﻿96 
  0. 
  Holtedahl 
  — 
  Paleozoic 
  Formations 
  of 
  

  

  structures, 
  occurring 
  in 
  limestones 
  and 
  dolomites. 
  In 
  

   fact, 
  some 
  of 
  the 
  stromatolites 
  from 
  the 
  Triassic 
  figured 
  

   by 
  Kalkowsky 
  are 
  extraordinarily 
  similar 
  to 
  the 
  Gymno- 
  

   solen 
  structure 
  and 
  thus 
  probably 
  also 
  to 
  Cryptozoon 
  

   minnesotense. 
  Walcott 
  's 
  Collenia 
  from 
  the 
  Algonkian 
  

   should 
  likewise 
  be 
  considered 
  as 
  a 
  stromatolitic 
  structure. 
  

  

  As 
  to 
  the 
  nature 
  of 
  these 
  structures, 
  I 
  can 
  not 
  agree 
  

   with 
  Steinmann, 
  who 
  regards 
  Gymnosolen 
  as 
  a 
  coelenter- 
  

   ate, 
  probably 
  some 
  sort 
  of 
  a 
  coral 
  ; 
  or 
  with 
  Grabau 
  and 
  

   Shinier, 
  who 
  in 
  their 
  " 
  North 
  American 
  Index 
  Fossils," 
  

   1, 
  page 
  46, 
  place 
  Cryptozoon 
  among 
  the 
  Stromatopo- 
  

   roidea; 
  or 
  with 
  Wieland, 
  who 
  thinks 
  Cryptozoon 
  bassleri 
  

   is 
  nearly 
  related 
  to 
  Lithothamnium. 
  9 
  The 
  laminated 
  

   structure 
  without 
  any 
  indication 
  of 
  a 
  radial 
  one, 
  and 
  with 
  

   inclusions 
  of 
  oolite 
  and 
  clastic 
  mineral 
  particles, 
  can, 
  in 
  

   the 
  writer's 
  opinion, 
  only 
  mean 
  that 
  it 
  is 
  in 
  the 
  main 
  to 
  

   be 
  regarded 
  as 
  a 
  chemical 
  precipitation, 
  one, 
  however, 
  

   that 
  probably 
  came 
  into 
  existence 
  through 
  the 
  organic 
  

   processes 
  of 
  living 
  organisms. 
  I 
  consider 
  this 
  type 
  of 
  

   rock 
  to 
  belong 
  to 
  what 
  Andree 
  in 
  "Ueber 
  Sediment- 
  

   bildung 
  am 
  Meeresboden" 
  10 
  calls 
  "physiologischer 
  Fal- 
  

   lungskalk." 
  It 
  is 
  natural 
  to 
  think 
  of 
  primitive 
  plants, 
  

   marine 
  algae, 
  as 
  the 
  most 
  active 
  organisms 
  in 
  these 
  

   respects, 
  and 
  we 
  have 
  here 
  a 
  strong 
  hint 
  in 
  the 
  known 
  

   calcium 
  carbonate-precipitating 
  activity 
  of 
  fresh-water 
  

   algae, 
  most 
  conspicuously 
  seen 
  in 
  hot 
  water 
  springs, 
  where 
  

   the 
  calcareous 
  deposits 
  may 
  be 
  very 
  like 
  the 
  stromato- 
  

   lites. 
  This 
  indirect 
  activity 
  of 
  algae 
  has 
  recently 
  been 
  

   emphasized 
  by 
  Walcott, 
  11 
  who 
  reports 
  the 
  inclusions 
  of 
  

   cells 
  of 
  blue-green 
  algae 
  in 
  the 
  Algonkian 
  Gamasia, 
  a 
  

   form, 
  however, 
  very 
  different 
  from 
  the 
  Cryptozo 
  on-like 
  

   Collenia. 
  We 
  may 
  also 
  be 
  reminded 
  of 
  Walcott's 
  

   announcement 
  of 
  having 
  found 
  bacteria 
  in 
  Algonkian 
  

   limestone, 
  12 
  and 
  the 
  well 
  known 
  statements 
  of 
  Drew 
  that 
  

   fine 
  calcareous 
  muds 
  are 
  precipitated 
  by 
  denitrifying 
  

   bacteria 
  in 
  the 
  Gulf 
  of 
  Mexico. 
  

  

  Even 
  if 
  the 
  stromatolites 
  most 
  strongly 
  resemble 
  cal- 
  

   careous 
  sediments 
  that 
  have 
  been 
  precipitated 
  by 
  plants 
  

   in 
  fresh-water 
  lakes 
  and 
  hot 
  springs, 
  it 
  seems 
  difficult 
  to 
  

   assume 
  with 
  Walcott 
  that 
  all 
  the 
  limestones 
  and 
  dolo- 
  

  

  Bull. 
  Amer. 
  Mus. 
  Nat. 
  Hist., 
  vol. 
  33, 
  1914 
  

  

  IO 
  Geol. 
  Kundschau, 
  7, 
  p. 
  282, 
  1916. 
  

  

  u 
  Algonkian 
  algal 
  flora, 
  Smithson. 
  Misc. 
  Coll., 
  64, 
  1914. 
  

  

  12 
  Proc. 
  Nat. 
  Acad. 
  Sci., 
  1, 
  1915. 
  

  

  