SPURR   AND 
GARREY. 
GEORGETOWN     MINING    DISTRICT,    COLORADO.  109 
a  "  horse  "  of  rock.  This  may  take  place  in  both  a  vertical  and  a 
horizontal  direction.  Most  of  the  Silver  Plume  veins  (Seven  Thirty, 
Mendota,  Pay  Rock,  etc.)  belong  to  this  type.  This  system  is  due 
to  faulting,  the  movement  having  been  horizontal  along  usually 
steeply  dipping  fault  planes,  as  proved  by  numerous  observed  move- 
ment striae  whose  dips  range  from  very  nearly  horizontal  to  25°  or 
80°  from  the  horizontal. 
(2)  The  crossing  or  Democrat  Mountain  system.  This  consists 
of  at  least  two  distinct  sets  of  strong  veins  crossing  each  other — 
for  example,  a  northwest  set  and  a  northeast  set  (exemplified  by 
the  Fletcher-Polar  Star  veins  and  the  Rogers  vein).  The  lodes  cross 
each  other,  or  may  run  into  each  other  and  stop.  The  branching 
type  of  veins  is  rare  in  the  Democrat  Mountain  locality.  This  vein 
system  is  different  from  that  just  described  as  following  branching 
fissures  due  to  horizontal  faulting.  It  resembles  more  the  fractures 
in  a  solid  shattered  by  concussion.  Moreover,  the  rock  appears  to 
have  been  fractured  without  a  great  amount  of  slipping.  Although 
slickensides  and  movement  stria1  were  not  observed  in  these  veins, 
observations  lead  one  to  suspect  that  the  force  causing  the  fracturing 
was  an  almost  vertical  one. 
(3)  The  Empire  system  of  combined  branching  and  crossing 
veins  is  interniediate  between  the  Silver  Plume  branching  type  and 
the  Democrat  Mountain  crossing  type.  The  lodes  undoubtedly 
cross  in  many  cases;  in  others  they  merge  at  the  junctions.  The 
(jold  Dirt  group  of  veins  at  Empire  is  the  best  representative  of  this 
system.  Two  steeply  inclined  (50°  to  75°  from  the  horizontal) 
crossing  sets  of  movement  striae  on  the  vein  walls  indicate  strains 
intermediate  between  those  which  caused  the  vein  system  of  Silver 
Plume  and  those  which  probably  affected  the  rocks  of  Democrat 
[Mountain. 
(4)  A  fourth  system  consists  of  one  or  more  practically  non- 
branching  veins  arranged  along  or  parallel  to  the  contact  between  a 
porphyry  dike  and  the  neighboring  rocks.  The  Sunburst-Sceptre 
Tem  illustrates  this  class. 
Grouping  according  to  miner  alogical  differences. — A  classification 
nay  also  be  made  on  characteristic  mineralogical  and  chemical  differ- 
nces,  often  with  attendant  physical  differences.  Among  the  charac- 
eristic  ore  types  resulting  from  such  a  study  are  the  following: 
(1)  Seven  Thirty-Bismarck  type  (Dunkirk,  Wisconsin,  Pay 
lock,  etc.)  is  marked  by  relatively  high-grade  silver  ores  containing 
considerable  galena,  some  zinc  blende,  usually  of  a  dark-brown  color, 
onsiderable  polybasite  and  gray  copper,  a  little  copper-bearing 
yrite,  and  a  small  amount  of  pyrite.  There  is  little  evidence  of 
ssure-filling  in  this  group.  Junctions  of  different  veins  or  branches 
sually  make  ore  bodies. 
