﻿438 
  Lane 
  — 
  Geothermal 
  Gradient 
  in 
  Michigan. 
  

  

  little 
  water 
  in 
  the 
  deep 
  mines. 
  Darton's 
  researches 
  may 
  throw 
  

   light 
  on 
  this. 
  

  

  11. 
  The 
  transmission 
  of 
  heat 
  by 
  igneous 
  rocks 
  is 
  different 
  

   from 
  that 
  by 
  water 
  in 
  that 
  they 
  do 
  not 
  first 
  work 
  down, 
  being 
  

   slowly 
  heated 
  up. 
  Igneous 
  intrusions 
  will 
  produce, 
  or 
  tend 
  to 
  

   produce, 
  steeper 
  gradients 
  in 
  rocks 
  overlying 
  them, 
  and 
  igne- 
  

   ous 
  outbursts 
  or 
  intrusions 
  will 
  produce 
  lower 
  gradients 
  in 
  the 
  

   rocks 
  through 
  or 
  over 
  which 
  they 
  work. 
  When, 
  however, 
  the 
  

   igneous 
  rock 
  has 
  cooled 
  off 
  so 
  as 
  not 
  to 
  be 
  above 
  its 
  environ- 
  

   ment 
  perceptibly, 
  then 
  the 
  whole 
  series 
  will 
  have 
  temperatures 
  

   lower 
  than 
  normal, 
  owing 
  to 
  the 
  freer 
  and 
  more 
  rapid 
  loss 
  of 
  

   heat 
  thereby. 
  

  

  This 
  seems 
  to 
  me 
  worth 
  considering. 
  Both 
  Keweenaw 
  

   Point 
  and 
  the 
  Vulcan 
  Mine 
  are 
  in 
  regions 
  in 
  which 
  there 
  was 
  

   intense 
  volcanic 
  activity 
  in 
  the 
  dawn 
  of 
  the 
  earth's 
  history, 
  so 
  

   long 
  ago 
  that 
  the 
  local 
  volcanic 
  heat 
  may 
  be 
  supposed 
  to 
  be 
  

   entirely 
  dissipated. 
  But 
  much 
  heat 
  must 
  have 
  been 
  thereby 
  

   taken 
  from 
  beneath. 
  Hence 
  the 
  rate 
  of 
  increase 
  should 
  be 
  

   low, 
  as 
  it 
  is. 
  The 
  Lower 
  Peninsula 
  is 
  not 
  known 
  to 
  have 
  any 
  

   trace 
  of 
  volcanic 
  activity. 
  Hence 
  its 
  thermal 
  gradient 
  should 
  

   be 
  more 
  nearly 
  normal, 
  as 
  it 
  is. 
  For 
  instance, 
  it 
  is 
  nearly 
  in 
  

   accord 
  with 
  Hallock's 
  West 
  Virginia* 
  observations. 
  Any 
  

   gneiss 
  of 
  the 
  original 
  crust 
  should 
  have 
  a 
  higher 
  gradient, 
  

   according 
  to 
  this 
  theory, 
  than 
  any 
  old 
  cooled 
  off 
  granite 
  massif. 
  

  

  In 
  conclusion, 
  it 
  seems 
  likely 
  that 
  among 
  the 
  three 
  causes 
  

   (§§9 
  to 
  11) 
  which 
  seek 
  the 
  cause 
  of 
  low 
  gradients 
  in 
  a 
  more 
  

   rapid 
  conduction 
  or 
  convection 
  away 
  of 
  the 
  heat 
  of 
  the 
  earth 
  

   from 
  the 
  interior, 
  the 
  efficient 
  causes 
  of 
  varying 
  geothermal 
  

   gradients 
  will 
  be 
  found. 
  

  

  * 
  School 
  of 
  Mines 
  Quarterly, 
  xviii, 
  Jan 
  , 
  1897, 
  p. 
  148. 
  

   Geol. 
  Survey 
  of 
  Michigan, 
  Lansing. 
  

  

  