﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  II7 
  

  

  the 
  limestone 
  folds 
  in 
  this 
  district, 
  as 
  well 
  as 
  others 
  in 
  shales 
  in 
  

   western 
  New 
  York 
  and 
  Ohio, 
  demonstrated 
  that 
  they 
  were 
  super- 
  

   ficial 
  and 
  postglacial, 
  and 
  attributed 
  them 
  to 
  *' 
  horizontal 
  expansion 
  

   of 
  superficial 
  strata, 
  consequent 
  on 
  postglacial 
  amelioration 
  of 
  cli- 
  

   mate."^ 
  The 
  writer 
  does 
  not 
  question 
  the 
  correctness 
  of 
  this 
  ex- 
  

   planation 
  as 
  applied 
  to 
  the 
  folds 
  in 
  shales 
  and 
  shaly 
  rocks, 
  which 
  

   Gilbert 
  describes, 
  but 
  is 
  not 
  so 
  sure 
  as 
  to 
  its 
  adequacy 
  in 
  the 
  case 
  of 
  

   quite 
  massive, 
  rigid 
  limestones 
  such 
  as 
  the 
  Lowville, 
  and 
  is 
  especially 
  

   doubtful 
  of 
  it 
  as 
  applied 
  to 
  a 
  well 
  cemented, 
  massive 
  sandstone 
  like 
  

   the 
  Potsdam, 
  which 
  is 
  an 
  exceedingly 
  rigid 
  and 
  resistant 
  rock. 
  Post- 
  

   glacial 
  climate 
  is 
  no 
  warmer 
  than 
  was 
  preglacial 
  climate. 
  Unless 
  

   therefore 
  the 
  weight 
  of 
  the 
  overlying 
  ice 
  was 
  sufficient 
  to 
  cause 
  some 
  

   lateral 
  spreading 
  of 
  the 
  rocks, 
  at 
  the 
  same 
  time 
  that 
  it 
  was 
  producing 
  

   contraction 
  in 
  them 
  by 
  lowering 
  of 
  their 
  temperature, 
  postglacial 
  

   warming 
  would 
  merely 
  reexpand 
  them 
  to 
  their 
  preglacial 
  condition. 
  

   There 
  is 
  no 
  question 
  as 
  to 
  the 
  competency 
  of 
  the 
  ice 
  weight 
  to 
  pro- 
  

   duce 
  lateral 
  spread 
  in 
  shales 
  and 
  shaly 
  rocks. 
  Many 
  shales 
  are 
  

   known 
  to 
  spread 
  and 
  to 
  give 
  rise 
  to 
  buckles 
  under 
  much 
  smaller 
  

   pressures, 
  hence 
  the 
  cause 
  suggested 
  by 
  Gilbert 
  would 
  seem 
  ample 
  

   to 
  account 
  for 
  the 
  results. 
  But 
  the 
  pressure 
  necessary 
  to 
  produce 
  

   spread 
  in 
  a 
  massive, 
  rigid 
  limestone 
  is 
  quite 
  another 
  matter, 
  and 
  that 
  

   required 
  in 
  the 
  case 
  of 
  such 
  a 
  rock 
  as 
  the 
  Potsdam 
  sandstone 
  is 
  of 
  

   a 
  still 
  higher 
  order. 
  The 
  weight 
  of 
  an 
  ice 
  sheet 
  i 
  mile 
  thick 
  would 
  

   be 
  equal 
  to 
  that 
  of 
  from 
  1700 
  to 
  1800 
  feet 
  of 
  average 
  sedimentary 
  

   rock. 
  We 
  do 
  not 
  know 
  the 
  thickness 
  which 
  the 
  ice 
  attained 
  over 
  this 
  

   region 
  but 
  even 
  the 
  supposition 
  that 
  it 
  was 
  much 
  more 
  than 
  a 
  mile 
  

   thick 
  does 
  not 
  greatly 
  enhance 
  our 
  figures 
  of 
  rock 
  thickness. 
  Are 
  

   such 
  pressures, 
  even 
  if 
  applied 
  continuously 
  for 
  a 
  long 
  time, 
  suffi- 
  

   cient 
  to 
  bring 
  about 
  lateral 
  spreading 
  in 
  such 
  a 
  rock 
  as 
  the 
  Potsdam 
  ? 
  

   So 
  far 
  as 
  known 
  to 
  the 
  writer 
  there 
  are 
  no 
  direct, 
  positive 
  data 
  

   which 
  warrant 
  a 
  definite 
  answer 
  to 
  this 
  question. 
  It 
  is 
  certain, 
  how- 
  

   ever, 
  that 
  at 
  such 
  depths 
  below 
  the 
  surface 
  such 
  rocks 
  are 
  abundantly 
  

   fissured, 
  are 
  often 
  porous, 
  and 
  permit 
  free 
  passage 
  of 
  fluids. 
  This 
  

   certainly 
  suggests 
  that 
  they 
  are 
  not 
  under 
  sufficient 
  weight 
  to 
  close 
  

   up 
  cracks. 
  

  

  ' 
  If, 
  However, 
  this 
  pressure 
  due 
  to 
  the 
  ice 
  load 
  could 
  be 
  rein- 
  

   forced 
  by 
  .pressure 
  from 
  some 
  other 
  source 
  in 
  sufficient 
  amount, 
  

   the 
  necessary 
  lateral 
  spreading 
  could 
  be 
  brought 
  about, 
  A 
  

  

  1 
  Gilbert, 
  G. 
  K. 
  Am. 
  Ass'n 
  Ad\^ 
  Sci. 
  Proc. 
  35:227; 
  40:249. 
  

  

  Am. 
  Joiif." 
  Sci. 
  Ser. 
  3, 
  32:324. 
  

  

  The 
  writer 
  is 
  under 
  great 
  obligations 
  to 
  Dr 
  G. 
  K. 
  Gilbert, 
  J. 
  , 
  C. 
  'Branner 
  

   and 
  H. 
  F. 
  Reid 
  for 
  references 
  to 
  the 
  literature 
  and 
  for 
  personal 
  discussion 
  

   of 
  these 
  folds. 
  

  

  