92  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  26Q. 
STRUCTURAL  CHARACTER  OF  DEPOSITS. 
With  few  exceptions  the  ore  bodies,  of  whatever  shape,  are  causally 
connected  with  fissures,  and  most  of  them  constitute  fissure  veins  of 
various  types.  The  fissure  system  of  the  district  appears  to  radiate 
from  a  point  near  the  northern  limit  of  the  volcanic  area.  In  the 
eastern  part  the  prevailing-  directions  are  northwest  or  north  -north- 
west, gradually  changing  to  a  northerly  strike  in  the  southern  por- 
tion and  to  predominate  north-northeast  or  northeast  courses  in  the 
western  side  of  the  district. 
Individual  veins  are  rarely  over  half  a  mile  in  length,  but  linked 
vein  systems  often  extend  for  a  mile  in  the  same  direction.  The  dip 
is  generally  very  steep.  The  movement  along  these  fissure  planes 
appears  in  all  cases  to  have  been  very  slight.  The  fissures  charged 
with  ore  are  sometimes  simple  veins  with  one  fracture  plane;  much 
more  commonly,  however,  they  are  composite  veins  or  lodes  which 
consist  of  several  closely  spaced  and  frequently  linked  fissures,  all 
more  or  less  ore  bearing.  A  better  expression  for  this  structural  typei 
as  it  appears  in  Cripple  Creek  is  the  term  "  sheeted  zone." 
TYFES   OF  DEPOSITS. 
The  most  important  types  of  auriferous  ore  bodies  occurring  in  the 
district  are : 
1.  Tabular  in  form  and  strictly  following  simple  fissures  or  sheeted 
zones.  A  subtype  comprises  lodes  in  which  the  sheeted  zone  follows 
"  basalt  "  or  phonolite  dikes. 
2.  Irregular  bodies  adjacent  to  fissures  and  formed  by  replacement 
and  recrystallization  of  the  country  rock — usually  granite. 
These  types  are  not  always  sharply  distinct,  but  may  be  connectec* 
by  deposits  of  intermediate  character. 
All  the  ore  bodies,  of  whatever  type,  exhibit  certain  common  fea  i 
tures  which  serve  to  distinguish  the  deposits  of  Cripple  Creek  frorr 
those  of  most  other  mining  districts.  In  the  first  place,  the  aetua  j 
openings  in  the  rocks  available  for  the  deposition  of  ore  are,  as  a  rule 
remarkably  narrow.  In  the  second  place,  the  amount  of  materia 
carried  in  the  mineralizing  solutions  and  deposited  as  gangue  an< 
ore  minerals  was  comparatively  small.  In  consequence  of  thes 
two  conditions,  the  district  contains  no  such  massive  veins,  solid! 
filled  with  quartz  or  other  vein  minerals,  as  are  characteristic  of  th 
San  Juan  region  in  Colorado  or  the  Mother  Lode  region  in  Cali 
fornia.  Even  the  small  fissures  of  the  Cripple  Creek  district  ar 
rarely  completely  filled,  but  exhibit  a  characteristic  open  or  vugg 
structure.  Where  the  fractures  are  of  unusual  width,  or  where  th 
rocks  are  extensively  shattered,  as  in  the  Midget  and  Moose  mine:: 
