SPtTRR   AND 
GABEEY. 
GEOEGETOWN    MINING    DISTRICT,    COLORADO.  113 
Veins  which  are  usually  straight  and  regular  single  fissures  some- 
Limes  locally  split  up  into  small  stringer  lodes.  This  is  likely  to 
happen  near  the  ends,  especially  if  the  vein  enters  soft  micaceous 
gneiss.  In  this  latter  case  stringers  of  ore  often  spread  out  between 
the  laminae  of  the  gneiss,  and  dwindle  in  size  until  they  disappear. 
The  ore  in  the  veins  is  sometimes  found  adhering  or  "  frozen  "  to 
the  walls.    Again,  however,  there  has  been  sufficient  movement  along 
the  vein  to  cause  the  ore  to  come  away  readily  from  the  Avails,  due  to 
the  formation  of  a  narrow  streak  of  gouge  or  selvage  between  the  two. 
In  many   instances   veins  consist   entirely   of   a   zone   of  crushed, 
pulverized,  and  more  or  less  altered  country  rock.     This  vein  filling 
differs  little  from  the  wall  rocks  except  in  being  slightly  more  silici- 
fied  and  kaolinized.     A  crushed  zone  of  this  kind  offers  a  natural 
channel  for  the  circulation  of  underground  waters;  but  as  a  result 
)f  the  retardation  to  the  flowage  of  water,  due  to  the  obstructed  con- 
it  ion  of  the  passage,  these  zones  would  be  favorable  to  ore  depo- 
ition.     Instances  were  noted  where  parts  of  the  crushed  zone,  more 
)articularly  next  to  the  walls,  had  become  changed  to  quartz;  others 
^here  quartz  and  ore  formed  a  sort  of  network  with  regular  outlines 
etween  the  two  walls.     It  is  eas}^  to  imagine  this  process  continued 
urther  until  the  whole  space  between  the  two  walls  is  filled  with 
re.     This,  then,  may  be  the  origin  of  some  of  the  lodes  which  now 
ave  the  appearance  of  having  formed  in  open  cavities  and  are  com- 
letely  filled  with  ore.    Occasionally  such  veins  contain  small  patches 
f  quartz  which  show  crustification,  and  comb  quartz,  which  give 
pe  impression  of  fissure   filling.     However,   these  spaces   are    very 
regular  in  shape  and  are  not  true  fissures,  but  undoubtedly  cavi- 
fl.es   of   dissolution   resulting   from   the   action   of   the  mineralizing 
aters. 
Another  type  of  ore  is  that  resulting  from  the  impregnation  and 
iplacement  of  wall  rocks  bordering  fractures  which  acted  as  water- 
urses   for   mineralizing   solutions.     As   a   combined   result   of   the 
*ocess  of  replacement  and   impregnation   there   are  all  gradations 
tween  rock  sprinkled  with  small  amounts  of  the  ore  minerals  to 
lid  ore.     The  ore  bodies  of  the  Empire  mining  region  are  usually 
ry  irregular  in  outline,  have  ill-defined  boundaries,  and  are  corn- 
led  of  ore  gradations  similar  to  those  above  described. 
ocation  of  ore  bodies. — Among  the  factors  which  are  especially 
orable   to   the   formation    of   ore   bodies   in   the    district    are    the 
lowing: 
.  Junctions  of  two  large  veins,  or  junctions  of  branches  or  feeders 
h  the  main  vein,  where  different  solutions  have  mingled  and  thus 
(.rJBised  precipitation. 
Intersections  or  crossings  of  veins,  for  the  same  reason  as  No.  1. 
Bull.  260—05  m 8 
