﻿TYPES 
  OF 
  THE 
  CAiVIBKIAIf 
  BEDS. 
  213 
  

  

  The 
  Cambrian 
  beds 
  of 
  St. 
  David's 
  are 
  similar 
  in 
  composition 
  to 
  

   the 
  above, 
  and 
  have 
  yielded 
  to 
  Dr. 
  Hicks 
  a 
  peculiar 
  marine 
  fauna, 
  

   consisting 
  of 
  Trilobites, 
  Brachiopods, 
  &c, 
  the 
  most 
  ancient 
  group 
  

   of 
  these 
  forms 
  known 
  in 
  Britain. 
  

  

  The 
  beds 
  of 
  the 
  same 
  horizon 
  in 
  the 
  east 
  of 
  Ireland 
  are 
  not 
  dis- 
  

   similar, 
  allowance 
  being 
  made 
  for 
  geographical 
  space. 
  The}- 
  are 
  of 
  

   great 
  but 
  unknown 
  thickness, 
  consistiug 
  of 
  green 
  and 
  purple 
  grits, 
  

   quartzites, 
  and 
  rough 
  slates. 
  Conglomerate 
  beds 
  are 
  rare. 
  The 
  

   fossils 
  yielded 
  consist 
  only 
  of 
  tracks 
  and 
  burrows 
  of 
  marine 
  worms 
  

   and 
  two 
  species 
  of 
  a 
  peculiar 
  form 
  of 
  zoophyte 
  called, 
  after 
  its 
  dis- 
  

   coverer, 
  Olclhamia. 
  These 
  beds 
  are 
  unconformably 
  overlain 
  by 
  the 
  

   Lower 
  Silurian 
  slates 
  and 
  grits, 
  the 
  hiatus 
  here 
  recognizable 
  being 
  

   due 
  to 
  the 
  absence 
  of 
  the 
  Menevian, 
  Lingula, 
  and 
  Tremadoc 
  stages. 
  

  

  All 
  the 
  palseontological 
  evidence 
  at 
  present 
  existing 
  goes 
  to 
  show 
  

   that 
  the 
  Cambrian 
  beds 
  of 
  the 
  Hiberno-Cambrian 
  type 
  of 
  England, 
  

   "Wales, 
  and 
  Ireland 
  were 
  deposited 
  in 
  one 
  connected 
  basin, 
  and 
  in 
  

   the 
  waters 
  of 
  the 
  ocean; 
  and 
  in 
  this 
  as 
  well 
  as 
  in 
  their 
  peno- 
  

   logical 
  characters 
  they 
  offer 
  a 
  strong 
  contrast 
  to 
  the 
  beds 
  of 
  the 
  

   Caledonian 
  type, 
  which 
  I 
  concur 
  with 
  Professor 
  Ramsay 
  in 
  consi- 
  

   dering 
  to 
  have 
  been 
  deposited 
  in 
  a 
  lake-basin*. 
  

  

  These 
  differences 
  are 
  so 
  great 
  that 
  they 
  seem 
  of 
  themselves 
  to 
  

   point 
  to 
  deposition 
  within 
  the 
  limits 
  of 
  distinct 
  basins 
  ; 
  and 
  this 
  

   view 
  receives 
  further 
  confirmation 
  from 
  the 
  second 
  kind 
  of 
  evidence, 
  

   that 
  depending 
  on 
  certain 
  geometrical 
  considerations 
  I 
  am 
  now 
  

   about 
  to 
  adduce. 
  

  

  It 
  is 
  known 
  that 
  all 
  along 
  the 
  western 
  outcrop 
  of 
  the 
  Lower 
  

   Silurian 
  quartzites 
  and 
  limestones, 
  from 
  Loch 
  Emboli 
  in 
  the 
  north 
  

   to 
  Loch 
  Carron 
  in 
  the 
  south, 
  the 
  dip 
  is 
  easterly, 
  and 
  these 
  beds 
  pass 
  

   transgressively 
  across 
  the 
  truncated 
  edges 
  of 
  the 
  horizontal 
  beds 
  of 
  

   Cambrian 
  sandstonef. 
  These 
  sandstones 
  rest 
  upon 
  an 
  eroded 
  surface 
  

   of 
  Laurentian 
  beds, 
  with 
  a 
  (generally) 
  very 
  slight 
  inclination 
  east- 
  

   ward. 
  Between 
  the 
  slope 
  of 
  the 
  Silurian 
  beds 
  and 
  that 
  of 
  the 
  

   Laurentian 
  floor 
  there 
  is 
  generally 
  a 
  considerable 
  angle, 
  so 
  that 
  the 
  

   two 
  planes 
  ultimately 
  collide 
  — 
  as, 
  for 
  instance, 
  at 
  Loch 
  M~aree 
  in 
  

   the 
  south, 
  and 
  Loch 
  Assynt 
  in 
  the 
  north 
  %. 
  If, 
  therefore, 
  we 
  reduce 
  

   the 
  plane 
  of 
  the 
  Lower-Silurian 
  beds 
  to 
  its 
  original 
  nearly 
  horizontal 
  

   position, 
  the 
  other 
  plane, 
  formed 
  by 
  the 
  Laurentian 
  floor, 
  becomes 
  

   tilted 
  upwards 
  towards 
  the 
  east; 
  in 
  other 
  words, 
  it 
  rises 
  in 
  the 
  

   direction 
  of 
  the 
  central 
  Highlands. 
  This 
  is 
  illustrated 
  by 
  the 
  fol- 
  

   lowing 
  diagram 
  (fig. 
  2) 
  : 
  — 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxviii. 
  Dr. 
  Mourlon 
  refers 
  the 
  rocks 
  of 
  the 
  

   Ardennes 
  in 
  which 
  Oldhamia 
  radiata 
  occurs 
  to 
  the 
  "Cambrian 
  system" 
  (Geol. 
  

   de 
  la 
  Belgique, 
  t 
  i. 
  p. 
  31, 
  1880). 
  

  

  t 
  Murchison 
  and 
  Geikie, 
  Quart. 
  Journ. 
  Geol, 
  Soc. 
  vol. 
  xvii. 
  pp. 
  184, 
  11)0. 
  

   I 
  have 
  endeavoured 
  to 
  illustrate 
  this 
  point 
  by 
  a 
  diagram 
  in 
  my 
  paper 
  on 
  the 
  

   Northern 
  Highlands 
  of 
  Scotland 
  in 
  Scient. 
  Proc. 
  Roy. 
  Dubl. 
  Soc. 
  vol. 
  iii. 
  

   pi. 
  x. 
  fig. 
  5. 
  

  

  % 
  Murchison 
  and 
  Geikie, 
  supra 
  cit. 
  p. 
  191. 
  

  

  