﻿240 
  J. 
  P. 
  Kimball 
  — 
  Genesis 
  of 
  Iron-ores 
  by 
  Isomorphous 
  

  

  with 
  horizons 
  of 
  limestone 
  by 
  stratigraphic 
  relations. 
  Imprac- 
  

   ticable 
  though 
  this 
  may 
  be 
  in 
  certain 
  cases, 
  it 
  should 
  not 
  fail 
  

   to 
  be 
  considered 
  that 
  as 
  the 
  thinner 
  and 
  less 
  persistent 
  lime- 
  

   stones 
  are 
  the 
  only 
  ones 
  liable 
  to 
  complete 
  replacement, 
  actual 
  

   stratigraphic 
  or 
  even 
  inferential 
  identification 
  is 
  not 
  in 
  all 
  cases 
  

   to 
  be 
  expected. 
  

  

  (23.) 
  Calling 
  attention 
  to 
  the 
  possible 
  application 
  of 
  the 
  

   theory 
  of 
  the 
  formation 
  of 
  ore-deposits 
  by 
  replacement 
  or 
  

   substitution, 
  Emmons 
  expresses 
  the 
  possibility 
  that 
  "in 
  the 
  

   older 
  and 
  more 
  crystalline 
  rocks, 
  where 
  the 
  calcareous 
  beds 
  are 
  

   of 
  limited 
  extent, 
  metallic 
  deposits 
  in 
  large 
  masses 
  like 
  those 
  

   of 
  iron, 
  may 
  have 
  so 
  completely 
  replaced 
  the 
  calcareous 
  mate- 
  

   rial 
  that 
  little 
  or 
  no 
  trace 
  of 
  it 
  remains."* 
  Complete 
  ultimate 
  

   replacement 
  of 
  isolated 
  masses 
  of 
  emerged 
  coral-reef 
  by 
  ferric 
  

   oxide 
  on 
  the 
  island 
  of 
  Cuba 
  was 
  described 
  by 
  me 
  in 
  1884. 
  To 
  

   this 
  example 
  I 
  shall 
  again 
  take 
  occasion 
  to 
  refer. 
  

  

  " 
  The 
  limits 
  of 
  the 
  actually 
  demonstrated 
  application 
  of 
  the 
  

   theory 
  of 
  the 
  formation 
  of 
  ore 
  deposits," 
  as 
  remarked 
  by 
  Em- 
  

   mons 
  in 
  the 
  paper 
  just 
  quoted, 
  " 
  are 
  being 
  every 
  day 
  extended, 
  

   not 
  only 
  by 
  studies 
  of 
  new 
  districts, 
  but 
  by 
  more 
  careful 
  and 
  

   unbiased 
  studies 
  of 
  old 
  districts 
  in 
  which 
  a 
  different 
  method 
  of 
  

   formation 
  had 
  previously 
  been 
  determined 
  upon." 
  

  

  (24.) 
  Argillaceous 
  shales 
  and 
  other 
  miscellaneous 
  ferriferous 
  

   sediments 
  commingled 
  with 
  carbonate 
  of 
  lime, 
  originally 
  accu- 
  

   mulated, 
  or 
  resulting 
  from 
  decomposition 
  of 
  component 
  basic 
  

   silicates 
  or 
  left 
  behind 
  from 
  evaporation 
  of 
  circulating 
  waters, 
  

   may 
  in 
  whole 
  or 
  in 
  part 
  be 
  transformed 
  into 
  clay 
  iron-stone 
  or 
  

   siderite, 
  containing 
  insoluble 
  residues 
  of 
  the 
  original 
  beds. 
  

   This 
  process 
  is 
  again 
  one 
  of 
  replacement. 
  Divisional 
  parts 
  or 
  

   prisms 
  of 
  such 
  beds 
  separated 
  by 
  planes 
  of 
  cleavage 
  and 
  

   stratification, 
  and 
  by 
  anfractuosities 
  from 
  shrinkage, 
  pass 
  by 
  

   progressive 
  superficial 
  oxidation 
  into 
  concretionary 
  or 
  nodular 
  

   limonite. 
  This 
  process 
  has 
  often 
  been 
  described. 
  f 
  

  

  (25.) 
  Diffused 
  ferrous 
  carbonate 
  resulting 
  from 
  replacement 
  

   of 
  calcic 
  carbonate, 
  also 
  diffused 
  and 
  more 
  or 
  less 
  commingled 
  

   with 
  clay 
  containing 
  other 
  insoluble 
  residues 
  of 
  sub-aerial 
  

   decay 
  of 
  basic 
  rocks, 
  may, 
  especially 
  in 
  sediments 
  as 
  yet 
  unin- 
  

   durated, 
  be 
  involved 
  in 
  what 
  may 
  be 
  termed 
  the 
  extra-molecu- 
  

   lar 
  tendency 
  of 
  fine 
  clays 
  to 
  form 
  concretionary 
  aggregations. 
  

   Thus 
  it 
  appears 
  that 
  impure 
  ferrous 
  carbonate 
  in 
  nodular 
  form, 
  

   so 
  frequently 
  imbedded 
  in 
  clays, 
  shales 
  and 
  grits, 
  is 
  probably 
  a 
  

   product 
  of 
  secular 
  metasomatic 
  interchange 
  and 
  substitution 
  

   under 
  genetic 
  conditions 
  varying 
  only 
  slightly 
  with 
  circum- 
  

   stances 
  of 
  environment 
  from 
  conditions 
  governing 
  replacement 
  

   of 
  limestone 
  beds 
  by 
  siderite. 
  

  

  * 
  Trans. 
  Am. 
  Inst. 
  Min. 
  Eng. 
  1886. 
  Extract 
  p. 
  7. 
  

   f 
  See 
  Hunt, 
  this 
  Jour., 
  xxvi, 
  1883, 
  pp. 
  202, 
  206. 
  

  

  