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  Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  

  

  It 
  hardly 
  needs 
  to 
  be 
  stated 
  that 
  the 
  fossils 
  in 
  the 
  basal 
  

   chert 
  layer 
  of 
  the 
  Wreford 
  are 
  replacements, 
  and 
  the 
  

   theory 
  that 
  this 
  replacement 
  preceded, 
  or 
  was 
  contempo- 
  

   raneous 
  with, 
  the 
  solidification, 
  encounters 
  none 
  of 
  the 
  

   objections 
  urged 
  against 
  the 
  others. 
  This 
  theory 
  also 
  

   quite 
  readily 
  explains 
  the 
  labyrinthine 
  intergrowth 
  of 
  

   limestone 
  and 
  chert 
  in 
  the 
  second 
  chert 
  zone 
  of 
  the 
  "Wre- 
  

   ford, 
  as 
  well 
  as 
  the 
  fact 
  that 
  the 
  chert 
  contains 
  an 
  abun- 
  

   dance 
  of 
  spicules, 
  though 
  there 
  are 
  few 
  in 
  the 
  limestone. 
  

   That 
  this 
  chert 
  is 
  not 
  concentrically 
  banded 
  may 
  be 
  

   referred 
  to 
  a 
  continuity 
  of 
  uniform 
  conditions. 
  The 
  

   obscure 
  horizontal 
  banding 
  is 
  an 
  inheritance 
  of 
  banding 
  

   in 
  the 
  unconsolidated 
  lime 
  sediments. 
  

  

  The 
  facts 
  relating 
  to 
  the 
  third 
  chert 
  of 
  the 
  Wreford 
  

   also 
  harmonize 
  with 
  the 
  theory. 
  All 
  the 
  characteristics 
  

   are 
  readily 
  explained, 
  and 
  the 
  globular 
  and 
  irregular 
  

   nodules 
  offer 
  no 
  difficulty; 
  neither 
  do 
  those 
  whose 
  long 
  

   axes 
  are 
  perpendicular 
  or 
  transverse 
  to 
  the 
  bedding. 
  

   The 
  abundant 
  calcite 
  in 
  this 
  chert 
  is 
  thus 
  easily 
  accounted 
  

   for. 
  

  

  It 
  may 
  well 
  be, 
  however, 
  that 
  original 
  precipitation 
  and 
  

   also 
  other 
  processes 
  played 
  some 
  part 
  in 
  the 
  development 
  

   of 
  these 
  nodules. 
  Nature 
  forms 
  the 
  same 
  things 
  in 
  dif- 
  

   ferent 
  ways 
  and 
  rare 
  indeed 
  is 
  the 
  occurrence 
  of 
  a 
  geolog- 
  

   ical 
  result 
  which 
  was 
  produced 
  through 
  the 
  operation 
  of 
  

   a 
  single 
  factor. 
  History, 
  beginning 
  with 
  the 
  founders 
  

   of 
  geology, 
  notably 
  Werner, 
  has 
  recorded 
  the 
  errors 
  of 
  

   those 
  who 
  have 
  thus 
  assumed. 
  

  

  One 
  other 
  theory 
  deserves 
  consideration, 
  and 
  this 
  con- 
  

   cerns 
  the 
  method 
  of 
  origin 
  of 
  the 
  concentric 
  structure 
  of 
  

   the 
  nodules 
  of 
  the 
  third 
  chert 
  zone 
  of 
  the 
  Wreford 
  lime- 
  

   stone. 
  While 
  studying 
  these 
  nodules 
  in 
  the 
  field 
  and 
  in 
  

   the 
  laboratory, 
  the 
  suggestion 
  recurred 
  again 
  and 
  again 
  : 
  

   Might 
  not 
  these 
  structures 
  be 
  of 
  organic 
  origin? 
  They 
  

   are 
  not 
  markedly 
  different 
  in 
  general 
  outward 
  appear- 
  

   ance, 
  irregularities 
  of 
  the 
  surface, 
  and 
  microscopic 
  inter- 
  

   nal 
  structure 
  from 
  Cryptozoa. 
  Moreover, 
  algae 
  are 
  

   known 
  to 
  have 
  existed 
  in 
  large 
  numbers 
  in 
  some 
  parts 
  

   of 
  the 
  world 
  during 
  the 
  Carboniferous. 
  14 
  Search 
  for 
  

   microscopic 
  structures 
  of 
  a 
  type 
  approximating 
  those 
  

   possessed 
  by 
  algae 
  has, 
  however, 
  been 
  absolutely 
  barren 
  

   of 
  results. 
  If 
  ever 
  present, 
  they 
  have 
  been 
  totally 
  

  

  14 
  Garwood, 
  E. 
  J., 
  Geol. 
  Mag., 
  Dec. 
  V, 
  10, 
  497-498, 
  1913; 
  Dec. 
  VI, 
  1, 
  

   265-271, 
  1914. 
  

  

  