﻿ALDERNEY 
  AND 
  THE 
  CASQTTETS. 
  387 
  

  

  small 
  inclusion 
  showing 
  the 
  striped 
  structure 
  so 
  remarkable 
  in 
  that 
  

   island, 
  and 
  in 
  the 
  Alderney 
  Quarry, 
  north 
  of 
  Coque 
  Lihou, 
  I 
  saw 
  a 
  

   fragment 
  larger, 
  but 
  less 
  characteristically 
  marked. 
  

  

  5. 
  Age 
  op 
  the 
  Grits. 
  

  

  Of 
  this 
  nothing 
  has 
  hitherto 
  been 
  known. 
  Ansted 
  only 
  con- 
  

   jectured 
  that 
  they 
  might 
  be 
  Triassic. 
  Now, 
  however, 
  a 
  fairly 
  

   definite 
  horizon 
  can 
  be 
  assigned 
  to 
  them. 
  It 
  has 
  been 
  shown 
  that 
  

   they 
  lie 
  unconformably 
  on 
  the 
  granites, 
  and 
  also 
  contain 
  pebbles 
  of 
  

   the 
  dykes 
  which 
  cut 
  these. 
  Moreover, 
  they 
  are 
  in 
  general 
  not 
  

   penetrated 
  by 
  the 
  abundant 
  series 
  of 
  dykes. 
  Ansted 
  mentions 
  that 
  

   " 
  the 
  veins 
  traversing 
  the 
  syenite 
  rock 
  of 
  which 
  the 
  Casquets 
  are 
  

   mainly 
  composed 
  are 
  said 
  to 
  extend 
  into 
  the 
  sandstone 
  overlying." 
  

   At 
  my 
  visit 
  not 
  a 
  vestige 
  of 
  igneous 
  rock 
  was 
  to 
  be 
  seen, 
  either 
  as 
  

   underlying 
  mass 
  or 
  as 
  dyke. 
  On 
  Burhou 
  I 
  saw 
  no 
  dyke. 
  Along 
  

   miles 
  of 
  the 
  Alderney 
  shore 
  I 
  saw 
  no 
  dykes. 
  The 
  insufficiency 
  of 
  

   negative 
  evidence 
  is 
  shown 
  by 
  the 
  two 
  dykes 
  already 
  described 
  as 
  seen 
  

   in 
  the 
  Mannez 
  Quarry 
  and 
  by 
  one 
  on 
  the 
  shore 
  east 
  of 
  the 
  quarry, 
  

   which 
  perhaps 
  is 
  a 
  continuation 
  : 
  there 
  is, 
  however, 
  abundant 
  

   evidence 
  that 
  the 
  series 
  is 
  later 
  than 
  the 
  majority 
  of 
  the 
  intrusions. 
  

   But, 
  on 
  the 
  other 
  hand, 
  the 
  mica-trap 
  dyke 
  in 
  the 
  Mannez 
  Quarry, 
  

   as 
  it 
  cuts 
  them, 
  gives 
  a 
  posterior 
  limit 
  to 
  their 
  age, 
  for 
  it 
  clearly 
  

   belongs 
  to 
  the 
  "kersantons 
  " 
  of 
  Brittany, 
  which 
  Dr. 
  Barrois 
  assigns 
  to 
  

   the 
  close 
  of 
  the 
  Carboniferous 
  period 
  ; 
  thus 
  these 
  beds 
  must 
  be 
  far 
  

   earlier 
  than 
  the 
  Trias. 
  In 
  search 
  of 
  further 
  evidence, 
  I 
  made 
  a 
  

   traverse 
  of 
  the 
  Cotentin 
  from 
  St. 
  Malo 
  and 
  Avranches 
  up 
  to 
  Cher- 
  

   bourg, 
  and 
  so 
  discovered 
  the 
  beds 
  at 
  Omonville, 
  above 
  described 
  as 
  

   undistinguishable. 
  These 
  can 
  be 
  seen 
  to 
  rest 
  on 
  a 
  gneissic 
  rock, 
  

   and 
  are 
  described 
  by 
  M. 
  Bigot 
  as 
  intercalated 
  between 
  the 
  " 
  Phyl- 
  

   lades 
  " 
  (of 
  St. 
  Lo 
  ?) 
  and 
  the 
  " 
  Gres 
  Armoricain." 
  In 
  general, 
  on 
  the 
  

   mainland, 
  it 
  is 
  the 
  " 
  Conglomerat 
  Pourpre 
  " 
  which 
  precedes 
  the 
  " 
  Gres 
  

   Armoricain." 
  The 
  pebbles 
  in 
  that 
  conglomerate, 
  where 
  I 
  have 
  seen 
  

   it, 
  are 
  exclusively 
  quartz, 
  and 
  its 
  colour 
  is 
  extremely 
  deep. 
  But 
  the 
  

   Alderney 
  grits 
  contain 
  quartz 
  in 
  abundance, 
  though 
  other 
  materials 
  

   as 
  well 
  ; 
  and 
  their 
  colour, 
  as 
  has 
  been 
  mentioned, 
  sometimes 
  approaches 
  

   the 
  " 
  pourpre 
  " 
  tint. 
  We 
  m.'dj 
  therefore 
  assign 
  this 
  important 
  series 
  

   to 
  the 
  Upper 
  Cambrian 
  (of 
  Lapworth) 
  *. 
  

  

  6. 
  Comparisons 
  and 
  General 
  Conclusions. 
  

  

  The 
  north-east 
  corner 
  of 
  Jersey 
  is 
  occupied 
  by 
  a 
  remarkable 
  series 
  

   of 
  conglomerates. 
  They 
  have 
  attracted 
  the 
  attention 
  of 
  every 
  

   observer, 
  but 
  there 
  is 
  a 
  singular 
  absence 
  of 
  evidence 
  with 
  regard 
  to 
  

   their 
  age. 
  Ansted 
  conjectured 
  that 
  they 
  might 
  be 
  equivalent 
  to 
  

  

  * 
  At 
  several 
  points 
  about 
  high 
  water-mark, 
  especially 
  between 
  Forts 
  Quenard 
  

   and 
  Homeaux 
  Florains, 
  there 
  occur 
  on 
  bedding 
  faces 
  narkings 
  which 
  closely 
  

   resemble 
  organic 
  impressions, 
  and 
  which 
  high 
  authorities 
  thought 
  were 
  refer- 
  

   able 
  to 
  Dictyonema. 
  But 
  I 
  have 
  since 
  found 
  markings 
  extremely 
  similar 
  to 
  

   these 
  on 
  joint 
  faces 
  and 
  boulders, 
  which 
  must 
  be 
  of 
  recent 
  origin. 
  [Mr. 
  Ethe- 
  

   ridge 
  informs 
  me 
  that 
  they 
  are 
  produced 
  by 
  Limpets.] 
  

  

  2f2 
  

  

  