﻿the 
  Pre-Camhrian 
  Era. 
  45 
  

  

  "When 
  now 
  we 
  use 
  his 
  table 
  (Plate 
  1) 
  and 
  draw 
  lines 
  that 
  

   express 
  these 
  lithologic 
  correlations, 
  we 
  express 
  the 
  correlations 
  

   that 
  have 
  been 
  worked 
  out 
  stratigraphically 
  by 
  direct 
  observa- 
  

   tions 
  in 
  the 
  Held, 
  full 
  as 
  well 
  as 
  does 
  Lawson. 
  

  

  Is 
  there 
  an 
  explanation 
  for 
  this 
  lithologic 
  succession 
  which 
  

   will 
  make 
  it 
  more 
  than 
  a 
  mere 
  accident 
  ? 
  It 
  seems 
  to 
  me 
  so. 
  

   In 
  the 
  first 
  place 
  the 
  source 
  of 
  the 
  oxygen 
  of 
  the 
  atmosphere 
  

   of 
  which 
  we 
  are 
  sure 
  is 
  the 
  decomposition 
  of 
  carbon 
  dioxide, 
  

   C0 
  2 
  , 
  by 
  means 
  of 
  organisms. 
  Analyses 
  of 
  pure 
  volcanic 
  emana- 
  

   tions, 
  of 
  gases 
  given 
  off 
  by 
  igneous 
  rocks 
  in 
  vacuo, 
  and 
  of 
  

   meteorites, 
  while 
  they 
  show 
  oxides 
  of 
  carbon 
  show 
  no 
  free 
  

   oxygen, 
  and 
  it 
  is 
  hard 
  to 
  see 
  why 
  it 
  should 
  occur, 
  or, 
  if 
  

   a 
  little 
  did 
  wander 
  in 
  from 
  empty 
  space, 
  how 
  it 
  could 
  

   accumulate 
  in 
  the 
  presence 
  of 
  the 
  oxidizable 
  crust 
  of 
  the 
  earth 
  

   and 
  sunlight 
  and 
  water. 
  Only 
  three 
  hundred 
  feet 
  or 
  so 
  of 
  

   basalt 
  oxidized 
  as 
  the 
  Medford 
  diabase 
  has 
  been 
  would 
  use 
  up 
  

   all 
  the 
  oxygen 
  of 
  our 
  present 
  atmosphere. 
  It 
  is, 
  therefore, 
  

   reasonable 
  to 
  assume 
  that 
  in 
  azoic 
  time 
  there 
  was 
  no 
  atmosphere 
  

   largely 
  of 
  oxygen. 
  Rocks 
  accumulated 
  under 
  such 
  an 
  atmos- 
  

   phere 
  would 
  be 
  green 
  rather 
  than 
  red 
  as 
  indeed 
  the 
  Keewatin 
  

   rocks 
  are. 
  Under 
  such 
  an 
  atmosphere 
  the 
  volcanic 
  emanations 
  

   that 
  went 
  with 
  the 
  volcanic 
  eruptions 
  that 
  are 
  such 
  a 
  dominant 
  

   feature 
  of 
  the 
  Keewatin 
  could 
  accumulate 
  in 
  the 
  ocean 
  chlorides, 
  

   not 
  merely 
  of 
  line 
  and 
  magnesium 
  and 
  sodium, 
  but 
  of 
  ferrous 
  

   iron 
  as 
  well, 
  and 
  though 
  these 
  bases 
  were 
  liable 
  to 
  be 
  pre- 
  

   cipitated 
  as 
  carbonates 
  considerable 
  quantities 
  might 
  remain 
  in 
  

   solution 
  as 
  bicarbonate. 
  

  

  On 
  the 
  whole 
  the 
  Keewatin 
  "greenstone 
  schists" 
  are 
  the 
  

   kinds 
  of 
  rock 
  that 
  we 
  might 
  expect 
  under 
  this 
  hypothesis. 
  

   Will 
  it 
  not 
  be 
  less 
  hard 
  to 
  explain 
  the 
  occasionally 
  reported 
  

   exceptional 
  black 
  slates 
  or 
  iron-bearing 
  formations 
  as 
  fore- 
  

   runners 
  of 
  later 
  formations, 
  or 
  as 
  blocks 
  of 
  later 
  formations 
  

   whose 
  stratigraphic 
  relations 
  have 
  been 
  misunderstood 
  owing 
  

   to 
  early 
  faulting 
  before 
  the 
  rocks 
  were 
  buried, 
  deeply 
  folded, 
  

   and 
  metamorphosed, 
  than 
  for 
  Lawson 
  to 
  explain 
  away 
  his 
  dif- 
  

   ficulties 
  ? 
  Good 
  geologists 
  of 
  wide 
  experience 
  have 
  told 
  me 
  

   flatly 
  that 
  the 
  Keewatin 
  does 
  not 
  contain 
  carbonaceous 
  beds. 
  

   My 
  experience 
  agrees. 
  

  

  When, 
  however, 
  organic 
  life 
  entered 
  it 
  would 
  naturally 
  be 
  

   in 
  lower 
  forms 
  like 
  the 
  diatoms, 
  algae 
  and 
  stone 
  worts. 
  Such 
  

   life 
  could 
  precipitate 
  the 
  bicarbonates 
  as 
  carbonates, 
  even 
  as 
  

   Chas. 
  A. 
  Davis 
  has 
  pointed 
  out 
  that 
  at 
  present 
  the 
  thick 
  de- 
  

   posits 
  of 
  " 
  marl 
  " 
  (a 
  nearly 
  pure 
  limestone 
  dough) 
  are 
  formed 
  

   in 
  the 
  lakes 
  of 
  Michigan 
  and 
  other 
  regions. 
  Thus 
  deposits 
  of 
  

   cherty 
  iron 
  carbonate 
  and 
  ferrodolomite, 
  and 
  the 
  Grenville 
  

   limestone 
  would 
  be 
  formed, 
  while 
  at 
  the 
  same 
  time 
  such 
  

   organisms 
  could 
  be 
  splitting 
  up 
  the 
  C0 
  3 
  of 
  the 
  early 
  atmos- 
  

  

  