﻿of 
  Deep- 
  Sea 
  Sediments. 
  197 
  

  

  derived 
  from 
  the 
  land. 
  The 
  total 
  quantity 
  given 
  up 
  to 
  the 
  

   ocean 
  during 
  geological 
  time 
  must 
  be 
  enormous. 
  It 
  can 
  be 
  

   determined 
  within 
  limits 
  by 
  methods 
  which 
  I 
  have 
  described 
  

   when 
  dealing 
  with 
  the 
  derivation 
  of 
  sodium 
  from 
  the 
  land 
  in 
  

   connexion 
  with 
  the 
  age 
  of 
  the 
  earth. 
  Doubtless 
  much 
  of 
  it 
  

   has 
  at 
  one 
  time 
  existed 
  in 
  the 
  earlier 
  sediments 
  and 
  been 
  

   abstracted 
  by 
  secondary 
  denudation. 
  

  

  It 
  is 
  possible 
  to 
  show 
  that 
  a 
  very 
  slow 
  rate 
  of 
  collection 
  of 
  

   Calcareous 
  Oozes 
  on 
  those 
  areas 
  in 
  the 
  Pacific 
  from 
  which 
  

   arise 
  the 
  Coral 
  Islands, 
  might, 
  in 
  the 
  course 
  of 
  such 
  periods 
  

   as 
  geological 
  time 
  permits, 
  give 
  rise 
  to 
  depths 
  of 
  deposit 
  with 
  

   radio-thermal 
  effects 
  in 
  their 
  more 
  deeply 
  buried 
  parts, 
  

   adequate 
  to 
  elevate 
  considerably 
  the 
  temperature 
  in 
  the 
  

   underlying 
  crust 
  of 
  the 
  Earth. 
  This 
  effect 
  will 
  be 
  of 
  greater 
  

   intensity 
  if 
  the 
  buried 
  sub-oceanic 
  earth-crust 
  is 
  itself 
  

   radioactive, 
  as 
  it 
  very 
  probably 
  is. 
  The 
  mean 
  radioactivity 
  

   of 
  igneous 
  and 
  sedimentary 
  rocks 
  of 
  the 
  upper 
  crust 
  is 
  not 
  

   less 
  than 
  4 
  x 
  10~ 
  12 
  upon 
  my 
  experiments, 
  and 
  if 
  certain 
  

   specially 
  radioactive 
  rocks 
  are 
  taken 
  into 
  account 
  tbe 
  mean 
  

   result 
  would 
  be 
  higher 
  still. 
  The 
  igneous 
  rocks 
  stand 
  some- 
  

   what 
  higher 
  than 
  the 
  sedimentary, 
  and 
  the 
  sub-oceanic 
  crust 
  

   probably 
  partakes 
  of 
  the 
  qualities 
  of 
  the 
  latter. 
  The 
  thick- 
  

   ness 
  of 
  this 
  radioactive 
  layer 
  over 
  the 
  earth 
  we 
  do 
  not 
  know. 
  

   But 
  on 
  tbe 
  assumption 
  that 
  it 
  extends 
  to 
  a 
  depth 
  of 
  five 
  miles, 
  

   and 
  assuming 
  a 
  rate 
  of 
  accumulation 
  of 
  Calcareous 
  Oozes 
  of 
  

   five 
  centimetres 
  in 
  a 
  century 
  over 
  the 
  disturbed 
  and 
  relatively 
  

   shallow 
  areas 
  of 
  the 
  Pacific, 
  and 
  that 
  this 
  rate 
  has 
  persisted 
  over 
  

   10 
  millions 
  of 
  years 
  ; 
  I 
  find 
  the 
  resulting 
  radiothermal 
  effects 
  

   would 
  be 
  adequate 
  to 
  reduce 
  the 
  thickness 
  of 
  the 
  rigid 
  crust 
  

   by 
  as 
  much 
  as 
  25 
  or 
  even 
  30 
  per 
  cent. 
  That 
  is, 
  if 
  the 
  isogeo- 
  

   therm 
  of 
  800° 
  is 
  a 
  temperature 
  at 
  which 
  crustal 
  materials 
  

   lose 
  their 
  rigidity, 
  this 
  isogeotherm 
  will 
  be 
  shifted 
  upwards 
  

   from 
  its 
  probable 
  existing 
  depth 
  sufficiently 
  to 
  reduce 
  the 
  

   rigid 
  crust 
  as 
  stated. 
  

  

  Thus 
  it 
  seems 
  not 
  improbable 
  that 
  the 
  problem 
  of 
  the 
  

   unstable 
  areas 
  of 
  the 
  Pacific 
  is 
  in 
  its 
  nature 
  continuous 
  with 
  

   that 
  of 
  mountain 
  -building 
  along 
  the 
  continental 
  margins, 
  

   and 
  that 
  both 
  alike 
  are 
  dependent 
  on 
  the 
  thermal 
  effects 
  

   accompanying 
  radioactive 
  transformations 
  progressing 
  in 
  

   sedimentary 
  accumulations. 
  

  

  