610 
ELEMENTS OF GEOLOGY. 
Copper lode, near Redruth, enlarged at six 
successive periods. 
Fig. 632. 
cur where the vertical plates, 
or Gombs^ as they are there 
called, exhibit crystals so 
dovetailed as to prove that 
the same fissure has been 
often enlarged. Sir H. De 
la Beche gives the following 
curious and instructive ex¬ 
ample (Fig. 632), from a cop¬ 
per-mine in granite, near Red¬ 
ruth.* Each of the plates 
or combs (a, 5, c, 6, f) is 
double, having the points of their crystals turned inward 
along the axis of the comb. The sides or walls (2, 3, 4, 5, 
and 6) are parted by a thin covering of ochreous clay, so 
that each comb is readily separable from another by a mod¬ 
erate blow of the hammer. The breadth of each represents 
the whole width of the fissure at six successive periods, and 
the outer walls of the vein, where the first narrow rent was 
formed, consisted of the granitic surfaces 1 and 7. 
A somewhat analogous interpretation is applicable to many 
other cases, where clay, sand, or angular detritus, alternate 
with ores and vein-stones. Thus, we may imagine the sides 
of a fissure to be incrusted with siliceous matter, as Von 
Buch observed, in Lancerote, the walls of a volcanic crater 
formed in 1731 to be traversed by an open rent in which hot 
vapors had deposited hydrate of silica, the incrustation near¬ 
ly extending to the middle.f Such a vein may then be filled 
with clay or sand, and afterwards re-opened, the new rent di¬ 
viding the argillaceous deposit, and allowing a quantity of 
rubbish to fall down. Various metals and spars may then 
be precipitated from aqueous solutions among the interstices 
of this heterogeneous mass. 
That such changes have repeatedly occurred, is demon¬ 
strated by occasional cross-veins, implying the oblique frac¬ 
ture of previously formed chemical and mechanical deposits. 
Thus, for example, M. Fournet, in his description of some 
mines in Auvergne worked under his superintendence, ob¬ 
serves that the granite of that country was first penetrated 
by veins of granite, and then dislocated, so that open rents 
crossed both the granite and the granitic veins. Into such 
openings, quartz, accompanied by sulphurets of iron and ar¬ 
senical pyrites, was introduced. Another convulsion then 
burst open the rocks along the old line of fracture, and the 
* Geol. Rep. on Cornwall, p. 340. 
t Principles, ch. xxvii., 8th ed., p. 422. 
