94  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
Replacement  deposits  in  granite. — The  replacement  deposits  in 
granite  all  occur  in  close  proximity  to  the  contact  with  the  breccia, 
and  are  well  developed  in  the  Elkton  (Thompson),  Ajax,  Independ- 
ence, and  Portland  mines.  Although  these  bodies  of  ore  are  related 
to  fissures  and  occur  particularly  where  several  fissures  intersect,  or 
where  they  meet  a  dike,  the  ore  is  not  confined  to  the  actual  fractures. 
The  rock  in  the  vicinity  of  these  fissures  is  often  extensively  altered,; 
The  most  obvious  characteristic  of  the  metamorphosed  rock  is  a  porous 
texture  and  a  change  of  the  reddish  color  of  the  normal  granite  to 
grayish  or  greenish  tints.  Closer  examinations  shows  that  the  rock., 
consisting  originally  of  microcline,  oligoclase,  quartz,  and  biotite,  may 
be  completely  recrystallized  as  a  porous,  vugg}^  aggregate  of  second- 
ary orthoclase  (valencianite),  quartz,  fluorite,  pyrite,  calaverite  oil 
sylvanite,  and,  in  exceptional  cases,  sphalerite  and  galena.  The  ore 
minerals  are  partly  inclosed  in  the  other  secondary  minerals,  bull! 
occur  most  abundantly,  with  little  projecting  crystals  of  fluoritet 
quartz,  and  valencianite,  on  the  walls  of  the  irregular  pores  character! 
istic  of  the  altered  rock. 
Y\  nile  the  replacement  deposits  in  granite  are  important  because  o 
their  size  and  the  readiness  with  which  the  ore  may  be  mined  fre< 
from  waste,  the  ore  itself  is  usually  of  lower  grade  than  that  formec 
in  the  fissures  of  the  sheeted  zones. 
Mineralized  "  basalt  "  dikes. — The  ore  bodies  formed  by  the  miner 
alization  of  basic  dikes  are  in  some  ways  closely  related  to  th 
sheeted  zones  already  described.  Like  the  phonolite  dikes,  th 
"basalt"  exhibits  a  pronounced  tendency  to  split  into  thin  sheet 
parallel  with  the  dike  vails.  Normally,  the  minute  fissures  s 
formed  are  filled  with  veinlets  of  calcite  and  contain  no  ore.  Whei 
however,  a  zone  of  Assuring  coincides  with  the  dike  the  latter  may  bl 
traversed  by  veinlets  of  quartz  and  flourite  carrying  sylvanite  or  caWi 
verite,  while  the  body  of  the  dike  may  be  impregnated  with  pyrifcl 
Such  ore  differs  from  that  of  the  usual  sheeted  zones  in  breccia  ci 
phonolite  in  that  the  tellurides  are  not  so  clearly  confined  to  the  actut 
fissures,  but  appear  to  some  extent  to  permeate  the  rock  with  tl  I 
pyrite.  The  richest  portion  of  the  ore,  however,  undoubtedly  occu:  | 
in  the  small  veinlets  in  the  dike,  and  usually  near  one  or  both  wall 
Avhere  the  Assuring  is  best  developed. 
DEPTH    OF    OXIDIZED    ZONE. 
At  a  few  points,  as  in  the  Abe  Lincoln  and  El  Paso  mines,  tellurid  I 
are  found  almost  at  the  surface.     It  is  much  more  common,  howevt  i 
to  find  an  upper  zone,  from  w200  to  400  feet  deep,  in  which  free  go 
prevails  and  which  gradually  changes  to  the  zone  of  pure  telluri' 
ores.     As  may  be  expected  from  the  varying  surface  form  and  con( 
