﻿ALDEKNEY 
  AND 
  THE 
  CASQUETS. 
  389 
  

  

  S.S.W. 
  The 
  gneiss 
  of 
  Guernsey, 
  as 
  described 
  in 
  my 
  paper 
  on 
  that 
  

   island, 
  has 
  a 
  structure 
  which 
  runs 
  N. 
  to 
  S. 
  ; 
  and, 
  as 
  there 
  stated, 
  all 
  

   evidence 
  that 
  can 
  be 
  found 
  agrees 
  in 
  making 
  it 
  a 
  formation 
  prior 
  to 
  

   the 
  diorite 
  group. 
  Here, 
  then, 
  we 
  have 
  a 
  set 
  of 
  rock-masses 
  which 
  

   underwent 
  great 
  earth-movements 
  prior 
  to 
  times 
  which 
  themselves 
  

   were 
  Pre-Cambrian. 
  I 
  never 
  doubted 
  that 
  the 
  Guernsey 
  gneisses 
  

   were 
  Archaean, 
  but 
  I 
  did 
  not 
  feel 
  sure 
  that 
  proof 
  of 
  this 
  would 
  ever 
  be 
  

   obtained. 
  By 
  the 
  assistance 
  of 
  the 
  evidence 
  from 
  .Jersey, 
  Alderney 
  

   thus 
  leads 
  to 
  a 
  proof. 
  

  

  As 
  circumstances 
  render 
  it 
  improbable 
  that 
  I 
  shall 
  be 
  able 
  to 
  

   undertake 
  further 
  work 
  in 
  the 
  Channel 
  Islands, 
  I 
  wish 
  to 
  take 
  

   this 
  opportunity 
  of 
  making 
  some 
  corrections 
  in 
  my 
  former 
  paper 
  on 
  

   Guernsey 
  (Q. 
  J. 
  G. 
  S. 
  vol. 
  xl. 
  p. 
  404). 
  

  

  It 
  is 
  therein 
  assumed 
  throughout 
  that 
  the 
  main 
  mass 
  of 
  gneiss 
  

   is 
  a 
  non-igneous 
  rock. 
  I 
  have 
  now 
  no 
  doubt 
  that 
  much 
  of 
  it 
  is 
  

   igneous 
  and 
  crushed 
  ; 
  perhaps 
  all. 
  Ansted's 
  " 
  patch 
  of 
  clay-slate 
  in 
  

   Rocquaine 
  Bay 
  " 
  discussed 
  on 
  p. 
  406 
  may 
  be 
  explained 
  as 
  a 
  case 
  of 
  

   yielding 
  to 
  crush 
  on 
  a 
  very 
  large 
  scale, 
  and 
  the 
  pockets 
  of 
  coarser 
  

   material 
  there 
  described 
  can 
  now 
  be 
  fully 
  accounted 
  for 
  as 
  portions 
  

   which 
  have 
  offered 
  a 
  stouter 
  resistance 
  than 
  the 
  rest 
  ; 
  they 
  should 
  

   accordingly 
  afford 
  an 
  interesting 
  subject 
  of 
  study. 
  The 
  appearance 
  of 
  

   bedding 
  under 
  Port 
  Doyle 
  described 
  on 
  p. 
  409, 
  and 
  there 
  attributed 
  

   with 
  much 
  hesitation 
  to 
  an 
  inclusion 
  of 
  slaty 
  rock, 
  I 
  am 
  now 
  able 
  

   to 
  recognize 
  as 
  the 
  tongue 
  of 
  an 
  intruding 
  dyke 
  in 
  which 
  pressure 
  

   has 
  produced 
  the 
  semblance 
  of 
  a 
  slate. 
  

  

  Discussion. 
  

  

  The 
  Pkesident 
  remarked 
  that 
  the 
  evidence 
  as 
  to 
  the 
  age 
  of 
  the 
  

   Alderney 
  gneisses 
  was 
  clear. 
  

  

  Prof. 
  Bonnet 
  had 
  never 
  visited 
  Alderney, 
  though 
  he 
  was 
  acquainted 
  

   with 
  the 
  other 
  Channel 
  Islands, 
  but 
  he 
  knew 
  that 
  it 
  had 
  received 
  

   very 
  great 
  attention 
  from 
  Mr. 
  Hill. 
  

  

  The 
  age 
  of 
  these 
  gneissoid 
  rocks, 
  which 
  formed 
  an 
  uneven 
  floor 
  

   to 
  the 
  Lower 
  Palaeozoic 
  Hocks, 
  could 
  be 
  shown. 
  At 
  Omonville, 
  

   near 
  Cap 
  La 
  Hague, 
  the 
  " 
  gres 
  felspathique 
  " 
  rests 
  on 
  gneissoid 
  rock. 
  

   At 
  Tourlaville, 
  east 
  of 
  Cherbourg, 
  green 
  slates 
  (" 
  schistes 
  de 
  St. 
  Lo") 
  

   pass 
  up 
  into 
  the 
  "gres 
  felspathique," 
  which 
  is 
  overlain 
  by 
  the 
  " 
  gres 
  

   Armoricain," 
  perhaps 
  unconformably. 
  At 
  Coutances 
  gneissoid 
  rock 
  

   underlies 
  the 
  " 
  schistes 
  de 
  St. 
  Lo," 
  which 
  have 
  a 
  great 
  development 
  in 
  

   Western 
  JSTormandy. 
  These 
  gneissoid 
  rocks, 
  like 
  that 
  in 
  the 
  south 
  of 
  

   Guernsey, 
  are 
  doubtless 
  crushed 
  granites, 
  which 
  in 
  some 
  places, 
  as 
  

   at 
  Alderney, 
  are 
  little 
  modified. 
  The 
  strike 
  of 
  the 
  structure 
  is 
  very 
  

   roughly 
  IN", 
  and 
  S., 
  that 
  of 
  the 
  palaeozoics 
  about 
  E. 
  and 
  W. 
  There 
  is 
  

   no 
  opportunity 
  of 
  bringing 
  in 
  crushing 
  between 
  early 
  Cambrian 
  and 
  

   the 
  end 
  of 
  the 
  Lower 
  Palaeozoic, 
  so 
  that 
  these 
  gneissoid 
  rocks 
  must 
  be 
  

   Archaean. 
  The 
  " 
  gres 
  Armoricain 
  " 
  and 
  the 
  " 
  gres 
  felspathique 
  " 
  recall 
  

   the 
  quartzites 
  of 
  Northern 
  Scotland 
  and 
  the 
  Torridon 
  Sandstone, 
  and 
  

  

  