114  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
3.  "  Linked  "  nature  of  portions  of  a  vein,  producing  a  widening 
of  the  vein  zone  and  intersecting  channels  for  circulation,  with 
crushed  "  horses  "  between. 
4.  Broad  zones  of  weakness  in  rocks,  permitting  thorough  sat  urn 
tion  by  mineralizing  waters  (vicinity  of  Empire). 
5.  Mechanical  connection  with  porphyry  dike.  Along  such  dike: 
fault  movements  have  frequently  taken  place  and  vein  deposition  ha 
followed.  The  decomposed  porphyry  has  also  frequently  acted  a 
an  impervious  barrier  to  solutions,  inducing  precipitation  in  tli 
opened  zone  which  follows  along  it. 
G.  Rigid  wall  rocks,  producing  good  openings. 
7.  Abundance  of  water,  and  conditions  suitable  for  mingling  o 
chemically  different  waters. 
8.  Proximity  to  the  surface. 
While  the  last  three  of  these  factors,  (6)  nature  of  Avail  rocks,  (I 
abundance  of  water,  and  (8)  proximity  to  the  surface,  are  o 
importance  in  practically  all  parts  of  the  mining  district,  the  firr- 
five  factors  are  of  variable  importance  in  different  parts  of  the  dii 
trict.  For  example,  around  Silver  Plume  and  in  the  immediai 
vicinity  of  Georgetown  the  factor  "  junction  of  veins  "  is  very  ir 
portant.  At  least  three  phases  of  vein  juncture,  with  varying  loc 
tion  for  the  ore  bodies,  are  found  in  the  Silver  Plume  region : 
1.  When  two  minor  veins  of  like  dip  unite  to  form  a  main  or  trm 
vein,  or  when  a  branch  vein  unites  with  a  main  vein,  the  rich  ore  bo< 
may  be  located  on  the  main  vein  a  short  distance  beyond  the  junctit 
Often  the  two  veins  which  unite  carry  no  ore  whatever,  or  at  lerj| 
none  of  any  economic  value.  Examples  of  this  type  are  numeroi 
Among  them  may  be  mentioned  the  junction  which  caused  the  richu 
ore  body  on  the  Pay  Rock,  the  union  of  the  Brown  and  Philips  ve 
in  the  Burleigh  tunnel,  and  the  junctions  which  form  the  Carnahii 
stopes  in  the  Colorado  Central. 
2.  When  two  minor  veins  of  like  dip  unite  to  form  a  main  or  trSI 
vein,  or  when  a  branch  vein  unites  with  a  main  vein,  the  rich  ore  m 
be  located  directly  at  the  point  of  juncture  on  the  trunk  vein,  i 
usually  also  on  both  uniting  veins  for  a  short  distance  away  from 
junction.     Excellent  examples  of  this  type  occur  in  the  Lebanon  i\ 
Kirtley  mines.     (See  fig-  5.) 
3.  When  two  veins  of  different  dip  unite,  the  ore  shoot  ordinal 
occurs  in  the    V-shaped,  usually  pitching  trough  on  the  upper 
of  the  junction.     Examples  are  found  in  the  junction  of  the  Join 
Roe  vein  with  the  Seven  Thirty  and  the  Vice-President  lodes, 
union  of  the  flat  vein  with  the  South  vein  in  the  Colorado  Cen 
mine  is  another  illustration. 
Occasionally  ore  bodies  have  been  found  in  the  Silver  Plume  { 
