SPURR    A 
GARREY 
ND]       GEORGETOWN    MINING    DISTRICT,    COLORADO. 
117 
took  place,  for  normal  fresh  porphyry  ought  to  afford  as  good  frac- 
tures as  granite.  Fig.  7  illustrates  the  general  tendency  of  the  veins 
of  the  district  to  diminish  in  size  and  economic  importance  on  passing 
from  granite  or  hard  gneiss  to  somewhat  altered  porphyry.     An  ex- 
va  *'-'  ''/  "/"/^i,-;  ?.'/-'•'••  ';V'''V.C'^v:'-;'v:-.''Xv  '  \ N'  /  -  s/v  ^-v — -  /V  ' \' 'rrsr^'rT 
• 1  x  'N  '  '  '  v  / .  n  -  Vx^;!v/<y;;->/-\  /V/.^^o  _  > "~  /  *-  /    '  \  /  v  ^  x  "/   v '.  s '  - 
/    \            -      '      1        V        -      "        - '    '      "     '        ^    v'     "^^<.'-'-'  ■•' "'■jS^-     '       ■'/'><.                   V     '           '/     -•     N    '     '    W     »       N      '  ^    '           ' 
i,  V/V/1    0  "       '     ,\  ^  "    'i  /  i      '^~~L~~~<^'    '  ^  '     "' "' '         "~     v/  i  Granite  // N,  ^  s  i  / 
/  x  u  /    fdJ^'  '  /  •'  /  '  ,  ~  '  v" N  V^v-'-^ ■■',:^V-:':;^x'\  -\ '  ^  /  x  i  -  /  - 
L'/  v  -v  ^  /lx      /    v  Granite    /      /  N^     >  's  /  (X^^-^^^O^'^vvSci    '^  '   '\K  '    v 
L  '     '  /      'v^/     ^"'\i/n'/x''v/     w'\    /  x  /  ^^S4-  ''  :,'y':  '''^''V'^-V^sT    '  '  v  ""      '   s 
'     /n/^/n/^i/v    '    _/vS^'    /  x  ^   x  i    *  /N    'Sj  - v  :>""  -'''■•*'',  'V^T^N-.  '/  x  '/  '  / 
x-'i/l/    /    v/,n    '^',ivi    /    r  '  ,s  n    /i-  'iN/txV;'V/,V'.^V,,v?,V?v:<'',/H.'-'  v<-v^ 
Scale  of  feet 
200  300 
IG.  7.— Horizontal  plan  of  a  portion  of  the  Seven  Thirty  vein  on  the  80-foot  level,  showing  a 
diminution  in  size  of  the  vein  in  its  passage  from  granite  into  porphyry. 
eeption  to  the  general  lack  of  ore  in  the  porphyry,  which,  however, 
proves  the  main  contention,  is  illustrated  in  fig.  8,  which  represents 
the  vein  in  one  of  the  upper  tunnels  of  the  Maine  mine.     Here  the 
till  comparatively  fresh  porphyry  was  evidently  more  rigid  and  brit- 
i.  8.— Horizontal  plan  of  vein  in  tunnel  of  Maine  mine,  showing  thickening  of  veil 
from  gneiss  into  harder  porphyry. 
than  the  gneiss  which  lies  on  both  sides  of  the  dike,  and 
igly  fractured  better.     The  vein  in  the  porphyry  consists  of  i 
e  and  rock  lode,  varying  from  '1  to  4  feet  wide  and  containin 
w-'grade,  coarse  blende,  pyrite,  and  galena  ore  seams  up  to  1 
n  passing 
accord- 
i  mixed 
g  solid, 
foot  in 
! 
