﻿258 
  ANNUAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  the 
  absolute 
  age 
  of 
  the 
  sediments 
  within 
  tlie 
  earth's 
  crust 
  vary 
  

   greatly 
  among 
  themselves, 
  from 
  30 
  up 
  to 
  400 
  million 
  years.^ 
  This 
  

   should 
  not 
  be 
  surprising, 
  for 
  if 
  to-day 
  the 
  rate 
  of 
  formation 
  of 
  sedi- 
  

   mentary 
  matter 
  varies 
  greatly 
  in 
  different 
  localities 
  so 
  it 
  must 
  have 
  

   been 
  in 
  the 
  past. 
  It 
  is, 
  moreover, 
  difficult 
  in 
  the 
  estimation 
  of 
  the 
  

   lapse 
  of 
  time 
  to 
  make 
  satisfactory 
  premises. 
  

  

  It 
  is 
  an 
  undoubtable 
  supposition 
  that 
  the 
  salt 
  content 
  of 
  the 
  oceans 
  

   originates 
  from 
  the 
  rocks 
  of 
  the 
  continents 
  (the 
  last 
  end 
  of 
  volcanic 
  

   activity) 
  and 
  that 
  this 
  salt 
  was 
  carried 
  by 
  the 
  fresh 
  water 
  of 
  the 
  land 
  

   into 
  the 
  oceans, 
  increasing 
  their 
  salinity; 
  from 
  estimates 
  of 
  the 
  rate 
  

   of 
  import 
  and 
  deposition 
  we 
  can 
  determine 
  the 
  lapse 
  of 
  time 
  for 
  the 
  

   existence 
  of 
  the 
  oceans, 
  at 
  least 
  since 
  pre-Cambrian 
  times, 
  and 
  there- 
  

   with 
  the 
  ages 
  of 
  the 
  sediments 
  in 
  the 
  sea 
  regions 
  of 
  past 
  ages. 
  Values 
  

   of 
  100,000,000 
  to 
  340,000,000 
  years 
  have 
  thus 
  been 
  reached." 
  

  

  Apparently 
  very 
  trustworthy 
  estimations 
  of 
  the 
  lengths 
  of 
  smaller 
  

   durations 
  of 
  time 
  in 
  geological 
  history 
  have 
  been 
  recently 
  made. 
  

  

  The 
  water 
  melting 
  from 
  the 
  inland 
  Scandinavian 
  ice 
  has 
  resulted 
  

   in 
  the 
  formation 
  of 
  the 
  so-called 
  Biindertone. 
  De 
  Geer 
  recognizes 
  in 
  

   these, 
  in 
  their 
  alternation 
  of 
  fine 
  sand 
  and 
  clayey 
  material, 
  the 
  influ- 
  

   ence 
  of 
  yearly 
  variations 
  in 
  the 
  rate 
  of 
  melting 
  of 
  the 
  ice, 
  whence, 
  

   for 
  the 
  Scandinavian 
  peninsula, 
  he 
  has 
  computed 
  the 
  time 
  elapsing 
  

   from 
  the 
  beginning 
  of 
  the 
  retreat 
  of 
  the 
  diluvial 
  ice 
  until 
  the 
  present 
  

   as 
  12,000 
  years. 
  Sorgel 
  saw 
  in 
  the 
  rock 
  sequences 
  of 
  the 
  Thuringian 
  

   diluvium 
  the 
  decisive 
  influence 
  of 
  cosmic 
  factors, 
  the 
  cyclical 
  varia- 
  

   tion 
  in 
  the 
  obliquity 
  of 
  the 
  ecliptic, 
  the 
  eccentricity 
  of 
  the 
  earth's 
  

   orbit, 
  as 
  well 
  as 
  the 
  variation 
  of 
  the 
  perihelion 
  distance 
  of 
  the 
  earth 
  

   and 
  the 
  variation 
  in 
  the 
  intensity 
  in 
  the 
  sun's 
  heat 
  dependent 
  upon 
  

   these. 
  From 
  these 
  he 
  placed 
  the 
  beginning 
  of 
  the 
  north 
  German 
  

   diluvium 
  more 
  than 
  580,000 
  years 
  ago. 
  

  

  The 
  physical 
  chemists 
  consider 
  that 
  the 
  surest 
  method 
  of 
  estimating 
  

   the 
  ages 
  of 
  rocks 
  depends 
  upon 
  the 
  amount 
  of 
  the 
  radioactive 
  trans- 
  

   formation 
  products 
  present, 
  due 
  to 
  the 
  passage 
  from 
  uranium 
  and 
  

   thorium 
  to 
  lead 
  and 
  helium, 
  respectively, 
  in 
  the 
  minerals 
  of 
  volcanic 
  

   origin. 
  The 
  basis 
  for 
  this 
  surety 
  is 
  the 
  fact 
  that 
  radioactive 
  proc- 
  

   esses 
  are 
  wholly 
  independent 
  of 
  surrounding 
  chemical 
  and 
  physical 
  

   conditions 
  and 
  pursue 
  their 
  transformations 
  slowly 
  indeed, 
  but 
  at 
  

   the 
  same 
  speed 
  for 
  all 
  time. 
  Using 
  the 
  rate 
  of 
  transformation 
  of 
  

   uranium 
  or 
  thorium, 
  the 
  absolute 
  age 
  of 
  a 
  number 
  of 
  volcanic 
  rocks, 
  

   differing 
  in 
  geological 
  periods, 
  has 
  been 
  computed. 
  Although 
  the 
  

   values 
  obtained 
  are 
  not 
  fully 
  concordant, 
  generally 
  the 
  results 
  are 
  

  

  » 
  We 
  quote 
  only 
  a 
  fow 
  values 
  : 
  Sollas 
  reckoned 
  34,000,000 
  to 
  80,000,000 
  ; 
  rhillips, 
  

   38,000,000 
  to 
  96,000,000 
  ; 
  Walcott, 
  55,000,000 
  to 
  70,000,000 
  ; 
  de 
  Lapparent, 
  67,000,000 
  to 
  

   90,000,000; 
  Geike, 
  100,000,000 
  to 
  400,000,000 
  years. 
  Ami 
  Boue 
  sometime 
  since 
  pub- 
  

   lished 
  a 
  very 
  complete 
  summary 
  of 
  the 
  older 
  estimates. 
  

  

  » 
  New 
  estimates 
  due 
  to 
  Sollas 
  give 
  100,000,000 
  to 
  175,000,000 
  ; 
  to 
  Holmes, 
  210,000,000 
  

   to 
  340,000,000 
  ; 
  and 
  to 
  Scliraiedel 
  of 
  at 
  least 
  300,000,000 
  years. 
  

  

  