﻿Vol. 
  53-] 
  

  

  MICRO 
  PEGMATITE 
  EST 
  SOUTHERN 
  INDIA. 
  

  

  409 
  

  

  types 
  approaching 
  augite-andesite 
  in 
  structure. 
  

  

  Sections 
  of 
  both 
  

   For 
  the 
  analysis 
  

   of 
  the 
  rock 
  from 
  the 
  Seven 
  Pagodas 
  I 
  am 
  indebted 
  to 
  Mr. 
  P. 
  Bruhl, 
  

   Professor 
  of 
  Physics 
  in 
  the 
  Civil 
  Engineering 
  College, 
  Sibpur. 
  

  

  these 
  rocks 
  are 
  figured 
  in 
  PI. 
  XXIX. 
  figs. 
  1 
  & 
  3. 
  

  

  

  I. 
  Augite-dionte 
  with 
  \ 
  TT 
  . 
  ■. 
  -. 
  ■.„ 
  

   -, 
  T 
  . 
  & 
  ... 
  11. 
  Augite-andesite. 
  

   Micropeginatite. 
  ^ 
  

  

  SiO, 
  

  

  51-15 
  

  

  044 
  

   0-06 
  

  

  15-92 
  

   934 
  

   2-87 
  

   009 
  

  

  1040 
  

   6-48 
  

   119 
  

   1-61 
  

   011 
  

  

  50-86 
  

  

  0-63 
  

  

  trace 
  

  

  15-65 
  

  

  1 
  10-85 
  

  

  trace 
  

   11-76 
  

   603 
  

   201 
  

   1-56 
  

   020 
  

  

  Ti0" 
  9 
  

  

  p 
  2 
  o" 
  

  

  ALOo 
  

  

  Fe,0 
  Q 
  

  

  z, 
  27 
  3 
  

  

  FeO 
  

  

  MnO 
  

  

  CaO 
  

  

  MgO 
  

  

  Na,0 
  

  

  K 
  2 
  

  

  H 
  2 
  

  

  Sn. 
  2 
  r 
  

  

  99-66 
  

  

  99-55 
  

  

  319 
  

  

  301 
  

  

  

  Both 
  these 
  analyses 
  recall 
  the 
  composition 
  given 
  by 
  Mr. 
  Teall 
  for 
  

   the 
  Whin 
  Sill, 
  with 
  which 
  these 
  rocks 
  agree 
  so 
  strikingly 
  in 
  peno- 
  

   logical 
  characters. 
  1 
  The 
  comparatively 
  high 
  percentage 
  of 
  potash 
  

   among 
  the 
  alkalies 
  indicates 
  the 
  probable 
  presence 
  of 
  a 
  potash- 
  

   felspar 
  as 
  a 
  constituent 
  of 
  the 
  micropegmatite, 
  and 
  this 
  is 
  confirmed 
  

   by 
  the 
  detection 
  of 
  microcline 
  in 
  several 
  of 
  the 
  dykes. 
  

  

  If 
  the 
  silica, 
  alumina, 
  and 
  alkalies 
  due 
  to 
  the 
  micropegmatite, 
  

   which 
  possesses 
  a 
  silica-percentage 
  near 
  that 
  of 
  Bunseu's 
  ' 
  normal 
  

   trachytic 
  magma 
  ' 
  (76-67 
  Si0 
  2 
  ), 
  were 
  deducted 
  from 
  the 
  bulk-analysis 
  

   of 
  the 
  rock, 
  the 
  remainder 
  would 
  agree 
  fairly 
  closely 
  in 
  composition 
  

   with 
  the 
  ' 
  normal 
  pyroxenic 
  magma 
  ' 
  to 
  which 
  Bunsen 
  gave 
  the 
  

   hypothetical 
  composition 
  :— 
  Si0 
  2 
  , 
  48-47 
  ; 
  Ai 
  2 
  ? 
  , 
  14-78 
  ; 
  CaO, 
  11-87 
  ; 
  

   MgO, 
  6-89 
  ; 
  JFeO, 
  15-38 
  ; 
  alkalies, 
  2-61. 
  It 
  is 
  an 
  interesting 
  cir- 
  

   cumstance 
  that 
  the 
  numerous 
  associations 
  of 
  gabbro 
  with 
  granophyre 
  

   which 
  have 
  been 
  recorded 
  should 
  so 
  closely 
  approach 
  in 
  composition 
  

   Bunsen's 
  hypothetical 
  normal 
  pyroxenic 
  and 
  normal 
  trachytic 
  

   magmas, 
  and 
  the 
  similar 
  association 
  of 
  these 
  two 
  types 
  on 
  a 
  micro- 
  

   scopic 
  scale 
  in 
  the 
  South 
  Indian 
  dykes 
  suggests 
  the 
  derivation 
  of 
  

   these 
  two 
  distinct 
  rocks 
  by 
  segregative 
  consolidation 
  from 
  an 
  

   originally 
  common 
  magma. 
  

  

  IV. 
  Explanation 
  of 
  the 
  Structure 
  of 
  the 
  Dyice-rocks. 
  

  

  As 
  already 
  pointed 
  out, 
  the 
  augite-diorite 
  dyke-rocks 
  of 
  Southern 
  

   India 
  are 
  composed 
  principally 
  of 
  augite 
  and 
  plagioclasc, 
  wdth 
  

   subordinate 
  quantities 
  of 
  micropegmatite. 
  The 
  relations 
  of 
  the 
  

  

  1 
  See 
  Teall, 
  Quart. 
  Jjurn. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884) 
  p. 
  654. 
  

  

  