But  in  Cornwall,  some  lodes  occur  where  the  vertical  plates,  or

combs,  as  they  t  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
Redruth.*  Each  of  the
plates  or  combs  (a,  6,  c,  d,
/)  is  double,  having  the


Copper  lode,  near  Redruth,  enlarged  at  six
successive  periods.
points  of  their  crystals  turned  inwards  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  moderate  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  veinstones.  Thus,  we  may  imagine  the  sides  of
a  fissure  to  be  encrusted  with  siliceous  matter,  as  Von  Bucli
observed,  in  Lancerote,  the  walls  of  a  volcanic  crater  formed  in
1731  to  be  traversed  by  an  open  rent  in  which  hot  vapours  had
deposited  hydrate  of  silica,  the  incrustation  nearly  extending
to  the  middle,  f  Such  a  vein  may  then  be  filled  with  clay  or
sand,  and  afterwards  re-opened,  the  new  rent  dividing  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.
