﻿436 
  Lane 
  — 
  Geothermal 
  Gradient 
  in 
  Michigan. 
  

  

  4. 
  Appeal 
  lias 
  been 
  made 
  to 
  the 
  cooling 
  effect 
  of 
  Lake 
  Supe- 
  

   rior 
  on 
  the 
  flanks 
  of 
  the 
  Copper 
  Range. 
  

  

  But 
  draw 
  a 
  section 
  across 
  the 
  point 
  to 
  true 
  scale. 
  Then 
  

   remember 
  that 
  the 
  bottom 
  of 
  Lake 
  Superior 
  is 
  barely 
  two 
  

   degrees 
  cooler 
  than 
  the 
  mean 
  temperature 
  of 
  Calumet. 
  Con- 
  

   sider 
  that 
  B. 
  O. 
  Peirce* 
  has 
  shown 
  that 
  in 
  a 
  disk 
  six 
  to 
  ten 
  

   times 
  as 
  broad 
  as 
  high, 
  any 
  change 
  of 
  temperature 
  on 
  the 
  sides, 
  

   so 
  long 
  as 
  they 
  are 
  between 
  the 
  constant 
  temperatures 
  of 
  the 
  

   top 
  and 
  bottom, 
  will 
  hardly 
  affect 
  the 
  axial 
  temperatures. 
  We 
  

   shall 
  be 
  convinced 
  that 
  this 
  is 
  not 
  an 
  efficient 
  explanation. 
  

  

  5. 
  It 
  has 
  been 
  assumed 
  that 
  the 
  very 
  low 
  geothermal 
  gradi- 
  

   ent 
  was 
  the 
  normal 
  one 
  and 
  the 
  higher 
  due 
  to 
  local 
  chemical 
  

   action, 
  oxidation 
  of 
  pyrite, 
  etc. 
  But 
  we 
  should 
  expect 
  in 
  such 
  

   case 
  sharper 
  local 
  irregularities 
  from 
  bed 
  to 
  bed 
  than 
  we 
  have, 
  

   and 
  if 
  oxidation 
  of 
  pyrite, 
  or 
  gas, 
  or 
  oil, 
  were 
  the 
  causes 
  of 
  high 
  

   gradient, 
  there 
  is 
  no 
  little 
  of 
  both 
  in 
  the 
  Lower 
  Peninsula 
  of 
  

   Michigan. 
  

  

  6. 
  The 
  effect 
  of 
  deposition 
  as 
  such 
  would 
  be 
  to 
  depress 
  a 
  

   given 
  temperature 
  below 
  the 
  surface 
  and 
  lower 
  the 
  gradient, 
  

   and 
  that 
  of 
  erosion 
  to 
  raise 
  it. 
  But 
  the 
  lower 
  gradients 
  are 
  in 
  

   regions 
  of 
  erosion 
  rather 
  than 
  of 
  recent 
  or 
  great 
  deposition. 
  

  

  7. 
  I 
  have 
  seriously 
  considered 
  the 
  possibility 
  that 
  the 
  refrig- 
  

   eration 
  of 
  the 
  ice 
  age 
  was 
  still 
  perceptible. 
  If 
  for 
  instance 
  

   the 
  mean 
  annual 
  temperature 
  during 
  the 
  ice 
  age 
  was 
  18° 
  F. 
  at 
  

   Calumet 
  for 
  a 
  very 
  long 
  time, 
  so 
  that 
  the 
  gradient 
  was 
  adjusted 
  

   to 
  that 
  temperature 
  even 
  down 
  to 
  a 
  mile 
  or 
  more, 
  then 
  for 
  a 
  

   gradient 
  of 
  3° 
  in 
  200 
  feet 
  the 
  temperature 
  would 
  become 
  84°. 
  

   Then 
  if 
  at 
  the 
  end 
  of 
  the 
  ice 
  age, 
  so 
  recently 
  that 
  the 
  surface 
  

   change 
  of 
  temperature 
  was 
  still 
  unfelt 
  a 
  mile 
  below, 
  the 
  mean 
  

   annual 
  temperature 
  rose 
  to 
  40° 
  F., 
  the 
  gradient 
  would 
  be 
  only 
  

   what 
  it 
  is 
  now, 
  — 
  1° 
  F. 
  in 
  100 
  feet. 
  It 
  would 
  be 
  very 
  interest- 
  

   ing 
  if 
  we 
  could 
  find 
  the 
  cause 
  of 
  the 
  abnormal 
  gradient 
  here, 
  

   for 
  we 
  could 
  then 
  compute 
  the 
  time 
  since 
  the 
  ice 
  age. 
  Unfor- 
  

   tunately 
  lines 
  of 
  equal 
  geothermal 
  gradient 
  are 
  not, 
  as 
  this 
  

   theory 
  would 
  require, 
  parallel 
  to 
  the 
  successive 
  lines 
  of 
  the 
  

   retreating 
  ice 
  front 
  (?). 
  

  

  8. 
  Prof. 
  B. 
  O. 
  Peirce 
  suggests 
  that 
  an 
  inclined 
  band 
  of 
  very 
  

   conductive 
  rock 
  might 
  lead 
  the 
  heat 
  off 
  along 
  it 
  sideways 
  and 
  

   thus 
  make 
  very 
  low 
  gradients 
  along 
  it. 
  This 
  was 
  in 
  informal 
  

   talk 
  with 
  me. 
  For 
  instance, 
  the 
  iron-bearing 
  series 
  might 
  be 
  

   supposed 
  to 
  underlie 
  the 
  copper-bearing 
  series, 
  and 
  lead 
  off 
  the 
  

   heat 
  under 
  the 
  bottom 
  of 
  the 
  lake, 
  where 
  it 
  might 
  be 
  dis- 
  

   sipated 
  by 
  convection. 
  Unfortunately 
  like 
  so 
  many 
  previous 
  

   suggestions, 
  the 
  facts 
  we 
  have 
  now 
  to 
  study 
  do 
  not 
  support 
  

   this 
  suggestion. 
  A 
  cross 
  section 
  to 
  true 
  scale 
  must 
  be 
  used. 
  

  

  *Proc. 
  Am. 
  Acad, 
  of 
  Art 
  and 
  Sciences, 
  Aug. 
  1898, 
  p. 
  1. 
  

  

  