﻿210 
  Drs. 
  Smeeth 
  and 
  Watson 
  on 
  the 
  Radioactivity 
  of 
  

  

  work 
  showed 
  that 
  the 
  Santaveri 
  trap 
  of 
  the 
  Kadur 
  District 
  

   possessed 
  a 
  considerable 
  resemblance 
  to 
  the 
  Grey 
  trap 
  of 
  

   Chitaldrug, 
  so 
  much 
  so 
  that 
  it 
  had 
  almost 
  been 
  decided 
  to 
  

   class 
  these 
  two 
  formations 
  together 
  and 
  separate 
  them 
  from 
  

   the 
  Bellara 
  trap. 
  The 
  radium 
  determinations, 
  however, 
  

   show 
  that 
  the 
  Grey 
  trap 
  and 
  the 
  Bellara 
  trap 
  have 
  practically 
  

   the 
  same 
  radioactivity 
  {vide 
  Nos. 
  23 
  and 
  24), 
  which 
  is 
  about 
  

   one 
  third 
  of 
  that 
  of 
  any 
  of 
  the 
  schists 
  proper, 
  while 
  the 
  

   Santaveri 
  trap 
  (No. 
  19) 
  contains 
  about 
  three 
  times 
  as 
  much 
  

   radium 
  and 
  falls 
  within 
  the 
  limits 
  so 
  far 
  ascertained 
  for 
  the 
  

   schists. 
  This 
  appears 
  to 
  confirm 
  the 
  original 
  classification, 
  

   which 
  correlated 
  the 
  Grey 
  trap 
  with 
  the 
  Bellara 
  trap, 
  and 
  

   which, 
  consequently, 
  has 
  been 
  allowed 
  to 
  stand. 
  The 
  case 
  

   affords 
  an 
  illustration 
  of 
  the 
  possible 
  use 
  of 
  radium 
  deter- 
  

   minations 
  as 
  an 
  aid 
  to 
  correlation 
  of 
  the 
  highly 
  metamorphosed 
  

   members 
  of 
  the 
  Archsean 
  complex, 
  amongst 
  which 
  it 
  may 
  often 
  

   happen 
  that 
  the 
  chemical 
  and 
  mineral 
  composition, 
  and 
  the 
  

   field 
  relationships, 
  do 
  not 
  afford 
  sufficiently 
  definite 
  points 
  of 
  

   similarity 
  or 
  distinction. 
  The 
  value 
  of 
  such 
  determinations 
  

   will 
  depend 
  on 
  the 
  possibility 
  of 
  ascertaining 
  fairly 
  definite 
  

   limits 
  for 
  the 
  radium 
  contents 
  of 
  the 
  various 
  rock 
  groups 
  or 
  

   of 
  the 
  various 
  members 
  of 
  such 
  groups. 
  

  

  Amongst 
  the 
  various 
  gneisses 
  and 
  granites, 
  which 
  have 
  

   ! 
  een 
  divided 
  into 
  four 
  great 
  groups 
  of 
  distinctly 
  different 
  

   ages, 
  it 
  will 
  be 
  noted 
  that 
  the 
  Charnockites 
  stand 
  apart 
  from 
  

   the 
  others 
  in 
  virtue 
  of 
  their 
  excessively 
  low 
  radioactivity, 
  

   which 
  is 
  much 
  lower 
  even 
  than 
  that 
  of 
  the 
  Dharwar 
  schists. 
  

  

  The 
  Champion 
  gneiss, 
  Peninsular 
  gneiss 
  and 
  the 
  Closepet 
  

   granite 
  — 
  which 
  last 
  is 
  also 
  a 
  variable 
  complex 
  — 
  all 
  contain 
  

   from 
  12 
  to 
  15 
  times 
  as 
  much 
  radium 
  as 
  the 
  Charnockites 
  

   and 
  four 
  to 
  five 
  times 
  as 
  much 
  as 
  the 
  Dharwar 
  schists. 
  

   The 
  Charnockites 
  have 
  been 
  shown 
  by 
  Holland 
  to 
  form 
  a 
  

   distinct 
  petrographical 
  province 
  amongst 
  the 
  gneisses 
  of 
  

   Southern 
  India 
  and 
  vary, 
  as 
  the 
  result 
  ©f 
  magmatic 
  segre- 
  

   gation, 
  from 
  highly 
  acid 
  granites 
  to 
  norites 
  and 
  hypei 
  sthenites 
  

   — 
  all 
  characterized 
  by 
  the 
  presence 
  of 
  hypersthene 
  and 
  certain 
  

   physical 
  features 
  — 
  and 
  the 
  radium 
  determinations 
  fully 
  con- 
  

   firm 
  the 
  distinct 
  individuality 
  of 
  the 
  parent 
  magma. 
  The 
  

   varieties 
  of 
  Charnockite 
  which 
  have 
  been 
  examined 
  show 
  an 
  

   increase 
  of 
  radium 
  with 
  increasing 
  basicity, 
  but 
  the 
  hyper- 
  

   sthenite 
  and 
  quartz-magnetite 
  ore 
  — 
  which 
  are 
  considered 
  to 
  

   be 
  end 
  products 
  of 
  the 
  segregative 
  process 
  — 
  show 
  a 
  relapse 
  

   towards 
  the 
  mean 
  value. 
  

  

  The 
  other 
  gneisses 
  and 
  granites 
  are 
  very 
  complex, 
  and 
  the 
  

   determinations 
  are 
  not 
  sufficiently 
  numerous 
  to 
  permit 
  of 
  

  

  