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  0L 
  53*] 
  ATJGITE-DIORITES 
  WITH 
  MICROPEGMATITE. 
  405 
  

  

  29. 
  On 
  AUG1TE-DIORITES 
  ivith 
  MlCROPEGMATITE 
  in 
  SOUTHERN 
  INDIA. 
  

  

  By 
  Thomas 
  H. 
  Holland, 
  Esq., 
  A.R.C.S., 
  F.G.S., 
  Officiating 
  

   Superintendent, 
  Geological 
  Survey 
  of 
  India. 
  (Read 
  May 
  26th, 
  

   1897.) 
  

  

  [Plate 
  XXIX.] 
  

  

  Contents. 
  

  

  I. 
  Introduction 
  405 
  

  

  II. 
  Petrological 
  Characters 
  of 
  the 
  Rocks 
  406 
  

  

  III. 
  Chemical 
  Composition 
  of 
  the 
  Rocks 
  408 
  

  

  IV. 
  Explanation 
  of 
  the 
  Structure 
  of 
  the 
  Dyke-rocks 
  409 
  

  

  V. 
  Comparison 
  with 
  so-called 
  Granophyric 
  Gabbros 
  411 
  

  

  VI. 
  Summary 
  414 
  

  

  I. 
  Introduction. 
  

  

  Prior 
  to 
  the 
  great 
  outburst 
  of 
  Deccan 
  trap 
  at 
  the 
  close 
  of 
  the 
  

   -Cretaceous 
  period, 
  there 
  were 
  two 
  principal 
  periods 
  of 
  volcanic 
  

   activity 
  in 
  Peninsular 
  India. 
  One 
  of 
  these 
  is 
  indicated 
  by 
  the 
  

   contemporaneous 
  traps 
  of 
  the 
  Dharwars 
  — 
  the 
  oldest 
  of 
  our 
  recognized 
  

   Transition 
  systems 
  — 
  and 
  the 
  other 
  by 
  the 
  lava-flows 
  of 
  the 
  Cuddapah 
  

   system, 
  whose 
  precise 
  position 
  in 
  the 
  stratigraphical 
  succession 
  

   remains, 
  on 
  account 
  of 
  the 
  complete 
  absence 
  of 
  fossils, 
  still 
  un- 
  

   determined. 
  As 
  a 
  result 
  of 
  the 
  great 
  earth-movements 
  which 
  

   affected 
  the 
  strata 
  of 
  Peninsular 
  India 
  previous 
  to 
  the 
  deposition 
  of 
  

   the 
  Cuddapah 
  system, 
  the 
  igneous 
  rocks 
  of 
  Dharwar 
  age 
  have 
  been 
  

   greatly 
  metamorphosed, 
  and 
  in 
  this 
  respect 
  they 
  stand 
  in 
  striking 
  

   -contrast 
  to 
  the 
  Cuddapah 
  traps, 
  since 
  the 
  eruption 
  of 
  which 
  the 
  

   Peninsula 
  of 
  India 
  has 
  been 
  remarkably 
  free 
  from 
  geological 
  dis- 
  

   turbances. 
  It 
  is 
  evidently 
  in 
  consequence 
  of 
  this 
  remarkable 
  

   freedom, 
  both 
  from 
  extreme 
  changes 
  of 
  level 
  and 
  the 
  crushing 
  

   effects 
  of 
  earth-movements, 
  that 
  rocks 
  as 
  old 
  as 
  the 
  Cuddapahs 
  

   have 
  their 
  original 
  delicate 
  structures, 
  and 
  primary 
  constituents 
  so 
  

   susceptible 
  as 
  olivine 
  and 
  augite, 
  preserved 
  with 
  striking 
  freshness. 
  

  

  The 
  numerous 
  dykes 
  of 
  basic 
  igneous 
  rocks 
  which 
  break 
  through 
  

   the 
  ' 
  pyroxene-granulites 
  ' 
  and 
  gneisses 
  of 
  the 
  Madras 
  Presidency, 
  

   and 
  which, 
  for 
  reasons 
  that 
  need 
  not 
  now 
  be 
  stated, 
  are 
  regarded 
  

   as 
  the 
  dyke-representatives 
  of 
  the 
  Cuddapah 
  lava-flows, 
  1 
  vary 
  in 
  

   composition 
  from 
  very 
  basic 
  olivine-augite 
  norites 
  approaching 
  

   saxonites, 
  through 
  augite-norites, 
  to 
  augite-diorites 
  with 
  micro- 
  

   pegmatite. 
  Detailed 
  descriptions 
  of 
  these 
  and 
  the 
  peculiar 
  hemi- 
  

   crystalline 
  and 
  vitreous 
  varieties, 
  which 
  form 
  the 
  selvages 
  of 
  the 
  

   larger 
  masses, 
  or 
  occur 
  as 
  narrow 
  apophyses, 
  are 
  published 
  in 
  the 
  

   Eecords 
  of 
  the 
  Geological 
  Survey 
  of 
  India 
  (vol. 
  xxx. 
  pt. 
  i. 
  1897). 
  It 
  

   is 
  only 
  with 
  the 
  interesting 
  features 
  displayed 
  by 
  the 
  rocks 
  which 
  I 
  

   have 
  grouped 
  under 
  Prof. 
  Cole's 
  very 
  convenient 
  term 
  augite- 
  

   diorite 
  that 
  the 
  present 
  paper 
  is 
  intended 
  to 
  deal. 
  

  

  The 
  augite-diorite 
  dykes 
  occur 
  in 
  various 
  parts 
  of 
  the 
  Madras 
  

  

  1 
  'Manual 
  of 
  the 
  Geology 
  of 
  India,' 
  2nd 
  ed. 
  (1893) 
  p. 
  40. 
  

  

  