104  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
In  the  second  thin  section  chalcocite  as  well  as  malachite  forms  th 
cement.  Further,  both  minerals  till  many  fractures  in  the  quart 
grains.  The  areas  of  the  copper  minerals  in  the  sandstone  are  si 
large  and  many  of  the  quartz  grains  have  been  so  greatly  eaten  int< 
by  malachite  and  chalcocite  that  it  is  evident  that  both  copper  mm 
erals  have  replaced  the  quartz  to  an  important  extent. 
Silver  Cliff  mine. — The  Silver  Cliff  mine  is  three-fourths  of  a  mil 
southwest  of  Lusk,  the  deposit  crossing  low  hills  in  a  southerly  direc 
tion.  Ore  from  this  mine  is  reported  to  have  assayed  as  high  a 
$150  per  ton  in  silver  and  copper.  The  pre-Cambrian  formal io 
here  is  a  steeply  dipping  muscovite  schist  interbedded  with  thi 
lenses  of  limy  schist.  The  flat-lying  (iuernsey  formation  above  it  i 
dropped  down  by  a  fault  on  the  west  side  against  the  pre-Can 
brian.  This  fault  courses  S.  10°  W.  and  has  a  displacement  of  2 
feet. 
The  pre-Cambrian  schist  along  this  fault  contains  malachih 
chrysocolla,  and  chalcocite,  and  a  few  leaves  of  native  silver  occi 
on  fracture  planes  in  the  schist.  The  exposures  are  not  sufficient  t 
determine  definitely  whether  this  is  a  pre-Cambrian  vein,  whetlu 
the  ore  in  the  pre-Cambrian  rocks  was  leached  from  the4  overlym 
Guernsey  formation,  or  whether  the  copper  ores  in  both  pre-Can 
brian  and  Carboniferous  terranes  were  deposited  by  waters  ascenc 
big  along  the  fault  plane. 
In  the  iron-stained  sandstone  at  the  base  of  the  Guernsey  form) 
tion  along  the  fault  line  is  a  blanket  of  copper  ore  from  20to40fe< 
wide,  2  to  5  feet  thick,  and  several  hundred  feet  long.  In  miner;) 
ogical  composition  the  ore  is  in  all  essential  particulars  similar  1 
thai  of  the  Green  Hope  mine  previously  described,  except  th; 
native  copper  is  reported  to  occur  near  the  surface.  A  thin  section 
the  Guernsey  ore  shows  that  limonite,  malachite,  and  chalcoci 
replace  the  matrix  of  the  sandstone,  which  is  kaolinitic  material  ar 
calcite.  Kaolinixed  grains  of  feldspar  are  likewise  heavily  staim 
by  malachite  that  apparently  replaces  the  kaolinitic  material.  T1 
copper  minerals  also  replace  quartz  and  occur  in  fractures  in  t. 
quartz  grains. 
Origin  of  deposits.  In  considering  the  origin  of  these  blank 
deposits  the  following  facts  must  be  taken  into  account:  First,  th 
lie  upon  an  impervious  stratum,  and,  with  the  exception  of  the  Silv 
Cliff  deposit,  have  a  definite  downward  limit:  second,  the  ores  a 
minera logically  like  those  of  the  Sunrise  and  Green  Mountain  B< 
mines,  which  are  known  to  be  deposited  by  descending  waters;  thii 
the  deposits  are,  with  the  exception  of  that  at  the  Silver  Cliff  mil 
unconnected  with  fractures:  fourth,  they  occur  in  a  formation  whi 
is  younger  than  any  known  igneous  rock  in  the  Ilartville  uplift  :  a 
fifth,  in  many  places  they  are  associated  with  and  structurally  rese 
