606 
ELEMENTS OF GEOLOGY. 
hypogene and fossiliferous, and extend downward to indefi¬ 
nite or unknown depths. We may assume that they corre¬ 
spond with such rents as we see caused from time to time by 
the shock of an earthquake. Metalliferous veins referable to 
such agency are occasionally a few inches wide, but more 
commonly three or four feet. They hold their course con¬ 
tinuously in a certain prevailing direction for miles or 
leagues, passing.through rocks varying in mineral composi¬ 
tion. 
That metalliferous Veins were Fissures.—As some intelli¬ 
gent miners, after an attentive study of metalliferous veins, 
have been unable to reconcile many of their characteristics 
with the hypothesis of fissures, I shall begin by stating the 
evidence in its favor. The most striking fact, perhaps, which 
can be adduced in its support is, the coincidence of a consid¬ 
erable proportion of mineral veins with faults^ or those dislo¬ 
cations of rocks which are indisputably due to mechanical 
force, as above explained (p. 87). There are even proofs in 
almost every mining district of a succession of faults, by 
which the opposite walls of rents, now the receptacles of me¬ 
tallic substances, have suffered displacement. Thus, for ex¬ 
ample, suppose a a. Fig. 629, to be a tin lode in Cornw^all, the 
term lode being applied to veins containing metallic ores. 
This lode, running east and west, is a yard wide, and is shift¬ 
ed by a copper lode (5 h) of similar width. The first fissure 
{a a) has been filled with various materials, partly of chemic¬ 
al origin, such as quartz, fluor-spar, peroxide of tin, sulphuret 
of copper, arsenical pyrites, bismuth, and sulphuret of nickel, 
and partly of mechanical origin, comprising clay and angular 
fragments or detritus of the intersected rocks. The plates 
of quartz and the ores are, in some places, parallel to the 
vertical sides or walls of the vein, being divided from each 
other by alternating layers of clay or other earthy matter. 
Occasionally the metallic ores are disseminated in detached 
masses among the vein-stones. 
It is clear that, after the gradual introduction of the tin 
and other substances, the second rent {h h) was produced by 
another fracture accompanied by a displacement of the rocks 
along the plane of h h. This new opening was then filled 
with minerals, some of them resembling those in a a, as fluor¬ 
spar (or fluate of lime) and quartz ; others different, the cop¬ 
per being plentiful and the tin wanting or very scarce. We 
must next suppose a third movement to occur, breaking 
asunder all the rocks along the line c c, Fig. 630; the fissure, 
in this instance, being only six inches wide, and simply filled 
with clay, derived, probabl}^ from the friction of the walls 
