﻿382 
  REV. 
  E. 
  HILL 
  ON 
  THE 
  ROCKS 
  OF 
  

  

  the 
  isolated 
  dome 
  of 
  Ortac. 
  Then 
  follows 
  the 
  deep 
  and 
  clear 
  Ortac 
  

   Channel, 
  in 
  breadth 
  abont 
  a 
  mile, 
  and 
  across 
  this 
  to 
  the 
  west 
  

   another 
  similar 
  ridge, 
  two 
  miles 
  long, 
  emerges 
  at 
  its 
  extremity 
  into 
  

   the 
  lonely 
  group 
  of 
  rocks 
  called 
  the 
  Casquets, 
  which 
  support 
  the 
  

   great 
  lighthouse 
  of 
  that 
  name, 
  outermost 
  sentinel 
  of 
  the 
  Archipelago. 
  

   The 
  dry-land 
  surface 
  in 
  the 
  region 
  treated 
  of 
  cannot 
  reach 
  five 
  

   square 
  miles, 
  but 
  it 
  is 
  scattered 
  over 
  an 
  area 
  ten 
  miles 
  long. 
  This 
  

   paper 
  accordingly 
  deals 
  with 
  the 
  geology 
  of 
  a 
  considerable 
  tract, 
  

   not 
  much 
  inferior 
  to 
  Charnwood 
  Forest 
  in 
  dimensions, 
  and 
  with 
  

   nearly 
  as 
  much 
  rock 
  above 
  the 
  water 
  as 
  in 
  Charnwood 
  emerges 
  

   from 
  the 
  Triassic 
  clay. 
  

  

  2. 
  Principal 
  Igneous 
  Masses. 
  

  

  Alderney 
  itself 
  consists 
  mainly 
  of 
  a 
  rock 
  commonly 
  called 
  granite. 
  

   This 
  constitutes 
  the 
  whole 
  of 
  the 
  west 
  and 
  centre 
  of 
  the 
  island. 
  

   It 
  has 
  been 
  worked 
  in 
  various 
  quarries, 
  especially 
  the 
  great 
  one 
  

   west 
  of 
  the 
  harbour, 
  and 
  is 
  well 
  exposed 
  along 
  the 
  north-west 
  coast. 
  

   To 
  the 
  eye 
  it 
  shows 
  felspar, 
  often 
  conspicuously 
  striped, 
  much 
  black 
  

   mica, 
  hornblende 
  in 
  very 
  vaiwing 
  amounts, 
  and 
  some 
  quartz, 
  gene- 
  

   rally 
  sufficient 
  in 
  quantity 
  to 
  render 
  the 
  popular 
  name 
  correct. 
  The 
  

   constituent 
  minerals 
  occur 
  in 
  particles 
  about 
  J 
  inch 
  across 
  over 
  

   most 
  of 
  the 
  area, 
  but 
  east 
  of 
  Port 
  Albert 
  the 
  rock 
  is 
  rather 
  finer 
  

   in 
  grain. 
  It 
  is 
  generally 
  very 
  well 
  crystallized 
  : 
  one 
  specimen 
  

   only, 
  from 
  Telegraph-Cable 
  Pay, 
  has 
  an 
  amorphous 
  appearance, 
  

   perhaps 
  an 
  effect 
  of 
  some 
  intrusion. 
  Under 
  the 
  microscope 
  the 
  

   biotite 
  is 
  seen 
  to 
  be 
  sometimes 
  replaced 
  by 
  aggregated 
  flakes, 
  possibly 
  

   chlorite. 
  

  

  On 
  the 
  shore 
  beneath 
  Port 
  Tourgis 
  the 
  usual 
  dark 
  rock 
  weather- 
  

   ing 
  grey 
  is 
  intermixed 
  with 
  a 
  paler 
  rock 
  weathering 
  pink 
  which 
  

   contains 
  much 
  more 
  quartz 
  than 
  is 
  usual. 
  I 
  am 
  inclined 
  to 
  regard 
  

   this 
  as 
  different, 
  but 
  the 
  question 
  requires 
  closer 
  examination. 
  It 
  

   is 
  surely 
  not 
  necessary 
  to 
  assume 
  that 
  every 
  coarsely 
  crystalline 
  

   mass 
  is 
  throughout 
  of 
  simultaneous 
  date, 
  any 
  more 
  than 
  are 
  the 
  

   basaltic 
  plateaus 
  of 
  Antrim 
  and 
  Mull. 
  The 
  owner 
  of 
  one 
  large 
  

   quarry 
  declared 
  to 
  me 
  that 
  it 
  furnished 
  eighteen 
  different 
  qualities 
  

   of 
  stone. 
  The 
  quarry 
  referred 
  to 
  contains 
  several 
  dykes 
  ; 
  still, 
  

   most 
  of 
  these 
  qualities 
  must 
  have 
  been 
  varieties 
  of 
  the 
  mass 
  which 
  

   those 
  penetrate. 
  

  

  At 
  one 
  locality 
  there 
  can 
  be 
  no 
  doubt 
  that 
  we 
  find 
  a 
  granite 
  dis- 
  

   tinguishable 
  from 
  the 
  rest. 
  This 
  constitutes 
  the 
  extremity 
  of 
  the 
  

   promontory 
  on 
  which 
  stands 
  the 
  fort 
  called 
  Chateau 
  a 
  l'Etoc, 
  Bibette 
  

   Head, 
  and 
  the 
  rocks 
  and 
  islets 
  in 
  the 
  intervening 
  Baie 
  de 
  Saie. 
  

   Most 
  is 
  much 
  decomposed, 
  but 
  on 
  Bibette 
  Head 
  it 
  has 
  been 
  quarried, 
  

   and 
  can 
  be 
  seen 
  to 
  consist 
  normally 
  of 
  white 
  felspar, 
  quartz, 
  black 
  

   mica, 
  and 
  hornblende 
  in 
  unusually 
  well-shaped 
  crystals, 
  nearly 
  

   \ 
  inch 
  in 
  length. 
  The 
  assemblage 
  forms 
  a 
  beautiful 
  white 
  granite. 
  

   Under 
  the 
  microscope 
  sphene 
  and 
  haematite 
  or 
  magnetite 
  can 
  be 
  

   recognized, 
  and 
  the 
  occasional 
  imbedding 
  of 
  small 
  felspar 
  crystals 
  in 
  

   quartz 
  is 
  interesting. 
  The 
  line 
  of 
  division 
  between 
  this 
  white 
  

  

  