﻿P 
  re-Cambrian 
  and 
  Associated 
  Rocks 
  of 
  Mozambique. 
  137 
  

  

  The 
  above 
  correlations 
  of 
  certain 
  groups 
  of 
  rocks 
  with 
  the 
  

   Lower 
  and 
  Middle 
  Pre- 
  Cambrian 
  of 
  other 
  regions 
  are 
  based 
  on 
  

   the 
  determination 
  of 
  lead-uraniuni 
  ratios 
  of 
  zircons 
  derived 
  from 
  

   the 
  gneisses 
  and 
  granulitic 
  granites 
  respectively, 
  the 
  zircons 
  

   having 
  been 
  obtained 
  by 
  crushing 
  and 
  panning 
  the 
  rocks 
  in 
  the 
  

   field. 
  The 
  gneisses 
  give 
  a 
  ratio 
  of 
  0*21, 
  comparable 
  with 
  a 
  ratio 
  

   of 
  0*24 
  obtained 
  for 
  Canadian 
  zircons 
  of 
  Laurentian 
  age. 
  The 
  

   granulitic 
  granites 
  give 
  ratios 
  of 
  014 
  to 
  0*17, 
  comparable 
  to 
  those 
  

   of 
  radioactive 
  minerals 
  of 
  late 
  Archaean 
  : 
  that 
  is, 
  late 
  Middle 
  Pre- 
  

   Cambrian, 
  age 
  in 
  Scandinavia 
  (Moss 
  0*12 
  to 
  015, 
  Arendal 
  0*16 
  to 
  

   0-18, 
  and 
  Ytterby 
  0*15 
  toO'17), 
  Canada 
  (Villeneuve, 
  Quebec, 
  0*17), 
  

   and 
  India 
  (Singar 
  0*14). 
  

  

  The 
  rocks 
  are 
  described 
  in 
  detail, 
  with 
  tables 
  giving 
  the 
  quanti- 
  

   tative 
  mineral 
  composition 
  and 
  the 
  specific 
  gravities 
  and 
  radium 
  

   contents. 
  Numerous 
  examples 
  of 
  contact 
  -phenomena 
  between 
  

   crystalline 
  limestones 
  and 
  various 
  types 
  of 
  igneous 
  rock 
  are 
  

   recorded: 
  pyroxene, 
  amphibole, 
  sphene, 
  and 
  soda-lime 
  felspar 
  

   being 
  the 
  new 
  minerals 
  chiefly 
  developed 
  between 
  granite 
  and 
  

   limestone, 
  with 
  garnet 
  and 
  scapolite 
  also 
  in 
  special 
  cases. 
  

  

  With 
  reference 
  to 
  the 
  origin 
  of 
  the 
  crystalline 
  limestones 
  and 
  

   gneisses, 
  the 
  following 
  conclusions 
  are 
  arrived 
  at 
  : 
  — 
  

  

  (a) 
  The 
  crystalline 
  schists 
  and 
  limestones 
  are 
  interpreted 
  as 
  arenaceous 
  

  

  and 
  calcareous 
  f 
  acies 
  respectively 
  of 
  an 
  ancient 
  sedimentary 
  series, 
  

   their 
  argillaceous 
  complements 
  being 
  unrepresented 
  unless 
  they 
  

   enter 
  into 
  the 
  composition 
  of 
  the 
  biotite 
  -gneisses. 
  

  

  (b) 
  The 
  limestones 
  have 
  controlled 
  the 
  formation 
  of 
  hornblende 
  - 
  

  

  gneiss 
  and 
  amphibolite 
  by 
  their 
  interaction 
  with 
  a 
  granitic 
  

   magma 
  that 
  elsewhere 
  is 
  represented 
  by 
  biotite-gneisses. 
  The 
  

   cores 
  of 
  the 
  limestones 
  have 
  been 
  enabled 
  to 
  resist 
  further 
  

   silicification 
  by 
  being 
  thus 
  enclosed 
  within 
  a 
  blanket 
  of 
  rocks 
  

   impoverished 
  in 
  silica. 
  

  

  (c) 
  If 
  the 
  ancient 
  sedimentary 
  series 
  included 
  argillaceous 
  formations, 
  

  

  it 
  is 
  thought 
  probable 
  that 
  the 
  gneisses 
  are 
  composite 
  rocks 
  

   produced 
  by 
  the 
  concordant 
  injection 
  of 
  granitic 
  magma 
  into 
  

   such 
  formations. 
  This 
  view, 
  although 
  not 
  proved, 
  is 
  supported 
  

   by 
  mineralogical 
  and 
  radioactive 
  evidence, 
  and 
  by 
  the 
  fact 
  that 
  

   in 
  certain 
  inselberg 
  peaks 
  the 
  banding 
  of 
  the 
  gneisses 
  gradually 
  

   dies 
  away 
  as 
  the 
  slopes 
  are 
  ascended, 
  the 
  rocks 
  passing 
  into 
  

   granulitic 
  granite 
  nearly 
  free 
  from 
  biotite 
  and 
  showing 
  few 
  

   traces 
  of 
  foliation. 
  These 
  peaks 
  are 
  interpreted 
  as 
  the 
  irruptive 
  

   foci 
  of 
  granulitic 
  magmas 
  which 
  fed 
  the 
  lateral 
  intrusions 
  repre- 
  

   sented 
  by 
  the 
  surrounding 
  gneisses. 
  

  

  It 
  is 
  shown 
  that 
  there 
  are 
  at 
  least 
  three 
  types 
  of 
  inselberg 
  

   peaks 
  that 
  owe 
  their 
  survival 
  to 
  peculiarities 
  of 
  structure 
  and 
  

   composition. 
  The 
  first 
  type 
  is 
  that 
  just 
  mentioned, 
  in 
  which 
  

   the 
  foliation 
  is 
  less 
  marked 
  and 
  the 
  biotite-content 
  appreciably 
  

   lower 
  than 
  in 
  the 
  surrounding 
  gneisses. 
  In 
  the 
  second, 
  the 
  

   peaks 
  are 
  mainly 
  composed 
  of 
  granulitic 
  granite 
  (again 
  poor 
  

   in 
  biotite 
  compared 
  with 
  the 
  gneisses), 
  and 
  in 
  the 
  third 
  type 
  

   the 
  peaks 
  are 
  riddled 
  with 
  tongues 
  and 
  apophyses 
  of 
  pegmatite 
  

  

  Phil 
  Mag. 
  S. 
  6. 
  Vol. 
  35. 
  No. 
  205. 
  Jan. 
  1918. 
  L 
  

  

  