﻿AGE 
  OF 
  THE 
  EARTH 
  — 
  CHAMBERLIN 
  AND 
  OTHERS. 
  273 
  

  

  intervals 
  of 
  time 
  must 
  be 
  regarded 
  as 
  minimum 
  estimates. 
  The 
  

   uranium 
  and 
  helium 
  must 
  have 
  been 
  in 
  contact 
  with 
  each 
  other 
  for 
  

   at 
  least 
  as 
  long 
  as 
  the 
  periods 
  mentioned, 
  but 
  they 
  may 
  have 
  been 
  

   together 
  for 
  much 
  longer 
  intervals 
  of 
  time. 
  

  

  Calculations 
  based 
  on 
  the 
  quantity 
  of 
  lead 
  in 
  uranium 
  ores 
  vary 
  

   from 
  340 
  millions 
  to 
  1,700 
  millions 
  of 
  years, 
  according 
  to 
  the 
  locality 
  

   from 
  which 
  the 
  ore 
  is 
  obtained. 
  In 
  this 
  case 
  another 
  complication 
  

   appears. 
  We 
  have 
  learned 
  to 
  distinguish 
  several 
  different 
  kinds 
  of 
  

   lead 
  from 
  each 
  other. 
  The 
  various 
  kinds 
  of 
  lead 
  have 
  similar 
  chem- 
  

   ical 
  properties 
  but 
  differ 
  from 
  each 
  other 
  in 
  their 
  atomic 
  weights. 
  

   All 
  the 
  different 
  kinds 
  of 
  lead 
  do 
  not 
  come 
  from 
  uranium 
  ; 
  only 
  lead 
  

   of 
  atomic 
  weight 
  about 
  206 
  may 
  be 
  regarded 
  as 
  produced 
  from 
  ura- 
  

   nium. 
  Until, 
  therefore, 
  we 
  have 
  determined 
  exactly 
  what 
  the 
  atomic 
  

   weights 
  of 
  the 
  lead 
  in 
  the 
  various 
  ores 
  really 
  are, 
  we 
  can 
  not 
  be 
  sure 
  

   that 
  the 
  lead 
  came 
  from 
  the 
  uranium. 
  "We 
  can 
  assert, 
  however, 
  that 
  

   there 
  is 
  no 
  more 
  uranium 
  lead 
  in 
  a 
  given 
  uranium 
  ore 
  than 
  the 
  

   amount 
  of 
  lead 
  actually 
  found. 
  Unless, 
  therefore, 
  the 
  atomic 
  weight 
  

   of 
  the 
  lead 
  in 
  an 
  ore 
  has 
  been 
  actually 
  determined 
  and 
  found 
  to 
  be 
  

   about 
  206, 
  we 
  must 
  consider 
  the 
  estimate 
  of 
  the 
  age 
  of 
  the 
  ore 
  as 
  a 
  

   maximum 
  estimate 
  only. 
  The 
  lead 
  and 
  uranium 
  can 
  not 
  have 
  been 
  

   in 
  contact 
  with 
  each 
  other 
  for 
  a 
  period 
  of 
  time 
  longer 
  than 
  that 
  

   calculated 
  from 
  the 
  known 
  rate 
  of 
  transformation 
  of 
  uranium 
  into 
  

   lead. 
  

  

  The 
  atomic 
  weight 
  of 
  the 
  lead 
  in 
  a 
  few 
  ores 
  has 
  been 
  found 
  to 
  

   be 
  very 
  close 
  to 
  206. 
  In 
  one 
  of 
  these 
  the 
  age 
  of 
  the 
  mineral 
  has 
  

   been 
  estimated 
  at 
  a 
  little 
  over 
  900 
  millions 
  of 
  years. 
  

  

  The 
  calculation 
  of 
  the 
  age 
  of 
  uranium 
  deposits 
  by 
  means 
  of 
  radio- 
  

   active 
  data 
  rests 
  upon 
  the 
  laws 
  of 
  nature 
  as 
  we 
  now 
  believe 
  them 
  to 
  

   be. 
  It 
  would 
  be 
  a 
  waste 
  of 
  time 
  to 
  speculate 
  on 
  future 
  discoveries 
  

   (new 
  radio-active 
  elements, 
  for 
  instance, 
  or 
  alterations 
  in 
  the 
  rates 
  

   of 
  radio-active 
  processes) 
  or 
  on 
  a 
  possible 
  evolution 
  of 
  natural 
  law. 
  

  

  The 
  ages 
  calculated 
  from 
  radio-active 
  data 
  represent 
  the 
  length 
  

   of 
  time 
  during 
  which 
  we 
  may 
  suppose 
  the 
  chemical 
  elements 
  to 
  have 
  

   been 
  in 
  more 
  or 
  less 
  mechanical 
  contact 
  with 
  each 
  other. 
  They 
  do 
  

   not 
  represent 
  the 
  time 
  that 
  has 
  elapsed 
  since 
  the 
  earth 
  may 
  have 
  

   reached 
  a 
  state 
  capable 
  of 
  supporting 
  organic 
  life 
  as 
  we 
  now 
  know 
  it. 
  

  

  