98  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    1. 
their  entries.  Twenty  feet  immediately  beneath  the  ore  lens  a  tunnel 
w&s  driven  50  feet,  and  this,  with  the  exception  of  a  few  small 
stringers  of  ore,  is  also  barren. 
The  copper  ores  now  seen  here  occur  in  three  ways — as  replacements 
of  flint  nodules,  in  veinlets,  and  as  patchy  malachite  stains  in  the 
limestone.  Chrysocolla,  less  commonly  malachite,  and  still  more 
rarely  chalcocite  replace  the  flint  nodules.  The  three  ores  are  con] 
temporaneous.  In  a  number  of  places  the  exterior  of  the  flint  is 
unaltered,  the  replacement  having  begun  at  the  center.  As  seen  under 
the  microscope  the  copper  minerals  invade"  the  chalcedonie  quartz  as 
irregular  ramifying  bodies  with  rounded  ends,  resembling  the  fronds 
of  a  maidenhair  fern.  The  predominance  of  the  replacement  of  Hint 
by  chrysocolla  (a  copper  silicate)  is  probabrj  dm4  to  the  reaction  of  the 
siliceous  Hint  on  the  copper-bearing  solutions,  as  is  the  predominance 
of  chrysocolla  over  malachite  in  copper  Lenses  in  pre-Cambrian  schist. 
Since  replacement  the  Hint  nodules  have  been  cracked,  and  along 
these  fractures  arc  films  of  malachite.  Fractures  filled  with  veinlets 
of  malachite  and  chrysocolla  ace  common  on  the  sides  of  the  open  cut 
and  near  the  mouths  of  the  tunnels.  These  stringers  were  evidently 
leached  by  descending  waters  from  the  mam  ore  body  or  from  the 
replaced  Hint  nodules. 
The  ore  body  of  the  Green  Mountain  Boy  was  distinctly  limited 
below,  and  scarcely  a  copper  stain  is  visible  20  feet  beneath  the  bottom 
of  the  canoe-shaped  mass  of  chalcocite.  The  copper  ores  were  evi- 
dently deposited  by  descending  water,  t  he  mass  leached  probably  hav- 
ing been  the  upper  portion  of  the  Guernsey  formation.  The  ore  body 
was  largely  a  replacement  deposit,  in  part  of  limestone  and  in  part  of 
Hint.  Since  I  he  original  deposition  the  ore  body  has  been  fractured 
and  surface  water-  have  tilled  the  fractures  with  malachite  and  chryso- 
colla. 
Lenses  in  pre-Cambrian  limestone. — The  most  widely  distributed 
form  of  copper  deposit  in  the  Hartville  uplift  occurs  in  pre-Cambrian 
limestone,  as  a  rule  wit  hin  less  t  han  50  feet  of  t  he  base  of  the  overlying 
Guernsey  formation.  The  deposits,  in  places  at  least,  give  out 
within  20  feet  of  the  surface. 
The  ore  occurs  in  veinlets  along  bedding  planes,  many  of  which  are 
planes  of  differential  movement,  accompanied  by  brecciation ;  in 
veinlets  along  joint  faces;  and  as  replacements  of  limestone.  Mala- 
chite predominates  over  green  chrysocolla;  and  azurite,  brown  chryso- 
colla, chalcocite,  and  yellowish-green  arsenate  also  occur  locally.  Mr. 
Peter  Hoyer,  of  Hartville,  reports  that  a  canoe  of  chalcocite  12  to  14 
feet  long,  IV  feet  wTide,  and  1  to  2  feet  thick  was  encountered  at  the 
surface  at  the  Empire  mine,  half  a  mile  southwest  of  Sunrise.  Quart! 
and  calcite  are  the  chief  gangue  minerals,  and  the  gangue  material  is  in 
