﻿412 
  MR. 
  T. 
  H. 
  HOLLAND 
  ON 
  AUGrlTE-DIOEITES 
  WITH 
  [Aug. 
  1 
  89 
  7, 
  

  

  The 
  Madras 
  dykes 
  afford 
  a 
  f 
  airly 
  complete 
  answer 
  to 
  Prof. 
  Sollas's 
  

   difficulty. 
  The 
  cases 
  of 
  augite-diorite 
  dykes 
  containing 
  micro- 
  

   pegmatite, 
  which 
  have 
  been 
  carefully 
  examined 
  by 
  several 
  members 
  

   of 
  the 
  Geological 
  Survey 
  of 
  India, 
  are 
  not 
  one 
  or 
  two, 
  but 
  

   several 
  scores 
  ; 
  and 
  not 
  a 
  single 
  instance 
  has 
  been 
  recorded 
  of 
  a 
  

   dyke 
  crossed 
  by 
  a 
  later 
  igneous 
  intrusion 
  of 
  any 
  sort, 
  granitic 
  or 
  

   otherwise. 
  The 
  basic 
  dykes, 
  as 
  recently 
  remarked 
  by 
  my 
  colleague 
  

   Mr. 
  C. 
  S. 
  Miridlemiss, 
  who, 
  with 
  Mr. 
  F. 
  H. 
  Smith 
  and 
  myself, 
  has 
  

   during 
  the 
  last 
  season 
  kept 
  this 
  question 
  specially 
  in 
  view, 
  ' 
  show 
  

   nothing 
  to 
  suggest 
  a 
  veiniug 
  by 
  any 
  other 
  rock. 
  They 
  are 
  well- 
  

   defined, 
  weather 
  out 
  into 
  well-marked 
  boulders, 
  exfoliate 
  concen- 
  

   trically, 
  ring 
  like 
  metal 
  under 
  the 
  hammer, 
  and 
  completely 
  suggest 
  

   a 
  homogeneous 
  composition.' 
  It 
  would, 
  therefore, 
  be 
  quite 
  contrary 
  

   to 
  the 
  evidence 
  to 
  consider 
  the 
  micropegmatite 
  to 
  be 
  the 
  result 
  of 
  a 
  

   later 
  and 
  separate 
  intrusion, 
  while 
  for 
  regarding 
  it 
  as 
  derived 
  from 
  

   the 
  magma 
  which 
  gave 
  rise 
  to 
  the 
  augite 
  and 
  plagioclase 
  there 
  is 
  

   abundant 
  evidence, 
  both 
  positive 
  and 
  negative. 
  

  

  In 
  extending 
  his 
  explanation 
  of 
  the 
  relations 
  between 
  the 
  granite 
  

   and 
  gabbro 
  of 
  Carlingford 
  to 
  the 
  peculiar 
  structure 
  of 
  the 
  Whin 
  Sill, 
  

   Prof. 
  Soil 
  as 
  has 
  given 
  us 
  a 
  precedent 
  for, 
  per 
  contra, 
  extending 
  

   the 
  explanation 
  which 
  accounts 
  for 
  the 
  intimate 
  association 
  on 
  a 
  

   microscopic 
  scale 
  of 
  granophyric 
  material 
  with 
  augite-diorite 
  to 
  the 
  

   frequent 
  instances 
  which 
  have 
  been 
  recorded 
  of 
  larger 
  masses 
  of 
  

   granophyric 
  acid 
  rocks 
  occurring 
  in 
  intimate 
  association 
  with 
  basic 
  

   masses. 
  The 
  association 
  of 
  pyroxene-diorite 
  with 
  granophyre 
  in 
  

   such 
  well-known 
  instances 
  as 
  the 
  Stanner 
  Rock, 
  at 
  Penmaenmawr, 
  

   Carrock 
  Fell, 
  Carlingford, 
  and 
  the 
  "Western 
  Isles 
  of 
  Scotland 
  are 
  

   sufficiently 
  numerous 
  within 
  the 
  limited 
  area 
  of 
  the 
  British 
  Isles 
  to 
  

   suggest 
  some 
  sort 
  of 
  genetic 
  relationship 
  between 
  two 
  rocks 
  that 
  

   .are 
  found 
  so 
  often 
  together, 
  and 
  with 
  such 
  a 
  constancy 
  in 
  the 
  

   peculiarities 
  of 
  their 
  relations 
  one 
  to 
  the 
  other. 
  

  

  In 
  the 
  case 
  of 
  the 
  Madras 
  dykes 
  I 
  have 
  attempted 
  to 
  show 
  that 
  

   the 
  structure 
  of 
  the 
  rocks 
  indicates 
  the 
  formation 
  of 
  a 
  strong 
  

   framework 
  of 
  augite 
  and 
  plagioclase 
  before 
  the 
  consolidation 
  of 
  the 
  

   more 
  acid 
  residual 
  mother-liquor 
  in 
  the 
  interstices. 
  Such 
  an 
  

   occurrence 
  would 
  be 
  possible 
  where, 
  as 
  is 
  the 
  case 
  with 
  these 
  rocks, 
  

   the 
  augite 
  and 
  plagioclase 
  together 
  far 
  exceed 
  in 
  quantity 
  the 
  

   micropegmatite, 
  and 
  where 
  also 
  the 
  pressure 
  to 
  be 
  withstood 
  by 
  

   the 
  framework 
  so 
  formed 
  is 
  limited. 
  But 
  where 
  the 
  basic 
  minerals 
  

   are 
  in 
  subordinate 
  quantity, 
  and 
  where, 
  as 
  would 
  most 
  certainly 
  be 
  

   the 
  case 
  in 
  large 
  bosses, 
  the 
  pressure 
  is 
  much 
  greater, 
  it 
  would 
  be 
  

   only 
  natural 
  to 
  expect 
  that 
  the 
  formation 
  of 
  a 
  loose 
  framework 
  of 
  

   the 
  older 
  minerals 
  would 
  be 
  impossible, 
  and 
  there 
  would 
  as 
  a 
  

   consequence 
  be 
  a 
  more 
  complete 
  separation 
  into 
  distinct 
  masses 
  of 
  

   the 
  two 
  divisions 
  of 
  the 
  magma, 
  the 
  first-formed 
  giving 
  rise 
  to 
  

   an 
  augite-diorite, 
  while 
  the 
  second 
  phase 
  of 
  consolidation 
  would 
  be 
  

   represented 
  by 
  the 
  formation 
  of 
  an 
  acid 
  rock. 
  In 
  his 
  detailed 
  

   examination 
  of 
  the 
  gabbro 
  and 
  granophyre 
  of 
  Carrock 
  Fell, 
  Harker 
  

   approaches 
  a 
  similar 
  conclusion 
  in 
  stating 
  that 
  the 
  interval 
  between 
  

   the 
  consolidation 
  of 
  the 
  two 
  rocks 
  was 
  probably 
  a 
  short 
  one, 
  the 
  

  

  