﻿Report 
  of 
  the 
  State 
  Geologist. 
  699 
  

  

  show. 
  Near 
  the 
  Old 
  Sterling 
  mine, 
  between 
  it 
  and 
  the 
  Dixon, 
  a 
  

   small 
  drift 
  has 
  been 
  opened 
  in 
  a 
  low, 
  broad 
  ridge 
  of 
  dark, 
  rusty- 
  

   looking 
  rock. 
  This, 
  on 
  close 
  examination, 
  is 
  seen 
  to 
  be 
  quite 
  

   variable 
  in 
  composition, 
  but 
  as 
  a 
  rule 
  highly 
  pyritiferous, 
  the 
  

   pyrite 
  in 
  a 
  fresh 
  piece 
  constituting 
  a 
  considerable 
  percentage 
  of 
  

   the 
  rock. 
  The 
  pyrite 
  is 
  sometimes 
  replaced 
  by 
  a 
  black 
  mineral 
  

   which 
  proves 
  to 
  be 
  magnetite. 
  That 
  the 
  rock 
  is 
  profoundly 
  

   affected 
  by 
  weathering 
  is 
  shown 
  by 
  the 
  abundant 
  presence 
  of 
  

   portions 
  from 
  which 
  the 
  pyrite 
  has 
  been 
  entirely 
  removed 
  by 
  

   oxidation, 
  changing, 
  a 
  heavy, 
  compact 
  rock 
  to 
  a 
  light 
  -gray, 
  

   porous 
  mass, 
  somewhat 
  resembling 
  pumice. 
  

  

  This 
  oxidation 
  sets 
  in 
  circulation 
  dilute 
  solutions 
  of 
  ferrous 
  and 
  

   ferric 
  sulphate 
  and 
  sulphuric 
  acid. 
  In 
  other 
  words, 
  these 
  pyri- 
  

   tiferous 
  gneisses 
  are 
  precisely 
  what 
  is 
  needed 
  to 
  afford 
  a 
  source 
  

   of 
  iron 
  and 
  a 
  powerful 
  chemical 
  agent, 
  whose 
  action 
  is 
  confined 
  

   to 
  a 
  limited 
  area. 
  When 
  it 
  comes 
  to 
  using 
  these 
  rocks 
  as 
  an 
  ■ 
  

   explanation 
  of 
  the 
  ore 
  deposits, 
  difficulties 
  are 
  encountered 
  similar 
  

   to 
  those 
  met 
  with 
  in 
  investigating 
  the 
  origin 
  of 
  the 
  " 
  serpentine." 
  

   For 
  while 
  at 
  some 
  mines 
  the 
  pyrite 
  rocks 
  are 
  prominent, 
  at 
  others 
  

   outcrops 
  fail, 
  and 
  the 
  presence 
  of 
  these 
  rocks 
  can 
  only 
  be 
  iDferred. 
  

   But 
  as 
  they 
  may 
  always 
  be 
  found 
  in 
  the 
  vicinity 
  of 
  the 
  mines, 
  

   this 
  inference 
  seems 
  to 
  be 
  fully 
  justified. 
  

  

  The 
  pyritiferous 
  gneisses 
  standing 
  at 
  a 
  greater 
  or 
  less 
  angle 
  in 
  

   the 
  limestone 
  series 
  would, 
  as 
  above 
  stated, 
  supply, 
  at 
  and 
  near 
  

   the 
  surface, 
  dilute 
  solutions 
  of 
  iron 
  sulphates 
  and 
  sulphuric 
  acid. 
  

   A 
  portion 
  of 
  these 
  solutions 
  would 
  pass 
  away 
  in 
  surface 
  drainage 
  

   waters 
  ; 
  another 
  portion 
  would 
  sink 
  down, 
  with 
  a 
  tendency 
  to 
  

   follow 
  the 
  dip, 
  and 
  where 
  it 
  came 
  in 
  contact 
  with 
  limestone 
  

   would 
  produce 
  double 
  decomposition, 
  the 
  iron 
  being 
  precipitated 
  

   as 
  carbonate 
  or 
  hydrated 
  oxide 
  and 
  the 
  lime 
  and 
  magnesia 
  going 
  

   into 
  solution 
  as 
  sulphates.* 
  This 
  decomposition 
  of 
  limestone 
  by 
  

   the 
  ferruginous 
  solutions 
  would 
  be 
  promoted 
  by 
  the 
  presence 
  of 
  

   any 
  impervious 
  mass 
  which 
  would 
  check 
  the 
  downward 
  flow 
  of 
  

   the 
  solutions 
  and 
  cause 
  them 
  to 
  spread 
  laterally. 
  Such 
  a 
  mass 
  

   would 
  be 
  found 
  in 
  granite 
  cutting 
  through 
  the 
  pyritiferous 
  rocks 
  

   and 
  limestone, 
  which 
  explains 
  the 
  common 
  association 
  of 
  ore 
  and 
  

  

  *F. 
  P. 
  Dunnington.— 
  On 
  the 
  formation 
  of 
  deposits 
  of 
  Oxides 
  of 
  Manganese, 
  Amer. 
  Jour. 
  Scl. 
  

   (3) 
  XXXVI, 
  p. 
  177. 
  

  

  