FISSURES,—VEINS,—LODES,—DYKES.
9
tified  matter  discontinuities  of  various  widenefs.  These  rents  and  intervals ¬
  of  the  fractured  strata,  or  rock,  are  in  general  filled  with  a  matter ¬
  different  from  that  of  the  rock  or  stratum  in  which  the  rents  are
found.  And  rents  of  a  certain  wideness,  thus  filled  up  with  extraneous ¬
  matter,  are  called  veins,  or  in  Cornwall  lodes.  These  veins  are
fubject  to  all  the  Variations  of  wideness,  and  irregularity  of  direction,
which  you  can  easily  imagine  in  fractures  and  rents  of  hard  strata  and
rock;  and  they  go  down  to  a  depth  far  below  any  to  which  we  can
reach.  The  matter  which  fills  them  is  generally  a  whitiſh  ſtony  matter -
  named  spar.  It  is  more  pure  and  more  simple  in  its  composition
than  most  other  stones,  but  there  are  feveral  kinds  of  it  ;  and  intermixed ¬
  with  it,  we  often  find  also  different  metallic  and  mineral  substances. ¬
  It  is  indeed  in  thofe  veins  chiefly  that  the  ores  of  metals  are
found.
And  wherever  there  are  vacuities  left  in  the  vein,  by  its  being  im
perfectly  filled,  there  we  are  fure  to  meet  with  crystallized  matter  of
the  several  stony  and  mineral  substances  which  the  vein  contains.
In  a  great  many  examples,  however,  the  rents  and  discontinuities
I  have  described,  are  of  greater  wideness  than  what  are  commonly  called ¬
  veins.  The  wideness  of  veins  is  from  an  inch,  or  a  fraction  of  an
inch,  to  feveral  feet,  or  even  yards  in  some  cases.  But  we  have  examples ¬
  of  rents,  the  wideness  of  which  is  equal  to  many  fathoms  ;  and
in  this  case,  rock  or  rubbish,  or  other  extraneous  matter,  fills  up  the
interval.  When  this  occurs,  the  interval  and  materials  which  it  contains ¬
  is  not  called  a  vein  ;  our  miners  give  it  the  name  of  a  dyke,  or
partition.  The  extent  to  which  theſe  dykes,  as  well  as  the  veins,  are
continued  downwards  into  the  bowels  of  the  earth,  is  hitherto  unknown. ¬
  Their  depth  is  plainly  much  greater  than  what  human  art
and  labour  can  reach.
Theſe  rents  and  diflocations  of  the  strata,  with  the  intervals  thus
formed,  and  great  quantities  of  rock  or  rubbish  which  sometimes  fill
up  theſe  intervals,  are  well  known  to  our  colliers,  who  meet  with
B
VoL.  II.