1 86 The American Geologist. September, 1902 
Influence of Cafhonaccons Matter on the Formation of Ore. 
The well-known reducing action of carbon has long been 
an accepted explanation of the occurrence of ore-bodies in 
carbonaceous shales. The silver-veins of the Animikie slate 
are often cited as examples. The Australian veins, which carry 
rich gold-ores only where they cross "indicators," are a well- 
known instance, and the FahJbands of Norway another. 
Rickard ascribes the Australian ore-bodies to the reducing 
action of the carbonaceous shales. All the descriptions show, 
however, that although these indicator-reefs are strata of car- 
bonacious shale, they are remarkable for the amount of pyrite 
they contain, while other shale-beds crossed by 'the veins are 
also carbonaceous, but not pyritic. In previously published 
_ papers I have called attentitjn to the reducing action of pyrite 
on solutions carrying copper-salts : and believe that this min- 
eral is the real precipitating agent in both the Australian and 
Norwegian ores cited. This view, based upon field-observations 
and laboratory experiments, in connection with the work of the 
U. S. Geological Survey, is sti'engthened by the experiments 
made under the direction of my friend, Mr. H. V. Winchell, 
geologist of the Anaconda Copper Co., wdio finds that the 
mine-waters' of that company's properties, though strongly 
charged with cupric sulphate and ferric sulphate, also contain 
large amounts of carbonaceous matter from the old mine- 
timbers. It is evident that if the carbon were an active reduc- 
ing agent, it would reduce the ferric iron to the ferrous con- 
dition. On the other hand, experiments show that pyrite from 
the Butte veins left for several weeks in the natural mine- 
waters became coated with copper-glance (CUoS). 
Dependence of Vein-Minerals on the Character of Wall-Rock, 
Due to Metasomatic Chemical Reactions. 
The examples given show that a variation of mineral con- 
tents coincides with the change of country-rock in many places 
and in many kinds of veins. A study of the vein- and gangue- 
minerals and of the rock contiguous to the orebearing fissures 
is therefore essential to a complete understanding of the origin 
of ore-deposits. The rocks forming the vein-walls are com- 
monly altered. Where no such alteration is observable, it will 
usually be found that the veins are the result of the filling of 
cavities, and are thus excluded from the categorv discussed in 
