﻿and 
  Pseudomorphous 
  Replacement 
  of 
  Limestone, 
  etc. 
  239 
  

  

  probably 
  the 
  prevailing 
  one 
  in 
  the 
  circumstances 
  cited. 
  For 
  

   alkaline 
  mono-carbonates, 
  likewise 
  resulting 
  from 
  decomposition 
  

   of 
  silicates, 
  may 
  safely 
  be 
  assumed 
  to 
  be 
  present 
  partly 
  in 
  undis- 
  

   solved 
  or 
  diffused 
  form 
  wherever 
  ferrous 
  oxide 
  is 
  available, 
  or 
  

   wherever 
  ferrous 
  salts 
  are 
  displaced 
  from 
  solution. 
  

  

  (19.) 
  While 
  such 
  reactions 
  may 
  be 
  readily 
  believed 
  to 
  take 
  

   place 
  in 
  fissures, 
  particularly 
  in 
  contact 
  with 
  segregations 
  of 
  

   calcic 
  carbonate, 
  they 
  can 
  hardly 
  be 
  assumed 
  with 
  Senft 
  also 
  to 
  

   extensively 
  obtain 
  in 
  clay 
  bottoms 
  of 
  standing 
  water, 
  or 
  beneath 
  

   peat-bogs 
  and 
  marshes, 
  still 
  less 
  in 
  a 
  manner 
  to 
  result 
  in 
  direct 
  

   deposition 
  in 
  bedded 
  form 
  from 
  water. 
  In 
  such 
  circumstances 
  

   not 
  the 
  anhydrous 
  salt 
  but 
  the 
  hydrated 
  ferrous 
  carbonate, 
  if 
  

   either, 
  would 
  be 
  deposited 
  ; 
  this 
  however 
  quickly 
  passing 
  into 
  

   ferric 
  hydrate. 
  Still 
  more 
  likely, 
  ferric 
  hydrate 
  would 
  be 
  

   directly 
  deposited 
  from 
  solution 
  through 
  dissipation 
  of 
  free 
  

   carbonic 
  acid. 
  Yet 
  I 
  am 
  not 
  prepared 
  to 
  deny 
  that 
  from 
  the 
  

   condition 
  of 
  ferric 
  hydrate 
  however 
  accumulated 
  anhydrous 
  

   ferrous 
  carbonate 
  may 
  eventually 
  be 
  formed 
  by 
  de-oxidation 
  

   and 
  by 
  carbonating 
  processes. 
  If 
  so, 
  this 
  could 
  be 
  only 
  after 
  

   the 
  original 
  deposits 
  are 
  buried 
  deep 
  below 
  superficial 
  sedi- 
  

   ments 
  and 
  so 
  excluded 
  from 
  atmospheric 
  oxidation. 
  

  

  (20.) 
  Hence, 
  perhaps, 
  the 
  more 
  commonly 
  received 
  meta- 
  

   morphic 
  theory 
  of 
  the 
  genesis 
  of 
  stratiform 
  siderite, 
  generally 
  

   assumed 
  to 
  be 
  stratified. 
  This 
  theory, 
  based 
  on 
  the 
  assumption 
  

   of 
  relative 
  origin 
  corresponding 
  to 
  the 
  natural 
  order 
  of 
  enclos- 
  

   ing 
  strata, 
  involves, 
  in 
  short, 
  alteration 
  in 
  situ 
  of 
  ferric 
  hydrate 
  

   commingled 
  with 
  vegetable 
  matter 
  originally 
  accumulated 
  in 
  

   hydrographic 
  basins. 
  This 
  process 
  is 
  also 
  supposed 
  to 
  be 
  

   excluded 
  from 
  atmospheric 
  air 
  under 
  cover 
  of 
  successive 
  

   sediments. 
  

  

  (21.) 
  Some 
  of 
  the 
  objections 
  to 
  this 
  theory 
  as 
  a 
  general 
  expla- 
  

   nation 
  of 
  the 
  genesis 
  of 
  siderite 
  will 
  appear 
  farther 
  on. 
  Espe- 
  

   cially 
  will 
  it, 
  as 
  I 
  think, 
  be 
  found 
  to 
  fail 
  to 
  explain 
  the 
  prevail- 
  

   ing 
  occurrence 
  of 
  siderite 
  and 
  ferro-calcite 
  in 
  association 
  with 
  

   limestone, 
  or 
  on 
  horizons 
  of 
  limestone, 
  or 
  in 
  lenticular 
  form 
  

   otherwise 
  than 
  concretionary. 
  

  

  (22.) 
  On 
  the 
  other 
  hand, 
  a 
  theory 
  of 
  its 
  derivation 
  in 
  such 
  

   circumstances 
  at 
  least, 
  by 
  isomorphous 
  and 
  pseudomorphous 
  re- 
  

   placement 
  of 
  calcareous 
  material 
  in 
  situ, 
  not 
  only 
  seems 
  to 
  fit 
  

   the 
  greater 
  number 
  of 
  familiar 
  occurrences 
  of 
  siderite, 
  and 
  thus 
  

   to 
  explain 
  the 
  almost 
  universal 
  association 
  of 
  this 
  secondary 
  

   product 
  with 
  limestone, 
  and 
  the 
  graduation 
  into 
  each 
  other 
  of 
  

   these 
  two 
  materials 
  of 
  widely 
  opposite 
  derivation 
  ; 
  but 
  to 
  be 
  

   alone 
  adequate 
  to 
  explain 
  the 
  epigenesis 
  and 
  indeed 
  existence 
  

   of 
  the 
  anhydrous 
  salt. 
  Where 
  of 
  course 
  limestone 
  has 
  been 
  

   completely 
  transformed 
  into 
  siderite, 
  and 
  all 
  immediate 
  evi- 
  

   dences 
  of 
  their 
  relation 
  have 
  disappeared, 
  it 
  may 
  sometimes 
  be 
  

   found 
  practicable 
  to 
  identify 
  lenticular 
  developments 
  of 
  siderite 
  

  

  