﻿Lane 
  — 
  Geothermal 
  Gradient 
  in 
  Michigan. 
  437 
  

  

  9. 
  Differences 
  in 
  conductivity 
  may 
  be 
  appealed 
  to 
  to 
  account 
  

   for 
  differences 
  in 
  gradient. 
  We 
  shall 
  have 
  a 
  low 
  gradient 
  in 
  a 
  

   good 
  conductor 
  above 
  a 
  poor 
  conductor, 
  and 
  a 
  high 
  gradient 
  

   in 
  a 
  poor 
  conductor 
  above 
  a 
  good 
  conductor. 
  Water 
  is 
  a 
  

   poorer 
  conductor 
  than 
  most 
  rocks. 
  Air 
  is 
  vastly 
  poorer. 
  Hence 
  

   the 
  less 
  porous 
  the 
  rock 
  the 
  better 
  the 
  conductivity, 
  other 
  

   things 
  being 
  equal. 
  

  

  I 
  trust 
  we 
  may 
  soon 
  have 
  data 
  as 
  to 
  conductivity 
  which 
  will 
  

   enable 
  us 
  to 
  test 
  the 
  possibilities 
  along 
  this 
  line 
  better 
  than 
  we 
  

   can 
  at 
  present. 
  It 
  seems 
  likely 
  that 
  the 
  younger 
  rocks 
  are 
  in 
  

   general 
  less 
  condensed, 
  more 
  porous, 
  and 
  on 
  the 
  whole 
  poorer 
  

   conductors 
  than 
  the 
  older 
  rocks, 
  especially 
  the 
  crystallines. 
  

   We 
  might 
  therefore 
  expect 
  that 
  Keweenaw 
  Point 
  and 
  the 
  

   Archean 
  area 
  of 
  the 
  Upper 
  Peninsula, 
  where 
  heat 
  had 
  been 
  

   allowed 
  to 
  escape 
  more 
  freely 
  from 
  a 
  very 
  early 
  date 
  through 
  

   more 
  conductive 
  rocks, 
  might 
  show 
  a 
  lower 
  gradient 
  than 
  an 
  

   area 
  in 
  which 
  the 
  rocks 
  were 
  not 
  so 
  conductive. 
  

  

  Inasmuch 
  as 
  the 
  gradients 
  are 
  inversely 
  proportional 
  to 
  the 
  

   conductivity, 
  and 
  Peirce's 
  experiments 
  show 
  that 
  whereas 
  

   marbles 
  vary 
  in 
  conductivity 
  from 
  0*005 
  to 
  0*007, 
  glass 
  has 
  a 
  

   conductivity 
  of 
  0*002 
  to 
  0*003, 
  and 
  the 
  difference 
  in 
  conduc- 
  

   tivity 
  amounts 
  to 
  about 
  a 
  fifth 
  between 
  wet 
  and 
  dry 
  rock, 
  we 
  

   have 
  here 
  not 
  only 
  a 
  true 
  cause 
  of 
  different 
  gradients, 
  but 
  one 
  

   that 
  may 
  well 
  be 
  effective, 
  well 
  worthy 
  of 
  farther 
  experimental 
  

   study. 
  

  

  The 
  next 
  two 
  suggestions 
  may 
  be 
  considered 
  as 
  variations 
  

   on 
  the 
  same 
  theme, 
  in 
  which, 
  however, 
  the 
  "flow 
  of 
  heat 
  is 
  not 
  

   molecular, 
  but 
  in 
  a 
  larger 
  way, 
  the 
  net 
  effect 
  being 
  the 
  same. 
  

  

  10. 
  Circulating 
  waters 
  are 
  known 
  to 
  be 
  a 
  factor 
  in 
  affecting 
  

   heat 
  gradients, 
  as 
  the 
  Comstock 
  Lode. 
  In 
  particular 
  at 
  the 
  

   center 
  of 
  an 
  artesian 
  basin 
  like 
  that 
  of 
  Lower 
  Michigan, 
  the 
  

   tendency 
  will 
  be 
  for 
  water 
  to 
  work 
  up 
  by 
  joint 
  planes, 
  or 
  any 
  

   other 
  crevices. 
  If 
  it 
  emerges 
  hot 
  it 
  may 
  naturally 
  produce 
  a 
  

   steep 
  gradient 
  above 
  it, 
  if 
  it 
  assumes 
  an 
  oblique 
  course 
  ; 
  but 
  on 
  

   the 
  other 
  hand, 
  so 
  far 
  as 
  such 
  waters 
  act 
  in 
  exhausting 
  the 
  

   stores 
  of 
  heat 
  from 
  below, 
  they 
  would 
  serve 
  to 
  increase 
  the 
  

   conductivity 
  of 
  the 
  strata 
  as 
  a 
  whole, 
  so 
  that 
  any 
  local 
  increase 
  

   in 
  gradient 
  must 
  be 
  more 
  than 
  compensated 
  for 
  by 
  a 
  lower 
  

   gradient 
  elsewhere, 
  and 
  this 
  would 
  be 
  particularly 
  on 
  the 
  ele- 
  

   vated 
  margin 
  of 
  the 
  artesian 
  basin 
  where 
  the 
  water 
  was 
  taken, 
  

   and 
  down 
  which 
  it 
  worked. 
  

  

  The 
  effect 
  of 
  the 
  agency 
  just 
  described 
  is, 
  however, 
  limited 
  

   by 
  the 
  rapidity 
  of 
  the 
  circulation, 
  and 
  while 
  the 
  structure 
  of 
  

   Keweenaw 
  Point 
  and 
  some 
  things 
  about 
  the 
  alteration 
  of 
  the 
  

   rocks 
  favor 
  the 
  idea 
  that 
  downward 
  circulation 
  of 
  water 
  is 
  the 
  

   cause 
  of 
  the 
  unusual 
  coolness, 
  on 
  the 
  other 
  hand 
  there 
  is 
  very 
  

  

  