58 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bulb.  260. 
-"- Clay  shale 
==  Black  shaie 
Quartzite 
fissure  passes  into  shales.  In  some  mines,  such  as  the  Iowa  Chief 
in  Dexter  Creek,  this  is  due  to  the  very  narrow  character  of  the  vein 
in  the  clay  shales.  It  is  so  narrow  as  to  be  in  most  places  a  mere 
fracture  in  the  sediments,  with  no  appreciable  open  space.  On  pass- 
ing- into  the  quartzite,  a  rock  more  capable  of  supporting  large  cavi- 
ties, the  vein  widens  and  rises  so  much  in  value  as  to  furnish  very 
considerable  profits.  In  other  cases,  however,  the  vein  suffers  no 
appreciable  diminution  in  size,  but  the  ore  minerals  seem  to  have 
been  deposited  only  between  the  layers  of  quartzite,  the  portion  of 
the  vein  in  the  shales  being  occupied  by  barren-gangue  material  and 
clay. 
Again,  where  the  vein  passes  through  a  limestone  flat  shoots  of 
ore  are  developed  parallel  to  the  bedding  and  running  along  the  main 
fissure.     Such  shoots  often  extend  laterally  for  25  to  30  feet  or  more 
from  the  main  ore  body.  They 
are  formed  by  replacement  of  the 
limestone  with  ore  and  gangue 
minerals  and  are  clearly  derived 
from  the  main  vein.  Such  re- 
placement bodies  constitute  a 
class  of  deposits  so  distinct  from 
the  fissure  veins  in  their  gen- 
eral character  that  they  will  be 
further  described  under  the 
head  of  "  Replacement  deposits 
in  limestone." 
The  silver  veins  of  the  Dexter 
Creek  and  Uncompahgre  dis- 
tricts do  not  pass  upward  into 
the  andesite  breccia  of  the  San 
Juan  formation.  Those  in  the 
Portland  amphitheater,  on  the 
contrary,  extend  upward  into  that  formation.  The  reason  for  this 
difference  is  that  heavy  black  shales  of  the  Mancos  formation  lie  above 
the  ore  in  the  Dexter  Creek  country,  but  these  are  absent  in  the  Port- 
land amphitheater.  The  veins,  being  of  but  slight  displacement, 
seem  to  have  been  lost  in  these  shales.  This  is  particularly  true  of 
the  Bachelor  and  Wedge  mines,  where  the  ore  and  vein  terminate 
abruptly  and  bluntly  at  the  shale  horizon. 
The  fissures  have  been  frequently  affected  by  bedding  faults  which 
invariably  follow  the  shale  beds.  This  faulting  often,  as  in  the  Black 
Girl  and  Newsboy  mines,  shifted  the  upper  portion  of  the  vein  as 
much  as  30  feet  away  from  its  downward  continuation.  It  seems  to 
have  occurred  before  mineralization,  as  the  ore  is  usually  unbroken 
=^-^   Black  shale 
Gray  sandstone 
Fig.  2. — Type  of  silver-bearing  vein  modi- 
fied by  replacement  and  bedding  fault,  Ou- 
ray, Colo. 
