™ BANSOMBND ]  CRIPPLE    CREEK    DISTRICT,    COLORADO.  95 
tions  of  drainage,  there  is  groat  range  in  the  depth  attained  by  oxida- 
tion. Partial  oxidation  extends  in  many  mines  to  a  depth  of  over 
1,000  feet,  especially  along  the  often  more  or  less  open  fissures. 
RELATIONS  OF  ORE  BODIES  TO  DEPTH. 
It  is  well  known  that  the  payable  ores  in  auriferous  lodes  are  rarely 
equally  distributed  in  the  lode,  but  form  tabular  bodies  of  more  or  less 
regular  outline.  The  projections  of  these  ore  bodies  on  the  plane  of 
the  lode  often  appear  as  elongated  areas  with  greater  vertical  than 
horizontal  extent.  The  ore  bodies  or  shoots  of  Cripple  Creek  show 
great  similarity  to  those  of  other  gold-bearing  veins;  their  limit  in 
depth  is  usually  as  well  defined  as  their  extent  in  a  horizontal  direc- 
tion. 
In  the  case  of  shoots  reaching  the  surface,  a  certain  part  has 
probably  been  removed  by  erosion.  The  shoots  which  distinctly 
began  below  the  surface  show  the  normal  form  of  the  ore  bodies  to 
be  elongated,  vertical,  or  pitching  sharply  northward,  the  ratio  of 
vertical  to  horizontal  extension  varying  from  1-J:  1  to  5: 1.  Some  of 
these  shoots  are,  however,  of  about  equal  dimensions  vertically  and 
horizontally.,  while  in  a  few  the  horizontal  dimension  is  the  greater. 
Of  the  known  ore  bodies,  as  few  exceed  1,000  feet  in  length,  so 
very  few  exceed  1,000  feet  in  depth  or  extend  more  than  1,000  feet 
from  the  surface.  Speaking  broadly,  explorations  below  that  limit 
have  not  proved  very  satisfactory.  Drawing  the  lines  a  little  closer, 
it  may  be  said  that  in  proportion  to  the  amount  of  exploration  the 
upper  TOO  or  800  feet  have  yielded  more  than  the  interval  from  that 
limit  to  the  lowest  levels  reached — about  1,500  feet.  It  must  not  be 
overlooked,  however,  that  four  or  five  mines  still  have  good  ore  bodies 
at  a  depth  of  1,200  to  1,400  feet  from  the  surface.  The  develop- 
ments of  the  next  year  or  two  will  probably  give  a  safer  basis  for 
generalization. 
Roughly  speaking,  the  above-mentioned  distribution  holds  good 
for  any  elevation  within  the  district.  In  other  words,  the  principal 
productive  zone  everywhere  occupies  the  space  from  the  surface 
down  to  about  1,000  feet  below  it,  and  its  lower  limit  thus  forms  a 
curved  surface  approximately  parallel  to  the  surface  of  the  ground. 
The  general  features  of  the  vertical  distribution  of  the  known  ore 
bodies  recorded  above  have  of  late  years  received  more  or  less  recog- 
nition, and  there  has  been  a  decided  tendency  to  attribute  them  to  a 
process  of  secondary  enrichment  effected  by  waters  moving  generally 
downward  from  the  surface.     It  has  been  supposed0  that  such  waters 
i' 
«  Bancroft,  Geo.  J.,  Eng.  and  Min.  Jour.,  vol.  74,  1902,  pp.  752-753,  and  vol.  75,  1903, 
p.  111-112. 
Finch,  J.  W.,  Proc.  Colorado  Sci.  Soc,  vol.  7,  1904,  pp.  193-252. 
