176  CONTRIBUTIONS    TO   ECONOMIC   GEOLOGY,  1904.        [bull.  260. 
The  tin  deposits  here,  as  in  most  other  places,  are  found  in  close 
connection  with  granites.  Five  large  bosses  of  granite,  with  occa- 
sional smaller  ones,  extend  in  a  northeasterly  direction  from  Lands 
End  for  about  100  miles.  The  Scilly  Isles  are  a  sixth  boss  in  the 
southwestern  extension  of  this  line.  The  granite  is  overlain  by  a 
clay  slate  of  Devonian  age — the  "  killas  "  of  the  Cornish  miner — and 
both  are  cut  by  dikes  (Cornish  "  el  vans  "  or  "  el  van  courses")  of 
quartz-porphyry  and  serpentine. 
The  rocks  are  jointed  in  two  principal  directions,  one  set  of  joints 
following  the  line  of  granite  masses  northeast-southwest,  and  another 
crossing  this  set  at  right  angles,  with  some  medial  joints.  The 
veins  follow  these  sets  of  joint  planes. 
The  tin  veins  cut  slates,  granite,  and  dikes,  and  are  evidently 
younger  than  the  latter,  although  they  frequently  change  their  direc- 
tion somewhat  or  are  offset  in  passing  through  the  dikes.  The  gran- 
ites are  supposed  to  be  Carboniferous  or  Permian,  so  that  the  veins 
are  younger  than  Carboniferous.  Many  of  the  veins  outcrop  in  slate, 
but  all  can  be  followed  into  the  granites.  The  veins  are  sometimes 
in  fault  fissures,  as  shown  by  slickensides,  and  are  sometimes  in  brec- 
cia ted  zones. 
Some  of  the  veins  carried  copper  ores  in  the  slate  but  tin  in  the 
granites.  The  Dolcoath,  the  richest  of  the  English  tin  mines,  is  on 
such  a  vein.  This  mine  has  reached  a  depth  of  about  2,100  feet,  and 
is  still  in  good  ore,  although  most  of  the  mines  have  become  impov- 
erished at  lesser  depths.  The  Dolcoath  has  been  producing  tin  since 
1854. 
The  veins  in  the  slate  are  generally  composed  largely  of  quartz, 
while  in  the  granite  they  contain  feldspar  with  mica,  chlorite,  and 
tourmaline. 
In  the  granite  the  lodes  frequently  have  one  definite  wall,  while 
on  the  other  side  the  rock  gradually  loses  its  richness  until  it  can  not 
be  economically  mined.  This  is  probably  due  to  faulting,  during 
which  one  side  was  covered  with  gouge  comparatively  impermeable 
to  solutions.  The  cassiterite  often  replaces  the  feldspars,  as  does 
tourmaline,  which  occurs  in  great  amount.  Fluorite,  topaz,  zircon, 
copper,  iron,  and  arsenic  pyrites  are  accompanying  minerals. 
The  ore  mined  generally  runs  a  little  over,  but  sometimes  under,  1 
per  cent  tin.  The  average  richness  of  the  ores  from  four  of  the 
largest  mines  during  the  ten  years  from  1871  to  1881  was  as  follows: 
Dolcoath,  59  pounds;  Cooks  Kitchen,  43  pounds;  Tincroft,  53 
pounds,  and  Cam  Brea,  35  pounds,  of  black  tin  per  ton. 
The  output  of  the  English  mines  is  slowly  but  steadily  diminishing. 
There  are  no  placers  now  being  worked  in  England,  although  they 
were  formerly  of  much  importance. 
