was advanced the theory of the sccondarv deposition 
