﻿and 
  P 
  seudomorphous 
  Replacement 
  of 
  Limestone, 
  etc. 
  233 
  

  

  the 
  greater 
  number 
  of 
  instances 
  are 
  with 
  remarkably 
  pure 
  and 
  

   persistent 
  limestones, 
  comparatively 
  free 
  from 
  intercalations 
  of 
  

   argillaceous 
  matter, 
  also 
  a 
  residual 
  product 
  of 
  rock-decay, 
  and 
  

   invariably 
  accompanying 
  ochreous 
  matter 
  in 
  suspension. 
  Again, 
  

   replacement 
  of 
  limestone 
  naturally 
  progresses 
  from 
  exterior 
  

   and 
  divisional 
  surfaces. 
  This, 
  as 
  commonly 
  observed, 
  wherever 
  

   incomplete, 
  has 
  invariably 
  affected 
  superficial 
  or 
  upper 
  parts 
  

   of 
  formations 
  under 
  gentle 
  dips, 
  and 
  seldom 
  nether 
  parts 
  ex- 
  

   cept 
  under 
  steep 
  dips. 
  Lenticular 
  bodies 
  of 
  iron 
  ores, 
  not 
  

   purely 
  concretionary, 
  are 
  very 
  rarely 
  if 
  ever 
  found 
  completely 
  

   enclosed 
  in 
  pure 
  limestone 
  — 
  that 
  is, 
  in 
  any 
  form 
  corresponding 
  

   to 
  the 
  tilling 
  of 
  a 
  hydrographic 
  basin 
  of 
  marine 
  limestone. 
  

  

  Conditions 
  above 
  briefly 
  noticed 
  are 
  well 
  illustrated, 
  as 
  I 
  

   shall 
  endeavor 
  to 
  show, 
  by 
  the 
  more 
  important 
  developments 
  

   of 
  iron 
  ores 
  upon 
  horizons 
  of 
  limestones 
  and 
  adjacent 
  transi- 
  

   tion 
  strata 
  of 
  all 
  geologic 
  periods. 
  

  

  (2.) 
  The 
  geologic 
  importance 
  of 
  the 
  phenomena 
  of 
  displace- 
  

   ment 
  of 
  calcium 
  -carbon 
  ate 
  by 
  ferrous 
  carbonate 
  was 
  long 
  since 
  

   indicated 
  by 
  Bischof, 
  mainly, 
  as 
  it 
  appears, 
  on 
  mineral 
  ogic 
  or 
  

   a 
  priori 
  grounds.* 
  Pseudomorphous 
  siderite 
  after 
  calcite, 
  

   occurring 
  in 
  drusy 
  cavities 
  in 
  anamesite, 
  as 
  described 
  by 
  Blum 
  

   and 
  Sandberger, 
  was 
  attributed 
  to 
  removal 
  of 
  calcium 
  car- 
  

   bonate 
  by 
  carbonated 
  solutions 
  of 
  ferrous 
  carbonate 
  and 
  depo- 
  

   sition 
  of 
  this 
  salt 
  in 
  its 
  place. 
  The 
  same 
  result, 
  as 
  well 
  known, 
  

   is 
  produced 
  by 
  reaction 
  of 
  solutions 
  of 
  ferrous 
  sulphate, 
  calcium 
  

   sulphate 
  being 
  removed. 
  

  

  (3.) 
  Pseudomorphic 
  replacement 
  of 
  calcite 
  by 
  ochreous 
  ferric 
  

   oxyd 
  was 
  observed 
  by 
  Blum 
  to 
  have 
  taken 
  place 
  indirectly, 
  

   namely, 
  tirst 
  by 
  substitution 
  of 
  ferrous 
  carbonate 
  followed 
  by 
  

   alteration 
  of 
  this 
  comparatively 
  unstable 
  compound. 
  As 
  pointed 
  

   out 
  by 
  Bischof, 
  it 
  seems 
  probable 
  indeed 
  that 
  pseudomorphs 
  of 
  

   this 
  type 
  are 
  necessarily 
  indirect 
  — 
  never 
  direct.f 
  

  

  (4 
  ) 
  Aside 
  from 
  pseudomorphs 
  by 
  incrustation, 
  pseudomor- 
  

   phous 
  siderite 
  commonly 
  occurs 
  by 
  substitution 
  of 
  anhydrous 
  

   isomorphous 
  minerals. 
  Pseudomorphism 
  by 
  alteration 
  often 
  

   succeeds 
  pseudomorphism 
  by 
  substitution. 
  Both 
  processes, 
  as 
  

   inferred 
  from 
  relative 
  densities, 
  are 
  attended 
  with 
  contraction. 
  

   In 
  the 
  conversion 
  of 
  siderite 
  into 
  limonite, 
  this, 
  according 
  to 
  

   Hunt, 
  amounts 
  to 
  nearly 
  twenty 
  per 
  cent.J 
  Hence 
  the 
  exhibi- 
  

   tion 
  of 
  cavities, 
  anfractuosities 
  and 
  dislocations 
  in 
  products 
  of 
  

   either 
  transformation, 
  as 
  witnessed 
  both 
  on 
  a 
  mineralogic 
  and 
  

   petrographic 
  scale. 
  

  

  (5.) 
  Whatever 
  be 
  the 
  mode 
  of 
  accumulation 
  of 
  ferrous 
  car- 
  

   bonate 
  in 
  various 
  deposits, 
  it 
  can 
  scarcely 
  fail 
  to 
  be 
  recognized 
  

   as 
  invariably 
  a 
  secondary 
  product 
  universally 
  resulting 
  from 
  

   the 
  decomposition 
  of 
  diffused 
  proto-silicates 
  of 
  iron 
  by 
  means 
  

  

  * 
  Bd. 
  II, 
  1864, 
  p. 
  154. 
  f 
  Chem. 
  Geol., 
  Bd. 
  Ill, 
  1866, 
  8U. 
  

  

  \ 
  This 
  Journal, 
  xxvi, 
  1883, 
  202. 
  

  

  