﻿and 
  Pseudomorphous 
  Replacement 
  of 
  Limestone, 
  etc. 
  %&% 
  

  

  form, 
  and 
  perhaps 
  character 
  of 
  insoluble 
  contents, 
  on 
  the 
  

   one 
  hand 
  ; 
  and 
  local 
  developments 
  of 
  iron-ores, 
  void 
  of 
  any- 
  

   thing 
  like 
  regular 
  'form, 
  within 
  the 
  compass 
  of 
  a 
  thick 
  lime- 
  

   stone 
  formation, 
  and 
  graduating 
  into 
  that 
  materia!, 
  on 
  the 
  

   other 
  hand. 
  The 
  latter 
  modes 
  of 
  occurrence 
  (and 
  by 
  infer- 
  

   ence 
  the 
  former 
  also) 
  are 
  particularly 
  well 
  illustrated 
  by 
  

   numerous 
  stratiform 
  iron-ore 
  developments 
  in 
  strata 
  of 
  the 
  

   Carboniferous 
  period. 
  These 
  strata 
  are 
  understood 
  by 
  all 
  to 
  

   have 
  accumulated 
  under 
  specially 
  favorable 
  conditions 
  of 
  envi- 
  

   ronment 
  for 
  the 
  production 
  of 
  the 
  materials 
  of 
  iron-ores 
  

   through 
  internal 
  chemical 
  transmutations 
  ; 
  and 
  to 
  have 
  since 
  

   subsisted 
  under 
  equally 
  favorable 
  atmospheric 
  conditions 
  for 
  

   the 
  preservation 
  of 
  alterable 
  kinds 
  of 
  material 
  produced, 
  like 
  

   siderite 
  and 
  sphserosiderite. 
  

  

  (8.) 
  The 
  possession 
  of 
  many 
  physiographic 
  characters 
  by 
  

   stratiform 
  iron-ores 
  in 
  common 
  with 
  deposits 
  in 
  sit 
  a, 
  formed 
  

   in 
  the 
  natural 
  order 
  of 
  strata 
  between 
  which 
  they 
  are 
  imbed- 
  

   ded, 
  or 
  rather 
  enclosed, 
  has 
  naturally 
  led 
  geologists 
  to 
  seek 
  an 
  

   explanation 
  of 
  at 
  least 
  Carboniferous 
  iron-ores 
  of 
  this 
  descrip- 
  

   tion 
  on 
  theories 
  of 
  direct 
  deposition 
  either 
  chemical 
  or 
  me- 
  

   chanical 
  : 
  that 
  is, 
  according 
  to 
  one 
  theory, 
  in 
  original 
  form 
  of 
  

   ferrous 
  carbonate 
  ; 
  or, 
  according 
  to 
  another 
  theory, 
  as 
  the' 
  

   product 
  of 
  transmutation 
  of 
  ferric 
  hydrate 
  in 
  place 
  into 
  the 
  

   same 
  compound, 
  through 
  'successive 
  deoxidation 
  and 
  carbonat- 
  

   ing 
  agencies, 
  the 
  potential 
  influence 
  of 
  which 
  in 
  the 
  develop- 
  

   ment 
  of 
  this 
  particular 
  series 
  of 
  strata 
  it 
  may 
  not 
  seem 
  difficult 
  

   to 
  imagine 
  on 
  grounds 
  of 
  either 
  theory. 
  As 
  to 
  further 
  altera- 
  

   tion 
  from 
  weathering 
  action, 
  regulated 
  by 
  circumstances 
  of 
  

   topography 
  and 
  environment, 
  all 
  are 
  agreed. 
  

  

  The 
  same 
  agencies 
  however 
  may 
  well 
  be 
  believed 
  to 
  have 
  

   been 
  equally 
  potential 
  in 
  clearly 
  recognized 
  processes 
  resulting 
  

   in 
  pseudomorphous 
  replacement 
  of 
  limestone 
  by 
  ferrous 
  car- 
  

   bonate, 
  especially 
  in 
  the 
  preliminary 
  work 
  of 
  decomposing 
  

   diffused 
  clastic 
  ferrous 
  and 
  ferric 
  silicates, 
  dissolving 
  their 
  

   soluble 
  products, 
  and 
  in 
  the 
  generation 
  and 
  preservation 
  unal- 
  

   tered 
  of 
  anhydrous 
  ferrous 
  carbonate 
  in 
  concrete 
  form 
  however 
  

   produced. 
  

  

  (9.) 
  While 
  dissolved 
  alkaline 
  mono-carbonates, 
  as 
  well 
  known, 
  

   readily 
  precipitate 
  instable 
  hydrated 
  ferrous 
  carbonate 
  from 
  

   solutions 
  of 
  ferrous 
  salts, 
  no 
  artificial 
  method 
  appears 
  to 
  have 
  

   been 
  proposed 
  for 
  the 
  production 
  at 
  ordinary 
  temperatures 
  of 
  

   anhydrous 
  ferrous 
  carbonate. 
  

  

  (10.) 
  The 
  generation 
  of 
  this 
  natural 
  compound 
  in 
  the 
  form 
  

   of 
  siderite 
  and 
  sphserosiderite 
  is 
  sometimes 
  attributed 
  to 
  direct 
  

   precipitation 
  and 
  concentration 
  of 
  hydrous 
  ferrous 
  carbonate 
  

   in 
  the 
  presence 
  of 
  reducing 
  gases, 
  or 
  of 
  an 
  atmosphere 
  of 
  car- 
  

   bonic 
  anhydride. 
  This 
  presupposes 
  dehydration 
  at 
  ordinary 
  

   temperatures 
  by 
  some 
  natural 
  process 
  as 
  yet 
  unexplained. 
  

  

  