﻿584 
  Prof. 
  J. 
  Joly 
  on 
  the 
  

  

  regarded 
  as 
  primarily 
  traceable 
  to 
  the 
  effects 
  of 
  radioactive 
  

   energy. 
  

  

  -As 
  defining 
  a 
  general 
  connexion 
  between 
  radioactivity 
  

   and 
  vulcanicity, 
  this 
  view 
  appears 
  to 
  be 
  rather 
  strengthened 
  

   than 
  otherwise 
  by 
  recent 
  advances 
  in 
  our 
  knowledge 
  of 
  the 
  

   tidal 
  stability 
  of 
  the 
  earth. 
  For 
  if, 
  as 
  Professor 
  Love 
  con- 
  

   cludes 
  (Nature, 
  April 
  29, 
  1909), 
  we 
  may 
  not 
  postulate 
  the 
  

   general 
  extension 
  of 
  a 
  fluid 
  substratum, 
  we 
  are 
  evidently 
  

   driven 
  to 
  connect 
  vulcanicity 
  in 
  the 
  geosynclines 
  with 
  some 
  

   local 
  source 
  of 
  heat. 
  It 
  may 
  here 
  be 
  observed 
  that 
  there 
  

   does 
  not 
  seem 
  to 
  be 
  difficulty 
  in 
  reconciling 
  Prof. 
  Lovers 
  

   conditions 
  with 
  the 
  facts 
  attending 
  mountain 
  elevation. 
  

   Under 
  the 
  new 
  restrictions 
  we 
  are 
  compelled 
  to 
  assume 
  

   that 
  temperatures 
  deep 
  down 
  cannot 
  generally 
  amount 
  to 
  

   those 
  of 
  liquefaction. 
  However, 
  "crust-creep^' 
  on 
  a 
  large 
  

   scale 
  must 
  at 
  the 
  same 
  time 
  be 
  possible, 
  or 
  the 
  forces 
  

   acting 
  in 
  the 
  geosynclines 
  become 
  unaccountable. 
  In 
  the 
  

   wide 
  range 
  of 
  viscosity 
  of 
  siliceous 
  substances, 
  both 
  condi- 
  

   tions 
  seem 
  attainable. 
  While 
  generally 
  there 
  cannot 
  prevail 
  

   a 
  temperature 
  of 
  fusion, 
  conditions 
  of 
  rigidity 
  toward 
  tidal 
  

   stress 
  do 
  not 
  seem 
  inconsistent 
  with 
  sufficient 
  viscosity 
  to- 
  

   permit 
  yielding 
  under 
  the 
  more 
  prolonged, 
  unidirectional, 
  

   stress 
  of 
  secular 
  cooline;. 
  And 
  not 
  without 
  bearinor 
  on 
  the 
  

   connexion 
  between 
  radioactivity 
  and 
  vulcanicity 
  in 
  its 
  wider 
  

   aspect 
  is 
  the 
  fact 
  that 
  the 
  case 
  for 
  the 
  shallow, 
  superficial^ 
  

   character 
  of 
  the 
  radioactive 
  surface-materials 
  of 
  the 
  earth 
  

   seems 
  strengthened 
  by 
  the 
  limitation 
  of 
  temperature-rise 
  

   downward. 
  It 
  appears 
  that 
  the 
  gradients 
  must 
  ultimately 
  be 
  

   limited 
  to 
  such 
  a 
  gradual 
  slope 
  as 
  will 
  not 
  overtake 
  the 
  rise 
  

   under 
  pressure 
  of 
  the 
  melting-points 
  of 
  the 
  rock-forming 
  

   silicates 
  *. 
  Such 
  a 
  condition 
  is 
  absolutely 
  inconsistent 
  with 
  

   the 
  downward 
  extension 
  of 
  the 
  radium-richness 
  of 
  the 
  surf 
  ace- 
  

   I'ocks. 
  

  

  These 
  remarks 
  apply 
  generally 
  to 
  a 
  probable 
  connexion 
  

   between 
  radioactivity 
  and 
  volcanic 
  outbreak 
  ; 
  they 
  do 
  not 
  

   generally 
  deal 
  with 
  the 
  particular 
  case 
  to 
  which 
  I 
  have 
  called 
  

   attention 
  : 
  — 
  the 
  case 
  of 
  a 
  volcano 
  emitting 
  abnormally 
  radio- 
  

   active 
  lavas. 
  

  

  We 
  might 
  suppose 
  that 
  the 
  circumstances 
  connected 
  with 
  

   the 
  case 
  of 
  Vesuvius 
  were 
  to 
  a 
  certain 
  extent 
  fortuitous 
  ; 
  

   the 
  exceptionally 
  radioactive 
  materials 
  having 
  been 
  brought 
  

   to 
  light 
  by 
  a 
  regional 
  crustal 
  disturbance 
  unconnected 
  with 
  

   the 
  existence 
  of 
  a 
  local 
  deep-seated 
  radioactive 
  magma. 
  On 
  

   such 
  a 
  view 
  we 
  may 
  admit 
  that 
  the 
  locally 
  great 
  radio- 
  

   activity 
  has 
  perhaps 
  contributed 
  to 
  the 
  remarkable 
  persistence 
  

  

  * 
  For 
  data 
  see 
  Vog-t, 
  Tschermak's 
  Mm. 
  u. 
  Pet. 
  Mit. 
  xxvii. 
  p. 
  105. 
  

  

  