108 
CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,  1904.        [bull.  200. 
variety  of  ores  is  a  result.     Bornite  is  prominent  in  ores  of  the  Cen- 
tennial and  Gold  Fissure  mines. 
The  chief  gangue  mineral  is  quartz.  Siderite  appears  prominently 
as  a  gangue  mineral  in  the  Griffith,  Annette,  Sceptre,  and  some  other 
mines,  and  barite,  aragonite,  calcite,  magnesite,  and  fluorite  are  occa- 
sionally met  with  in  the  veins. 
VEIN   GROUPS   OF   THE  DISTRICT. 
Grouping  according  to  the  form  of  vein  systems. — In  the  George- 
town district,  geographically  different  groups  of  veins  differ  from 
Scale  of  feet 
50  100  150 
Fig.  5. — Horizontal  plan  of  portion  of  Kirtley  vein,  showing  the  branching  and  the  loc 
tion  of  the  ore  bodies  near  junction  of  branches. 
one  another  markedly  in  their  internal  arrangement.     Following  ai 
some  of  the  principal  types  of  vein  systems : 
(1)   The  branching  system  consists  of  a  main  or  master  vein  fro 
which  smaller  veins  branch   (see  fig.  5).     These  smaller  veins  msuj 
successively   subdivide  until  they   finally   die   out.     The   main    ve 
itself  dies  out  in  a  similar  manner.     In  exceptional  eases  a  brand 
may  curve  and  return  to  the  trunk  vein,  so  Unit  the  two  veins  inclo  'j 
