608 
ELEMENTS OF GEOLOGY. 
al features of Cornwall, are not affected by any of the dislo¬ 
cations, a powerful denuding force having clearly been exert¬ 
ed subsequently to all the faults. (See above, p. 93.) It is 
commonly said in Cornwall, that there are eight distinct sys¬ 
tems of veins, which can in like manner be referred to as many 
successive movements or fractures; and the German miners 
of the Hartz Mountains speak also of eight systems of veins, 
referable to as many periods. 
Besides the proofs of mechanical action already explained, 
the opposite walls of veins are often beautifully polished, as 
if glazed, and are not ^infrequently striated or scored with 
parallel furrows and ridges, such as would be produced by 
the continued rubbing together of surfaces of unequal hard¬ 
ness. These smoothed surfaces resemble the rocky floor over 
which a glacier has passed (see Fig. 106, p. 168). They are 
common even in cases where there has been no shift, and oc¬ 
cur equally in non-metalliferous fissures. They are called by 
miners “ slicken-sides,” from the German schlichten^ to plane, 
and seite^ side. It is supposed that the lines of the striae in¬ 
dicate the direction in which the rocks were moved. 
In some of the veins in the mountain limestone of Derby¬ 
shire, containing lead, the vein-stufi*, which is nearly compact, 
is occasionally traversed by what may be called a vertical 
crack passing down the middle of the vein. The two faces 
in contact are slicken-sides, well polished and fluted, and 
sometimes covered by a thin coating of lead-ore. When one 
side of the vein-stuff is removed, the other side cracks, espe¬ 
cially if small holes be made in it, and fragments fly off with 
loud explosions, and continue to do so for some days. The 
miner, availing himself of this circumstance, makes with his 
pick small holes about six inches apart, and four inches deep, 
and on his return in a few hours finds every part ready 
broken to his hand.* 
That a great many veins communicated originally with 
the surface of the country above, or with the bed of the sea, 
is proved by the occurrence in them of well-rounded pebbles, 
agreeing with those in superficial alluviums, as in Auvergne 
and Saxony. Marine fossil shells, also, have been found at 
great depths, having probably been ingulfed during subma¬ 
rine earthquakes. Thus, a gryphaea is stated by M. Virlet to 
have been met with in a lead-mine near Semur, in France, 
and a madrepore in a compact vein of cinnabar in Hun- 
gary.f In Bohemia, similar pebbles have been met with 
at the depth of 180 fathoms; and in Cornwall, Mr. Came men- 
* Conyb. and Phil. Geol., p. 401; and Farey’s Derbyshire, p. 243. 
t Fournet, ^Itudes sur les Depots Metalliferes. 
