138 
AMERICAN GEOLOGY. 
which the vein is exposed. In consequence of the changes of 
which I am speaking, many intelligent miners believe the vein 
has become poor; and in this opinion he is sustained by the fact 
that as usually worked, the profits have very materially dimin¬ 
ished. But this result is due to what nature has done for the 
upper part of the vein, by detaching the gold from its most 
intimate combination with the sulphurets, and no process has 
hitherto been invented by which all the gold has been separated 
from the mineral. Experience proves, however, that by repeated 
pulverizations, by means of stamps and Chilian mills, the aggre¬ 
gate amount of metal which can be obtained, is about as great 
below as above water. 
The changes referred to in the foregoing paragraphs are true 
of the auriferous veins, in connection with the sulphurets of 
iron and copper of this country; while those containing galena 
are far less chemically changeable. When to the sulphurets of 
iron and copper arsenic is added, as in the Ducktown mines in 
Tennessee, the changes are still more striking. 
The question whether mines are richer or poorer above than 
below water is not perhaps fully settled. For myself, I believe 
that the facts, when well determined, go to prove the undimin¬ 
ished value of the vein below water. But it is a question 
whether means sufficiently simple and cheap, can be devised by 
which mines can be made to pay a profit. Some will pay for 
forty, fifty or sixty feet, but ore may not be really rich enough 
to pay a profit below those points. The depth at which a vein is 
changed by atmospheric influence is variable. It depends un¬ 
doubtedly upon the depth of drainage. It is not a point which 
can be determined beforehand; we may reach water in twenty- 
five or fifty feet, or it may not be reached in sixty; but the 
mean is about forty-five or fifty feet. 
Note.—I t is not designed to intimate that water has any influence in in¬ 
creasing or diminishing the amount of what metal it carries. The reason why 
the expression above or below is used is sufficiently obvious from the explana¬ 
tions of the text. 
