﻿210 
  PROF. 
  E. 
  HULL 
  OX 
  THE 
  TWO 
  BRITISH 
  

  

  21. 
  On 
  the 
  two 
  British 
  Types 
  of 
  the 
  Lower 
  Cambrian 
  beds, 
  and 
  the 
  

   Conditions 
  under 
  which 
  they 
  were 
  respectively 
  deposited. 
  By 
  

   Prof. 
  Edward 
  Hull, 
  LL.D., 
  F.K.S., 
  F.G.S. 
  (Read 
  January 
  11, 
  

  

  1882.) 
  

  

  The 
  subject 
  of 
  this 
  paper 
  forced 
  itself 
  on 
  my 
  mind 
  when 
  drawing 
  

   up 
  an 
  essay 
  " 
  On 
  the 
  Laurentian 
  beds 
  of 
  Ireland 
  ;" 
  and 
  I 
  had 
  origi- 
  

   nally 
  intended 
  to 
  place 
  it 
  as 
  an 
  appendix 
  to 
  that 
  communication* 
  ; 
  

   but 
  on 
  further 
  consideration 
  I 
  have 
  thought 
  it 
  of 
  sufficient 
  interest 
  

   to 
  deserve 
  separate 
  treatment. 
  

  

  I 
  propose 
  to 
  compare 
  the 
  Cambrian 
  beds 
  of 
  the 
  North-west 
  

   Highlands 
  of 
  Scotland, 
  which 
  I 
  shall 
  call 
  those 
  of 
  the 
  " 
  Caledonian 
  

   type," 
  with 
  their 
  supposed 
  representatives 
  in 
  South 
  Britain 
  and 
  

   Ireland, 
  which 
  I 
  shall 
  call 
  those 
  of 
  the 
  " 
  Hiberno-Cambrian 
  type," 
  

   as 
  regards 
  both 
  their 
  petrological 
  characters 
  and 
  their 
  fossil 
  contents, 
  

   with 
  the 
  view 
  of 
  showing 
  that 
  they 
  were 
  formed 
  respectively 
  on 
  

   either 
  side 
  of 
  a 
  ridge 
  (or 
  barrier) 
  of 
  Archaean 
  rocks, 
  which 
  a 
  recent 
  

   examination 
  of 
  the 
  north 
  of 
  Ireland 
  enables 
  me 
  to 
  trace 
  along 
  its 
  

   whole 
  course 
  in 
  the 
  British 
  Isles. 
  

  

  Before 
  entering, 
  however, 
  on 
  this 
  special 
  subject, 
  I 
  ought 
  first 
  to 
  

   state 
  the 
  geological 
  position 
  of 
  the 
  beds 
  I 
  here 
  call 
  " 
  Cambrian," 
  as 
  

   the 
  term 
  is 
  unfortunately 
  rather 
  widely 
  applied 
  at 
  present 
  amongst 
  

   British 
  geologists, 
  

  

  Definition 
  of 
  " 
  Cambrian 
  " 
  Beds. 
  — 
  For 
  my 
  present 
  purpose 
  I 
  use 
  

   the 
  term 
  within 
  the 
  limits 
  in 
  which 
  it 
  is 
  used 
  by 
  the 
  Geological 
  

   Survey 
  — 
  that 
  is, 
  the 
  "Lower 
  Cambrian 
  " 
  of 
  Sedgwick, 
  including 
  the 
  

   beds 
  below 
  the 
  Lingula-flags 
  and 
  the 
  ^lenevian 
  stage 
  of 
  Salter 
  and 
  

   Hicks. 
  To 
  this 
  stage, 
  therefore, 
  the 
  Llanberris, 
  the 
  Harlech, 
  and 
  

   Longmynd 
  rocks 
  are 
  presumably 
  referable. 
  

  

  I 
  assume, 
  theu, 
  the 
  above 
  great 
  group 
  of 
  conglomerates, 
  grits, 
  and 
  

   slates 
  to 
  be 
  the 
  representatives 
  in 
  time 
  of 
  the 
  Cambrian 
  beds 
  of 
  the 
  

   North-west 
  Highlands 
  of 
  Scotland, 
  and 
  for 
  the 
  following 
  reasons 
  : 
  — 
  

  

  In 
  the 
  Highlands 
  the 
  Cambrian 
  beds 
  are 
  overlain 
  transgressively 
  

   by 
  quartzites 
  and 
  limestones, 
  the 
  latter 
  containing 
  fossils 
  chiefly 
  of 
  

   American 
  Lower-Silurian 
  types, 
  as 
  Salter 
  has 
  shown, 
  but, 
  so 
  far 
  as 
  

   the 
  evidence 
  goes, 
  referable 
  to 
  the 
  stage 
  of 
  the 
  Llandeilo 
  bedst 
  of 
  

   "Wales. 
  There 
  is 
  therefore 
  in 
  the 
  Highlands 
  a 
  hiatus 
  between 
  

   the 
  Lower 
  Silurian 
  beds 
  and 
  the 
  Cambrian 
  sandstone, 
  which 
  we 
  

   may 
  well 
  suppose 
  is 
  represented 
  elsewhere 
  by 
  the 
  " 
  Menevian," 
  

   " 
  Lingula," 
  " 
  Tremadoc," 
  and 
  possibly 
  " 
  Arenig 
  " 
  beds. 
  It 
  is, 
  of 
  

   course, 
  impossible 
  to 
  say 
  whether 
  all, 
  or 
  how 
  many, 
  of 
  these 
  stages 
  

   are 
  wanting 
  ; 
  but 
  certainly 
  some 
  of 
  them 
  are 
  absent, 
  owing 
  to 
  the 
  

   unconformity 
  observable 
  in 
  the 
  North-west 
  Highlands 
  — 
  an 
  uncon- 
  

   formity 
  which 
  is 
  itself 
  partially 
  represented 
  even 
  in 
  Wales, 
  where 
  

   all 
  these 
  stages 
  occur. 
  

  

  * 
  Scient. 
  Trans. 
  Boy. 
  Dublin 
  Soc. 
  vol. 
  i. 
  ser. 
  2, 
  p. 
  240. 
  

   t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xv. 
  p. 
  374, 
  with 
  plates 
  ; 
  also 
  ' 
  Siluria,' 
  

   4th 
  edit. 
  pp. 
  164-5. 
  

  

  