102  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    I. 
BLANKET    DEPOSITS    AT    THE    BASE    OF    THE    GUERNSEY    FORMATION. 
( 'h  a  rod eristics. — Copper  ores  are  segregated  in  the  sandstone  at  the 
base  of  the  Guernsey  formation  at  a  number  of  places  in  the  Hartvilll 
uplift.  Malachite  and  chrysocolla  and  less  constantly  azurite  and 
chalcocite  are  the  more  common  ores,  although  native  copper,  silver, 
and  copper  arsenate  also  occur.  Shipments  of  picked  ore  have  aver- 
aged over  17  per  cent  of  copper,  and  either  gold  or  silver  values  are 
present  in  much  of  the  ore.  These  ores  occur  as  lenticular  or  tabula! 
masses  in  the  sandstone,  where  the  copper  minerals  may  form  the 
cementing  material  of  the  sandstone.  They  also  fill  joint  cracks  andi! 
irregular  fractures  or  form  nodules.  The  ore  bodies  are  in  most 
places  unconnected  with  fissures  and  nowhere  with  igneous  rocks. 
Xo  igneous  rocks  other  than  those  of  pre-Cambrian  age  are  known  in 
the  Ilartville  uplift.  The  greatest  dimension  of  these  tabular  ore 
bodies  is  horizontal,  and  they  do  not  extend  into  the  overlying  lime! 
stone  member  of  the  Guernsey  formation.  In  many  places  stringer! 
of  ore  descend  into  the  pre-Cambrian  schist  or  limestone,  which  under- 
lies the  Guernsey  formation  unconformably,  and  at  least  in  most  of 
such  stringers  this  ore  has  been  leached  from  the  blanket  deposits. 
The  bedded  ore  body  is  in  general  but  2  or  3  feel  thick,  although 
where  it  lies  upon  pre-Cambrian  limestone  the  ore  in  the  ramifying 
sandstone  masses  extending  into  the  irregularities  of  the  pre-Cam- 
brian limestone  is  locally  20  feet  thick.  At  the  bottom  of  such 
pockel  -  and  small  sink  holes  the  ore  has  been  concentrated  against  the  ■ 
limestone,  forming  local  enrichments.  As  a  rule  the  lateral  extent  of! 
these  deposits  is  small,  although  some  of  them  are  considerably  over! 
LOO  feet    in  diameter. 
In    prospecting   these    deposits    the    lower   limit    of   the    flat-lying] 
Guernsey  formation  immediately  above  the  tilted  pre-Cambrian  lime- 
stones  and   schists  should   be   followed.      The  geographic   position   of 
this  line  is  shown  on  1  he  geologic  map  of  t  be  I  lart  ville  quadrangle  "  by 
W.  S.  Tangier  Smith  and  \.  II.  Darton.      Iron-stained  masses  of  the 
sandstone   especially   should    be   examined,   since   such    portions   are; 
more  favorable  for  copper  ores.     While  no  deposit  of  t  his  class  has  yet 
proved  to  be  of  great  size,  the  ore  is  cheaply  mined  by  tunneling,  and  < 
some  of  it  can  be  worked  at  a  profit. 
As  type  examples  of  t  his  form  of  deposit  the  Green  Hope  and  Silver] 
ClifT  mines  are  described. 
Green  Hope  mine. — The  Green  Hope  mine  is  situated  on  the  upper'] 
sl«»pe  of  a  valley  in  the  \\Y.  j  sec.  26,  T.  29  X.,  R.  65  W.      Its  product  J 
is  variously  estimated  at  400  to  525  tons,  carrying  from   15  to  27  pel 
cent  and   averaging  17  per  cent  of  copper.     The  deposit   has   been 
exploited  by  two  tunnels,  from  which  three  sinuous  galleries  extend. 
a  Geologic  Atlas  Q.  s..  folio  91,  U.  S.  Geol.  Survey    L903. 
