﻿414 
  ME. 
  T. 
  H. 
  HOLLAND 
  ON 
  AUGITE-DIOEITES 
  WITH 
  [Aug. 
  1897, 
  

  

  augite 
  to 
  hornblende 
  and 
  biotite 
  with 
  concomitant 
  separation 
  of 
  

   magnetite 
  does 
  not 
  necessarily 
  indicate 
  that 
  the 
  augite-diorite 
  has 
  

   been 
  altered 
  by 
  a 
  subsequent 
  and 
  distinct 
  igneous 
  intrusion 
  at 
  a 
  

   high 
  temperature, 
  

  

  If 
  the 
  separation 
  of 
  the 
  acid 
  from 
  the 
  basic 
  portions 
  of 
  a 
  magma 
  

   on 
  a 
  large 
  scale, 
  such 
  as 
  may 
  have 
  occurred 
  in 
  the 
  instances 
  quoted 
  

   above, 
  be 
  really 
  analogous 
  to 
  that 
  which 
  has 
  taken 
  place 
  on 
  a 
  

   microscopic 
  scale 
  in 
  the 
  Whin 
  Sill 
  and 
  the 
  Madras 
  dykes, 
  we 
  should 
  

   naturally 
  expect 
  to 
  find 
  that 
  the 
  acid 
  rock 
  is 
  later 
  than 
  its 
  basic 
  

   associate 
  in 
  completing 
  its 
  consolidation. 
  At 
  the 
  same 
  time, 
  the 
  

   close 
  agreement 
  in 
  the 
  ages 
  of 
  toe 
  two 
  rocks 
  might 
  very 
  well 
  give 
  

   rise 
  to 
  apparently 
  contradictory 
  phenomena 
  along 
  their 
  junction- 
  

   lines 
  (especially 
  if 
  earth-movements 
  disturbed 
  them 
  during, 
  or 
  sub- 
  

   sequent 
  to, 
  consolidation), 
  and 
  so 
  produce 
  isolation 
  of 
  injected 
  por- 
  

   tions 
  of 
  the 
  granophyre, 
  or 
  even 
  local 
  re-fusion 
  of 
  the 
  rocks 
  from 
  the 
  

   heat 
  produced 
  by 
  mechanical 
  movements. 
  

  

  In 
  the 
  minute 
  intercrystal 
  channels 
  and 
  lacunae 
  filled 
  with 
  micro- 
  

   miarolitic 
  micropegmatite 
  in 
  the 
  Madras 
  augite-diorite, 
  we 
  have 
  the 
  

   equivalents 
  of 
  what 
  the 
  older 
  geologists 
  understood 
  by 
  the 
  term 
  

   1 
  contemporaneous 
  veins,' 
  veins 
  which, 
  though 
  formed 
  after 
  suffi- 
  

   cient 
  consolidation 
  of 
  the 
  rock 
  in 
  which 
  they 
  occur, 
  are 
  yet 
  derived 
  

   from 
  the 
  same 
  magma 
  and 
  form 
  part 
  of 
  one 
  geological 
  unit. 
  

   Although 
  there 
  is 
  no 
  doubt 
  that 
  the 
  augite 
  and 
  plagioclase 
  were 
  

   separated 
  before 
  the 
  micropegmatite, 
  the 
  two 
  groups 
  of 
  minerals 
  

   have 
  been 
  separated 
  from 
  the 
  same 
  magma, 
  and 
  are 
  as 
  much 
  ' 
  con- 
  

   temporaneous 
  ' 
  as 
  are 
  two 
  twins, 
  which 
  for 
  obvious 
  reasons 
  are 
  not 
  

   born 
  simultaneously. 
  Such 
  was 
  the 
  view 
  taken 
  by 
  Waller 
  in 
  

   describing 
  the 
  augitic 
  acid 
  veins 
  penetrating 
  the 
  enstatite- 
  

   diorite 
  of 
  Penmaenmawr, 
  and 
  such 
  apparently 
  was 
  the 
  idea 
  in 
  

   Macculloch's 
  mind 
  when, 
  in 
  discussing 
  the 
  difficulties 
  of 
  settling 
  the 
  

   relative 
  ages 
  of 
  the 
  similar 
  rocks 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  

   he 
  concluded 
  that 
  the 
  ' 
  trap 
  ' 
  and 
  the 
  ' 
  syenite 
  ' 
  had 
  a 
  common 
  

   origin. 
  1 
  Haughton, 
  in 
  describing 
  the 
  granite 
  of 
  Barnavave, 
  has 
  also 
  

   classed 
  the 
  granophyric 
  veins 
  penetrating 
  the 
  gabbros 
  as 
  'con- 
  

   temporaneous.' 
  2 
  

  

  VI. 
  Summary. 
  

  

  In 
  the 
  Madras 
  Presidency 
  the 
  pyroxene-granulites 
  and 
  the 
  

   ordinary 
  gneisses 
  are 
  penetrated 
  by 
  a 
  very 
  large 
  number 
  of 
  basic 
  

   dykes, 
  which 
  are 
  regarded 
  as 
  the 
  underground 
  representatives 
  of 
  the 
  

   Cuddapah 
  lava-flows. 
  Since 
  the 
  intrusion 
  of 
  these 
  rocks 
  the 
  Indian 
  

   peninsula 
  has 
  been 
  remarkably 
  free 
  from 
  earth-movements, 
  and 
  the 
  

   structures 
  of 
  the 
  dykes 
  are 
  beautifully 
  preserved. 
  

  

  Many 
  of 
  these 
  rocks 
  are 
  augite-diorite 
  with 
  micropegmatite. 
  

   They 
  consist 
  mainly 
  of 
  augite, 
  approaching 
  hedenbergite, 
  and 
  

   plagioclase, 
  approaching 
  labradorite 
  in 
  composition. 
  The 
  crystal- 
  

   lization 
  of 
  these 
  two 
  minerals 
  has 
  been 
  approximately 
  simultaneous, 
  

  

  1 
  'Description 
  of 
  the 
  Western 
  Isles 
  of 
  Scotland,' 
  vol. 
  i. 
  (1819) 
  p. 
  363, 
  

   vol. 
  ii. 
  pp. 
  57 
  & 
  345. 
  

  

  2 
  See 
  also 
  Sollas, 
  Trans. 
  Roy. 
  Irish 
  Acad, 
  vol.xxx. 
  (1894) 
  p. 
  478. 
  

  

  