202 KANSAS CITY REVIEW OF SCIENCE. 
from different depths, and leaching rocks of various composition under great 
pressure and high temperature, having great solvent power, become loaded with 
various mineral matters. As they rise to the surface, the pressure and tempera- 
ture are reduced, and the materials held in solution are deposited to line and per- 
haps ultimately fill the channels through which they flow. This theory of the fill- 
ing of mineral veins—that is, by precipitation from heated chemical solutions 
coming from below—is supported by such an array of facts that it must be ac- 
cepted by all who will make a careful and unprejudiced study of the subject. It 
is true, however, that various other theories have been, at one time or another, 
put forth for the explanation of the phenomena. Among these, a few deserve a 
passing notice. They are: 
1. The theory of igneous ejection, according to which the matter filling mineral 
veins has been erupted like that of trap dikes, and such veins as those of Laké 
Superior containing metallic copper have been suggested as affording good exam- 
ples. But here we find metallic copper and silver associated, and each chemically 
pure; whereas if they had ever been fused, they certainly would have formed an 
alloy. The copper is also found in crystals of calc-spar and other minerals, where 
it must have been deposited with the other constituents of the crystal, and that 
crystal formed from solution. Other opposing facts might be cited; butit will be 
sufficient to say that not one sound argument can be advanced in favor of this 
theory. 
2. Aqueous deposition from above. ‘This theory, first advanced by Werner, 
but since generally abandoned, supposes the contents of mineral veins to have 
been deposited froma solution which flowed into the fissures from above; but in 
that case the vein-matter should be horizontally stratified, limited in extent down- 
ward, and spread over the surface adjacent to the fissure; whereas no one has yet 
reached the limits in depth of the ore in a true fissure-vein, and the characteristic 
banded structure can only have resulted from successive depositions of a long- 
continued flow of a hot solution. This theory has been recently advocated in this 
city, by Prof. Stewart of Nevada; but it is not only not sustained, but is really 
disproved by all the facts observed by the writer in some years devoted to the 
study of our western ore-deposits. 
3. Lateral secretion. According to this theory, the material filling all miner- 
al veins has leached into the cavity from the wall-rocks. While this is true of 
gash-veins, it can have played but a very subordinate part in the deposition 
of ore in fissure-veins. This is proved by the facts that different sets of fissures 
which cut the same formation frequently contain very different ores; and where 
the rocks of totally different character are, by faulting, brought to form opposite 
walls of a fissure, the ore may be symmetrically deposited in corresponding lay- 
ers. It may also be said that the same fissure frequently traverses several forma- 
tions, and yet its character may be essentially the same throughout. 
4. Sublimation. The facility with which certain metals are volatilized, and 
the fact that various minerals have been deposited from vapor, have formed the 
