24 Theories. 
by the dike C C; the movement always being in the 
same direction. 
Note—If C C is a vein, it is evident that it is 
younger than the veins through which it cuts. In this 
ease it would be called a Cross-Course. 
THEORIES. 
The most popular theories which claim to account 
for the filling of metalliferous veins are: 
(a) Theory of Ascension.—This theory supposes 
veins or lodes to have been formed in part only of min- 
erals dissolved out of rocks in the immediate horizon of 
vein fissures, and that the chief portion of the material 
has been derived from greater depth by solvents circu- 
lating through the fissure and subsequent precipitation 
of the minerals in solution on the walls of the cavity. 
(b) Theory of Lateral Secretion—This theory 
teaches that water perc@lating through the country rocks 
has by the aid of carbonic acid and other natural sol- 
vents, dissolved out of it all the material now forming 
the constituents of mineral veins. 
(c) Theory of Sublimation.—According to this 
theory vein fissures were filled by the volatilization of 
metalliferous minerals derived from the ignited interior 
of the earth. This theory is fast losing its advocates. 
(d) Theory of Replacement.—This is one of the 
most recent theories promulgated, and it has been much 
discussed of late. This theory claims that the metalli- 
ferous contents of deposits were obtained metasomati- 
cally, that is, there were a molecular substitution of the 
minerals contained in the circulating waters for parti- 
cles of the wall rocks. Thus the interchange was atom 
for atom until the walls of the fissure were impregnated 
with the metalliferous substances which now form the 
ore deposits. (See Fig. 7). 
“Ore Deposits,’”’ by J. A. Phillips. 
“Ore Deposits of the United States and Canada,” by J. F. Kemp. 
