﻿35J: 
  REV. 
  E. 
  HILL 
  OX 
  THE 
  ROCKS 
  OE 
  

  

  frequent 
  than 
  acid. 
  Diabase 
  dykes, 
  so 
  abundant 
  in 
  Guernsey. 
  

   occur, 
  but 
  not 
  in 
  the 
  same 
  profusion. 
  A 
  very 
  large 
  one 
  cuts 
  the 
  

   granite 
  west 
  of 
  the 
  fort 
  called 
  Chateau 
  a 
  L'Etoc 
  : 
  there 
  is 
  another 
  

   under 
  the 
  west 
  wall 
  of 
  Tort 
  Albert. 
  The 
  largest 
  I 
  have 
  seen 
  cuts 
  

   the 
  cliff 
  of 
  the 
  Alannez 
  Quarry, 
  the 
  great 
  quarry 
  where 
  the 
  railway 
  

   ends. 
  This 
  last 
  is 
  called 
  by 
  M. 
  Bigot 
  an 
  andesite. 
  It 
  is 
  so 
  decom- 
  

   posed 
  that 
  determination 
  is 
  difficult 
  : 
  my 
  slide 
  seems 
  to 
  be 
  a 
  fine- 
  

   grained 
  diabase. 
  It 
  will 
  be 
  shown 
  hereafter 
  that 
  this 
  is 
  certainly 
  

   later 
  than 
  most 
  of 
  the 
  intrusions. 
  

  

  Like 
  all 
  the 
  other 
  islands, 
  Alderney 
  has 
  its 
  mica-trap 
  dyke. 
  This 
  

   occurs 
  at 
  the 
  east 
  end 
  of 
  the 
  Mannez 
  Quarry, 
  where 
  the 
  cliff- 
  face, 
  

   not 
  now 
  worked, 
  is 
  only 
  some 
  15 
  feet 
  high. 
  It 
  forms 
  two 
  or 
  three 
  

   vertical 
  sheets 
  about 
  a 
  yard 
  thick, 
  and 
  does 
  not 
  seem 
  to 
  have 
  

   altered 
  the 
  rock 
  which 
  it 
  penetrates. 
  Decomposition 
  has 
  reduced 
  most 
  

   into 
  a 
  brown 
  earthy 
  mass, 
  but 
  the 
  centre 
  of 
  a 
  spheroid 
  contained 
  a 
  

   small 
  kernel 
  in 
  tolerable 
  condition, 
  from 
  which 
  a 
  slide 
  has 
  been 
  cut. 
  

   It 
  shows 
  abundant 
  brown 
  mica 
  in 
  well-shaped 
  hexagonal 
  plates 
  

   of 
  variable 
  size, 
  some 
  reaching 
  "5 
  inch 
  in 
  diameter. 
  The 
  felspathic 
  

   matrix 
  appears 
  to 
  have 
  been 
  a 
  glass 
  crowded 
  with 
  lath-shaped 
  

   felspar 
  crystallites. 
  Besides 
  granules 
  of 
  iron-oxide, 
  there 
  may 
  be 
  

   apatite 
  and 
  a 
  little 
  sphene, 
  and 
  there 
  may 
  have 
  once 
  been 
  a 
  pyro- 
  

   xenic 
  constituent. 
  The 
  rock 
  belongs 
  to 
  the 
  Kersantite 
  group 
  of 
  the 
  

   mica-traps. 
  

  

  The 
  most 
  interesting 
  intrusion 
  is 
  one 
  which 
  forms 
  a 
  thick 
  mass 
  

   on 
  the 
  beach 
  and 
  in 
  the 
  crags 
  on 
  the 
  west 
  side 
  of 
  Fort 
  Albert. 
  It 
  

   occupies 
  some 
  70 
  yards 
  of 
  the 
  shore, 
  but 
  immediately 
  inland 
  splits 
  up 
  

   into 
  dykes, 
  which 
  may 
  be 
  seen 
  in 
  the 
  scarp 
  beneath 
  the 
  outer 
  lines. 
  

   This 
  rock 
  I 
  have 
  not 
  found 
  elsewhere. 
  I 
  took 
  it 
  to 
  be 
  the 
  same 
  

   in 
  composition 
  as 
  the 
  great 
  mass 
  at 
  Bon 
  Bepos 
  Bay 
  in 
  Guernsey, 
  

   which 
  it 
  externally 
  resembles. 
  Professor 
  Bonney 
  recognized 
  its 
  

   true 
  nature, 
  and 
  has 
  kindly 
  given 
  me 
  the 
  materials 
  for 
  the 
  following 
  

   description. 
  Its 
  specific 
  gravity 
  has 
  the 
  high 
  value 
  of 
  3-003. 
  Ma- 
  

   croscopically, 
  it 
  is 
  a 
  very 
  dark 
  holo 
  crystalline 
  rock 
  containing 
  large 
  

   crystals 
  of 
  a 
  pyroxenic 
  mineral, 
  whose 
  glittering 
  divisional 
  planes 
  are 
  

   interrupted 
  by 
  dark 
  spots. 
  Under 
  the 
  microscope 
  the 
  pyroxenic 
  

   mineral 
  is 
  seen 
  to 
  dominate, 
  and 
  is 
  exhibited 
  in 
  various 
  stages 
  of 
  

   transition 
  from 
  an 
  almost 
  colourless 
  augite 
  to 
  a 
  brown 
  and, 
  occa- 
  

   sionally 
  as 
  a 
  final 
  stage, 
  green 
  hornblende. 
  This 
  specimen 
  confirms 
  

   the 
  view 
  that 
  there 
  is 
  a 
  true 
  conversion 
  of 
  augite 
  into 
  hornblende. 
  

   For 
  instance, 
  the 
  slide 
  cuts 
  through 
  a 
  large 
  pyroxenic 
  crystal 
  of 
  

   irregular 
  outline. 
  The 
  outer 
  and 
  greater 
  part 
  is 
  of 
  a 
  strong 
  brown, 
  

   with 
  a 
  slight 
  olive 
  tinge, 
  markedly 
  dichroic. 
  In 
  the 
  middle 
  is 
  a 
  

   grain 
  of 
  colourless 
  augite 
  exhibiting 
  the 
  close 
  cleavage 
  of 
  diallage, 
  

   and 
  with 
  the 
  proper 
  extinction. 
  Little 
  filmy 
  offshoots 
  from 
  the 
  

   hornblende 
  penetrate 
  its 
  boundary, 
  and 
  in 
  some 
  places 
  are 
  inter- 
  

   calated 
  irregularly 
  between 
  the 
  cleavage-planes, 
  in 
  a 
  way 
  that 
  

   resembles 
  a 
  formation 
  in 
  sit", 
  far 
  more 
  than 
  an 
  intercalation 
  by 
  

   enclosure. 
  Small 
  grains 
  of 
  olivine 
  are 
  enclosed 
  in 
  this 
  crystal 
  ; 
  

   larger 
  ones 
  are 
  intercrystallized 
  with 
  the 
  other 
  constituents, 
  and 
  are 
  

   more 
  or 
  less 
  converted 
  into 
  serpentine 
  in 
  the 
  usual 
  way. 
  There 
  are 
  

  

  