﻿and 
  Pseudomorphous 
  Replacement 
  of 
  Limestone, 
  etc, 
  237 
  

  

  imagine, 
  the 
  problem 
  still 
  remains. 
  — 
  Whence 
  the 
  production 
  

   of 
  the 
  anhydrous 
  carbonate? 
  

  

  (14.) 
  In 
  this 
  question 
  one 
  is 
  confronted 
  by 
  the 
  remarkable 
  

   fact 
  that 
  writers 
  within 
  the 
  field 
  of 
  chemical 
  geology 
  habitually 
  

   fail 
  to 
  discriminate 
  between 
  the 
  two 
  carbonates 
  either 
  in 
  not- 
  

   ing 
  rare 
  occurrences 
  of 
  hydrous 
  carbonate, 
  if 
  such 
  they 
  really 
  

   be, 
  developed 
  in 
  reactions 
  commonly 
  yielding 
  this 
  extremely 
  

   alterable 
  or 
  evanescent 
  form 
  ; 
  or 
  in 
  tracing 
  epigenesis 
  of 
  com- 
  

   paratively 
  stable 
  anhydrous 
  carbonate, 
  either 
  crystalline 
  or 
  

   amorphous, 
  from 
  like 
  reactions. 
  On 
  the 
  contrary, 
  it 
  seems 
  to 
  

   have 
  been 
  assumed 
  that 
  chemical 
  reactions, 
  geologically 
  con- 
  

   sidered, 
  producing 
  hydrous 
  carbonate, 
  might 
  equally 
  serve, 
  at 
  

   least 
  eventually, 
  to 
  produce 
  anhydrous 
  carbonate. 
  As 
  in 
  many 
  

   other 
  unexplained 
  instances 
  of 
  dehydration, 
  conceivable 
  only 
  at 
  

   ordinary 
  temperatures, 
  this 
  phenomenon 
  has 
  probably 
  been 
  

   supposed 
  to 
  be 
  an 
  effect 
  of 
  inscrutable 
  operations 
  of 
  time 
  

   Bischof, 
  for 
  instance, 
  to 
  whom 
  we 
  owe 
  what 
  still 
  stands 
  as 
  the 
  

   fullest 
  conspectus 
  of 
  this 
  subject, 
  fails 
  to 
  distinguish 
  as 
  such 
  

   the 
  hydrous 
  carbonate, 
  which 
  as 
  yet 
  appears 
  to 
  be 
  exclusively 
  

   the 
  product 
  of 
  well 
  understood 
  reactions. 
  

  

  (15.) 
  Now 
  there 
  seems 
  much 
  reason 
  to 
  doubt 
  that 
  anhydrous 
  

   ferrous 
  carbonate 
  is 
  ever 
  directly 
  deposited 
  from 
  acid 
  solutions 
  

   of 
  ferrous 
  salts 
  except 
  in 
  circumstances 
  of 
  contact 
  with 
  isolated 
  

   or 
  solid 
  anhydrous 
  alkaline 
  mono-carbonates, 
  probably 
  at 
  the 
  

   point 
  of 
  double 
  decomposition, 
  or 
  in 
  the 
  nascent 
  state 
  of 
  the 
  

   ferrous 
  salt. 
  Such 
  a 
  mode 
  of 
  development, 
  if 
  assumed, 
  must 
  

   be 
  considered 
  due 
  to 
  the 
  well 
  known 
  isomorphous 
  relations 
  of 
  

   anhydrous 
  ferrous 
  carbonate 
  and 
  its 
  pseudomorphic 
  tendencies. 
  

   This 
  explanation 
  appears 
  at 
  least 
  consistent 
  with 
  the 
  phenomena 
  

   of 
  replacement, 
  both 
  isomorphous 
  and 
  pseudomorphous, 
  of 
  

   amorphous 
  calcic 
  carbonate 
  ; 
  and 
  may 
  perhaps 
  be 
  found 
  ade- 
  

   quate 
  to 
  explain 
  most 
  occurrences 
  of 
  crystalline 
  siderite 
  on 
  the 
  

   theory 
  of 
  its 
  epigenic 
  origin 
  in 
  all 
  cases. 
  Some 
  of 
  these 
  points 
  

   will 
  now 
  be 
  further 
  considered. 
  

  

  (16.) 
  It 
  is 
  remarkable 
  that 
  although 
  in 
  the 
  earlier 
  volumes 
  

   of 
  his 
  great 
  work, 
  Bischof 
  was 
  the 
  first, 
  I 
  believe, 
  to 
  point 
  out 
  

   the 
  importance 
  of 
  replacement 
  of 
  limestone 
  as 
  one 
  mode 
  of 
  

   genesis 
  of 
  siderite, 
  he 
  assumes 
  in 
  his 
  supplementary 
  volume 
  

   stratiform 
  developments 
  of 
  this 
  epigenic 
  compound, 
  particularly 
  

   in 
  Carboniferous 
  series 
  of 
  strata, 
  to 
  have 
  been 
  directly 
  deposited 
  

   from 
  its 
  carbonated 
  water 
  solution 
  as 
  an 
  effect 
  of 
  volatilization 
  

   of 
  carbonic 
  acid, 
  and 
  to 
  have 
  been 
  preserved 
  from 
  oxidation 
  by 
  

   hydro- 
  carbon 
  gases. 
  Yet 
  the 
  constant 
  association 
  in 
  these 
  

   strata 
  of 
  carbonic 
  acid 
  along 
  with 
  those 
  gases 
  is 
  remarked 
  by 
  

   Bischof 
  in 
  the 
  same 
  place.* 
  Even 
  by 
  loss 
  of 
  half 
  combined 
  

   carbonic 
  acid, 
  however 
  difficult 
  to 
  imagine 
  as 
  taking 
  place 
  in 
  an 
  

   atmosphere 
  impregnated 
  with 
  the 
  same 
  gas, 
  it 
  is 
  extremely 
  

  

  * 
  Chem. 
  und 
  Phys., 
  Geol. 
  Suppl., 
  Band 
  1871, 
  p. 
  64. 
  

  

  