SPURR   AND 
GARREY. 
GEOKGETOWN    MINING    DISTEICT,    COLOKADO.  115 
which  apparently  have  no  connection  whatever  with  ordinary  vein 
junctions.  Some  of  these  have  every  appearance  of  having  origi- 
nated as  a  result  of  the  "  linked  "  nature  of  that  particular  portion  of 
the  vein.  By  the  "  linked  "  nature  is  meant  the  splitting  and  reunit- 
ing of  the  vein,  often  in  both  a  vertical  and  a  horizontal  direction, 
about  a  "horse"  of  country  rock.  The  ore  body  results  from  the 
gradual  impregnation  and  replacement  of  the  isolated  mass  of  rock 
by  quartz  and  ore  through  the  agency  of  the  mineralizing  solutions 
from  the  inclosing  channels. 
In  the  Republican  Mountain  group  the  richest  ore  shoots,  in  most 
cases,  have  some  connection  with  vein  junctions.  An  example  is  the 
junction  of  the  Boston  and  Beecher  veins. 
The  point  of  intersection  or  crossing  of  veins  is  the  favorite  loca- 
tion for  ore  bodies  in  the  Democrat  Mountain  area,  although  ore  in 
large  quantity  does  not  always  occur  at  such  a  place.  Proximity  to 
the  surface  is  also  a  characteristic  feature  of  the  Democrat  Mountain 
ore  deposits.  Good  ore  here  has  rarely  been  found  below  200  to  300 
feet,  and  the  ore  shoots  tend  to  pitch  with  the  slope  on  both  sides  of 
the  ridge  of  the  mountain.  Ore  shoots  also  occur  in  this  region  near 
but  not  at  the  junctions  of  branches  with  the  main  veins. 
In  the  mining  region  about  Empire  broad  zones  of  weakness  in 
rock  inclosed  between  intersecting  or  uniting  veins,  or  in  crushed 
rock  zones  extending  indefinitely  away  on  both  sides  of  a  central 
fault  plane,  are  locations  for  large  irregular  and  faintly  outlined  ore 
passes.  These  ore  bodies  represent  areas  of  strained  or  crushed  rock 
■where  there  has  been  impregnation  of  sulphide  ores  from  the  cen- 
trally located  small  fissures.  As  a  rule  the  ore  bodies  in  the  Empire 
Region  occur  away  from  the  vein  junctions. 
J  In  connection  with  porphyry  dikes  ore  shoots  have  several  modes 
If  occurrence.  According  to  Mr.  E.  A.  MaxAvell a  the  ore  on  the 
Ipchively  vein  at  Silver  Plume  all  occurred  in  the  portion  of  the  vein 
Jetween  two  parallel  porphyry  dikes  which  run  at  right  angles  to  and 
,  jre  intersected  by  the  vein.  The  explanation  of  this  is  probably  that 
|ie  dikes  became  decomposed  and  impervious,  and  that  the  waters 
ing  along  the  vein  fissures  were  confined  in  some  way  to  the  por- 
on  between  the  dikes,  and  so  rose  in  a  sort  of  chimney,  where  the 
rculation  was  unusually  vigorous. 
In  the  case  of  veins  parallel  with  and  at  or  near  the  contact  of  a 
)rphyry  dike  (such  as  the  Bismarck  or  Colorado  Central  veins), 
le  fault  movements  which  have  produced  the  vein  openings  have 
j»llowed  these  courses  on  account  of  their  being  zones  of  mechanical 
teakness.  Moreover,  after  the  formation  of  the  fault  opening,  the 
dutions  have  doubtless  often  been  checked  by  the  porphyry,  where 
°  Verbal  communication. 
isi 
