40  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
Movement  has  been  renewed  along  the  same  vein  fractures  since  the  ore  was  deposited 
and  has  crushed  the  vein  or  the  ore  breccia  just  described  into  a  secondary  breccia  of 
mixed  ore  and  wall  rock,  and  later  solutions  have  cemented  the  fragments  with  silica. 
Comparatively  recent  movements  have  in  turn  crushed  this  second  breccia  and  have  left 
the  fragments  of  ore,  quartz,  and  country  rock  in  an  unconsolidated  matrix  of  finely 
crushed  and  decomposed  material.  Some  of  the  movements  along  the  veins  already 
referred  to  have  been  very  pronounced,  as  is  indicated  by  the  well-rounded  form  and  often 
polished  surfaces  of  the  pebble-like  fragments  filling  the  vein.  Good  examples  of  these 
friction  conglomerates  occur  in  the  Stanley  and  Freeland  mines  and  in  the  Newhouse 
tunnel.  There  is  a  breccia  filling  in  the  Stanley  mine,  however,  which  can  not  be  ascribed 
wholly  to  movement,  but  rather  to  the  filling  by  detritus  of  a  water  course  along  an  open 
fissure. 
The  later  movements  also  produced  the  transverse  faults  already  described. 
RELATION    OF    ORE    DEPOSITION    TO    DEPTH. 
Ore  has  been  found  as  deep  as  1,800  feet  below  the  surface  in  the  mines  south  and  west 
of  Idaho  Springs.  Most  of  the  workings,  however,  reach  a  depth  ranging  from  100  to  800 
feet. 
In  many  of  the  mines  the  vein  practically  disappears  at  a  variable  depth  from  the  sur- 
face, only  an  uncemented  fracture  or  fault  zone  remaining.  It  is  not,  however,  fair  to 
ascribe  this  fact  in  all  cases  to  direct  dependence  of  the  mineralization  on  proximity  to  the 
surface,  for  a  similar  decrease  and  practical  disappearance  of  the  mineralization  is  frequently 
encountered  on  following  the  vein  horizontally.  The  fact  is  that  the  ores  have  been  depos- 
ited in  certain  areas,  limited  in  all  dimensions,  along  the  fault-zones.  Such  ore  bodies  may 
or  may  not  now  be  exposed  at  the  surface  as  a  consequence  of  erosion  subsequent  to  ore 
deposition.  In  the  majority  of  cases  they  probably  outcrop,  but  in  some  they  do  not .  The 
Lamartine  mine  affords  an  example  of  an  ore  shoot  that  does  not  extend  to  the  surface.  Crop- 
pings  of  the  vein  gave  assays  of  less  than  $4  to  the  ton  and  no  ore  was  encountered  until  the 
ore  body  was  reached  at  a  depth  of  100  feet  below  the  surface. 
On  the  whole  there  is  no  very  striking  or  complete  change  in  the  character  of  the  mineral- 
ization within  an  ore  body  from  top  to  bottom.  Id  several  mines  the  ore  at  depths  of  from 
800  to  1,600  feet  is  as  rich  as  that  near  the  surface. 
