﻿406 
  MR. 
  T. 
  H. 
  HOLLAND 
  ON 
  AT7GITE-DIORITES 
  WITH 
  [Aug. 
  1897, 
  

  

  Presidency, 
  but 
  are 
  particularly 
  well- 
  developed 
  in 
  the 
  districts 
  of 
  

   Chingelput 
  and 
  South 
  Arcot, 
  on 
  the 
  Coromandel 
  coast, 
  where 
  they 
  

   break 
  through 
  the 
  acid 
  members 
  of 
  the 
  ' 
  pyroxene-granulite' 
  

   series, 
  and 
  are 
  often 
  traceable 
  for 
  many 
  miles, 
  showing 
  their 
  actual 
  

   contacts 
  with 
  the 
  older 
  crystallines, 
  or 
  projecting 
  as 
  lines 
  of 
  black 
  

   hummocks 
  through 
  the 
  alluvium 
  and 
  soil 
  of 
  the 
  ' 
  paddy 
  ' 
  fields. 
  

  

  The 
  facts 
  revealed 
  by 
  the 
  microscopic 
  study 
  of 
  these 
  Indian 
  rocks 
  

   have, 
  it 
  seems 
  to 
  me, 
  the 
  most 
  interesting 
  bearing 
  on 
  the 
  questions 
  

   raised 
  by 
  the 
  distinguished 
  petrographers 
  who 
  have 
  examined 
  the 
  

   well-known 
  associations 
  of 
  micropegmatite 
  with 
  basic 
  rocks 
  near 
  

   Penmaenmawr, 
  1 
  in 
  the 
  Charnwood 
  Forest, 
  2 
  the 
  Whin 
  Sill, 
  3 
  the 
  

   Cheviot 
  district, 
  4 
  Carrock 
  Fell, 
  5 
  Carlingford, 
  6 
  and 
  Skye. 
  7 
  

  

  II. 
  Petrological 
  Characters 
  oe 
  the 
  Rocks. 
  

  

  The 
  central 
  masses 
  of 
  the 
  large 
  dykes 
  are 
  tough, 
  black, 
  even- 
  

   grained 
  rocks, 
  in 
  which 
  the 
  crystals 
  seldom 
  exceed 
  5 
  mm. 
  in 
  length. 
  

   The 
  specific 
  gravity 
  is 
  remarkably 
  uniform, 
  varying 
  between 
  3'0O 
  

   and 
  3*09. 
  Towards 
  the 
  margins 
  of 
  the 
  dykes 
  the 
  rocks 
  are 
  finer 
  

   in 
  grain, 
  while 
  the 
  selvages, 
  so 
  far 
  as 
  their 
  microscopic 
  characters 
  

   are 
  concerned, 
  could 
  not 
  be 
  distinguished 
  from 
  augite-andesites, 
  

   having 
  a 
  fine-grained, 
  pilotaxitic, 
  and 
  sometimes 
  distinctly 
  hyalo- 
  

   pilitic 
  groundmass, 
  through 
  which 
  irregular, 
  glomero-porphyritic 
  

   groups 
  of 
  augite 
  and 
  plagioclase 
  are 
  scattered. 
  

  

  Mineral 
  Composition. 
  — 
  The 
  rocks 
  are 
  composed 
  essentially 
  of 
  

   augite, 
  plagioclase, 
  and 
  micropegmatite. 
  Enstatite 
  is 
  often 
  present 
  

   in 
  small 
  quantities, 
  forming 
  the 
  cores 
  of 
  the 
  pyroxene-crystals. 
  

   Opaque, 
  black 
  iron-ores, 
  hornblende, 
  and 
  biotite 
  in 
  small 
  quantities, 
  

   and 
  either 
  wholly 
  or 
  in 
  part 
  secondary 
  in 
  origin, 
  present 
  a 
  constant 
  

   and 
  peculiar 
  relation 
  to 
  the 
  augite 
  and 
  micropegmatite, 
  the 
  signi- 
  

   ficance 
  of 
  which 
  is 
  pointed 
  out 
  below. 
  

  

  Order 
  of 
  Crystallization 
  of 
  the 
  Constituents. 
  — 
  Although 
  in 
  the 
  

   more 
  basic 
  dykes 
  associated 
  with 
  these 
  rocks, 
  and 
  in 
  which 
  enstatite 
  

   is 
  a 
  prominent 
  constituent, 
  the 
  consolidation 
  of 
  the 
  plagioclase 
  must 
  

   have 
  been 
  completed 
  invariably 
  after 
  that 
  of 
  the 
  pyroxene, 
  in 
  the 
  

   augite-diorites 
  now 
  under 
  consideration 
  there 
  is 
  no 
  such 
  distinct 
  

   difference 
  between 
  the 
  periods 
  of 
  the 
  consolidation 
  of 
  these 
  two 
  

   minerals. 
  They 
  are 
  intergrown 
  with 
  a 
  want 
  of 
  regularity 
  which 
  

   indicates 
  an 
  average 
  simultaneous 
  crystallization, 
  and 
  this 
  conclusion 
  

   is 
  confirmed 
  also 
  by 
  the 
  intergrowths 
  exhibited 
  in 
  the 
  glomero- 
  

   porphyritic 
  groups 
  of 
  augite- 
  and 
  plagioclase-phenocrysts 
  in 
  the 
  

   hemicrystalline 
  varieties 
  forming 
  the 
  selvages 
  of 
  the 
  dykes. 
  

  

  1 
  J. 
  A. 
  Phillips, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  toI. 
  xxxiii. 
  (1877) 
  p. 
  423 
  ; 
  Waller,. 
  

   ' 
  Midland 
  Naturalist,' 
  1885, 
  p. 
  4. 
  

  

  2 
  Hill 
  & 
  Bonney, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxiv. 
  (1878) 
  p. 
  199. 
  

  

  3 
  Teall, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884) 
  p. 
  640. 
  

  

  4 
  Id. 
  ' 
  British 
  Petrography 
  ' 
  (1888), 
  p. 
  272. 
  

  

  5 
  Ibid. 
  p. 
  179; 
  Harker, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  1. 
  (1894) 
  p. 
  311 
  &. 
  

   vol. 
  li. 
  (1895) 
  p. 
  125. 
  

  

  6 
  Sollas, 
  Trans. 
  Eoy. 
  Irish 
  Acad. 
  vol. 
  xxx. 
  (1894) 
  p. 
  477. 
  

  

  7 
  Harker, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  Hi. 
  (1896) 
  p. 
  320. 
  

  

  