﻿238 
  J. 
  P. 
  Klmhall 
  — 
  Genesis 
  of 
  Iron- 
  ores 
  hy 
  Isomorphous 
  

  

  doubtful 
  whether 
  the 
  anhydrous 
  salt 
  would 
  be 
  deposited. 
  A 
  

   no 
  less 
  important 
  difficulty 
  arises 
  as 
  to 
  the 
  locus 
  of 
  deposition. 
  

   If 
  this 
  take 
  place 
  at 
  the 
  surface, 
  the 
  presence 
  of 
  these 
  gases 
  

   can 
  scarcely 
  be 
  imagined 
  ; 
  and 
  if 
  below 
  — 
  conditions 
  are 
  pre- 
  

   cluded 
  for 
  lenticular 
  accumulations. 
  Beneath 
  the 
  surface 
  con- 
  

   ditions 
  exist 
  for 
  deposition 
  by 
  segregation 
  or 
  replacement 
  only. 
  

  

  (17.) 
  In 
  any 
  theory 
  of 
  the 
  genesis 
  of 
  siderite, 
  it 
  becomes 
  

   necessary 
  first 
  of 
  all 
  to 
  explain 
  occurrences 
  of 
  siderite 
  in 
  len- 
  

   ticular 
  form, 
  as 
  widely 
  distributed 
  : 
  that 
  is, 
  as 
  a 
  product 
  of 
  

   direct 
  superficial 
  deposition 
  in 
  hydrographic 
  basins 
  ; 
  or 
  else 
  of 
  

   chemical 
  replacement 
  of 
  lenticular 
  beds 
  originally 
  deposited 
  in 
  

   that 
  manner. 
  Between 
  these 
  alternatives 
  the 
  former 
  seems 
  to 
  

   me 
  to 
  be 
  quite 
  impracticable. 
  

  

  Lenticular 
  deposits 
  from 
  either 
  chemical 
  or 
  mechanical 
  pre- 
  

   cipitation 
  are 
  formed 
  exclusively 
  at 
  the 
  surface, 
  that 
  is, 
  in 
  

   hydrographical 
  basins 
  or 
  bottoms 
  where 
  conditions 
  essential 
  to 
  

   stability 
  of 
  hydrous 
  ferrous 
  carbonate 
  can 
  not 
  ordinarily 
  be 
  set 
  

   up, 
  or 
  at 
  least 
  long 
  maintained. 
  Besides, 
  wherever 
  this 
  salt 
  is 
  

   separated 
  from 
  standing 
  water 
  it 
  must 
  be 
  assumed 
  to 
  pass 
  

   spontaneously 
  into 
  a 
  higher 
  state 
  of 
  oxidation. 
  Not 
  only 
  does 
  

   it 
  appear, 
  then, 
  that 
  lenticular 
  developments 
  of 
  ferrous 
  car- 
  

   bonate 
  can 
  not 
  have 
  been 
  superficially 
  deposited, 
  but 
  that 
  this 
  

   compound 
  can 
  not 
  have 
  been 
  derived 
  from 
  direct 
  precipita- 
  

   tion. 
  

  

  (18.) 
  Senft's 
  theory 
  of 
  the 
  genesis 
  of 
  siderite 
  and 
  sphsero- 
  

   siderite 
  seems 
  to 
  have 
  been 
  founded 
  on 
  special 
  occurrences 
  of 
  

   stratiform 
  and 
  nodular 
  clay-ironstone 
  enclosed 
  in 
  clays 
  and 
  

   shales. 
  These 
  are 
  explained 
  as 
  epigenic 
  products 
  res 
  id 
  ting 
  

   from 
  saturation 
  of 
  buried 
  argillaceous 
  sediments 
  with 
  acid 
  

   solutions 
  of 
  ferrous 
  carbonate, 
  supposed 
  to 
  yield 
  the 
  neutral 
  salt 
  

   upon 
  evaporation 
  ; 
  or 
  again 
  by 
  interchange 
  with 
  stronger 
  bases 
  

   like 
  lime. 
  Spathic 
  carbonates 
  are 
  likewise 
  supposed 
  by 
  Senft 
  

   to 
  proceed 
  from 
  absorbents 
  like 
  calcareous 
  material, 
  clay 
  or 
  

   marl.* 
  However 
  applicable 
  may 
  seem 
  parts 
  of 
  this 
  theory 
  to 
  

   concretionary 
  lenses 
  and 
  nodules 
  of 
  clay-ironstone 
  contained 
  in 
  

   beds 
  of 
  residual 
  clay 
  and 
  shales, 
  it 
  must 
  be 
  seen 
  to 
  be 
  incompati- 
  

   ble 
  with 
  the 
  composition 
  of 
  spathic 
  siderite 
  of 
  considerable 
  

   purity, 
  that 
  is, 
  when 
  comparatively 
  free 
  from 
  earthy 
  admix- 
  

   tures, 
  as 
  well 
  as 
  with 
  conditions 
  of 
  deposition 
  in 
  the 
  form 
  of 
  

   lenticular 
  beds. 
  Like 
  other 
  explanations, 
  it 
  rests 
  on 
  the 
  

   assumption 
  that 
  anhydrous 
  ferrous 
  carbonate 
  may 
  be 
  separated 
  

   by 
  evaporation 
  as 
  well 
  as 
  by 
  precipitation 
  from 
  acid 
  solutions 
  

   of 
  ferrous 
  carbonate, 
  a 
  reaction 
  probably 
  true 
  only 
  in 
  a 
  limited 
  

   sense 
  as 
  above 
  pointed 
  out. 
  

  

  The 
  reaction 
  however 
  incidentally 
  mentioned 
  by 
  Senft, 
  

   namely, 
  the 
  isolation 
  of 
  ferrous 
  carbonate 
  by 
  interchange 
  of 
  

   solutions 
  of 
  ferrous 
  salts 
  with 
  stronger 
  bases 
  like 
  lime, 
  is 
  

   * 
  Gesteins 
  und 
  Bodenkunde, 
  1811, 
  28. 
  

  

  