﻿12 
  A. 
  C. 
  Lane 
  — 
  Lawsoii's 
  Correlation 
  of 
  

  

  Art. 
  IV. 
  — 
  Lawsoirs 
  Correlation 
  of 
  the 
  Pre- 
  Cambrian 
  Era 
  / 
  

   b} 
  T 
  Alfred 
  C. 
  Lane. 
  With 
  Plate 
  I. 
  

  

  A. 
  C. 
  Lawson 
  has 
  recently 
  issued 
  an 
  addition 
  to 
  his 
  important 
  

   contributions 
  to 
  "The 
  Correlation 
  of 
  the 
  Pre-Cambrian 
  Rocks 
  

   of 
  the 
  Pegion 
  of 
  the 
  Great 
  Lakes" 
  * 
  which, 
  written 
  in 
  his 
  usual 
  

   clear 
  and 
  forcible 
  style, 
  with 
  a 
  table 
  of 
  correlations, 
  herewith 
  

   reproduced 
  modified 
  so 
  as 
  (Plate 
  I) 
  to 
  show 
  my 
  views, 
  is 
  an 
  

   admirable 
  basis 
  for 
  showing 
  just 
  how 
  and 
  why 
  he 
  differs 
  from 
  

   many 
  of 
  the 
  others 
  of 
  us 
  who 
  are 
  familiar 
  with 
  the 
  problem. 
  

  

  He 
  assumes 
  that 
  in 
  a 
  region 
  extending 
  from 
  the 
  Adirondacks 
  

   to 
  the 
  Painy 
  Lakes, 
  there 
  were 
  two 
  and 
  only 
  two 
  Post- 
  

   Keewatin 
  periods 
  of 
  batholitic 
  intrusions 
  of 
  granite. 
  With 
  

   each 
  period 
  there 
  are 
  supposed 
  to 
  go 
  the 
  usual 
  pegmatites, 
  

   aplites 
  and 
  other 
  dikes, 
  and 
  products 
  magmatic 
  differentiation. 
  

   That 
  the 
  Keweenawan 
  red 
  rocks 
  would 
  be 
  a 
  third 
  period 
  if 
  

   they 
  were 
  batholitic, 
  he 
  grants, 
  but 
  assumes 
  that 
  they 
  are 
  

   ordinarily 
  easily 
  distinguishable 
  from 
  the 
  earlier 
  granites. 
  The 
  

   Embarrass 
  granite 
  and 
  the 
  aegirite 
  syenite 
  of 
  the 
  Mesabi 
  range, 
  

   and 
  other 
  cases 
  that 
  seem 
  to 
  contradict 
  his 
  fundamental 
  hypo- 
  

   thesis, 
  he 
  might 
  therefore 
  dispose 
  of 
  by 
  saying 
  that 
  these 
  are 
  

   really 
  exceptional 
  Keweenawan 
  intrnsives. 
  Indeed 
  I 
  do 
  not 
  

   doubt 
  that 
  this 
  is 
  at 
  times 
  a 
  fair 
  explanation. 
  For 
  instance, 
  in 
  

   the 
  original 
  Animikian 
  slate 
  of 
  Thunder 
  Pay, 
  not 
  far 
  from 
  

   Loon 
  Lake, 
  is 
  a 
  coarse 
  tourmaline-bearing 
  granite, 
  which 
  some 
  

   might 
  associate 
  with 
  the 
  Presque 
  Isle 
  granite 
  of 
  Michigan, 
  

   but 
  which 
  I 
  can 
  well 
  believe 
  to 
  be 
  Keweenawan. 
  

  

  Now 
  it 
  may 
  at 
  once 
  be 
  granted 
  that 
  it 
  will 
  be 
  rarely 
  if 
  ever 
  

   that 
  one 
  will 
  find 
  direct 
  proof 
  in 
  one 
  place 
  of 
  three 
  batholitic 
  

   granitic 
  invasions. 
  Such 
  invasions 
  alter 
  and 
  shatter 
  the 
  strata 
  

   too 
  much 
  to 
  leave 
  clear 
  records 
  of 
  the 
  earlier 
  assaults 
  after 
  two 
  

   later 
  have 
  swept 
  over 
  a 
  region. 
  After 
  double 
  granitic 
  invasions 
  

   strata 
  of 
  almost 
  anv 
  age 
  might 
  be 
  taken 
  for 
  Keewatin. 
  What 
  

   with 
  the 
  explanation 
  that 
  occasional 
  granites 
  are 
  Keweenawan, 
  

   and 
  the 
  difficulty 
  of 
  disproving 
  that 
  strata 
  thrice 
  invaded 
  by 
  

   granite 
  are 
  not 
  Keewatin, 
  it 
  will 
  be 
  hard 
  to 
  prove 
  in 
  a 
  coercive 
  

   way 
  that 
  his 
  fundamental 
  hypothesis 
  is 
  wrong. 
  

  

  Nevertheless, 
  every 
  case 
  of 
  granite-cutting 
  strata, 
  supposed 
  

   by 
  Lawson 
  to 
  be 
  later 
  than 
  his 
  later 
  granitic 
  Algoman 
  Revolu- 
  

   tion, 
  weakens 
  his 
  scheme 
  of 
  correlation. 
  

  

  The 
  facts 
  then 
  of 
  an 
  aplitic 
  dike 
  cutting 
  the 
  Virginia 
  slate, 
  

   of 
  granite 
  intrusions, 
  like 
  the 
  Embarrass 
  granite, 
  into 
  the 
  

   Mesabi 
  Range,f 
  Biwabik 
  and 
  later 
  formations, 
  as 
  Wolff 
  and 
  

  

  * 
  University 
  of 
  California 
  Publications, 
  Bulletin 
  of 
  the 
  Department 
  of 
  

   Geology, 
  vol. 
  x, 
  No. 
  1, 
  pp. 
  1-19, 
  April 
  27, 
  1916. 
  

  

  fSee 
  the 
  October 
  Bulletin 
  of 
  the 
  American 
  Institute 
  of 
  Mining 
  Engineers, 
  

   p. 
  1766. 
  

  

  