﻿282 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  will 
  serve 
  to 
  illustrate 
  the 
  peculiar 
  relations 
  of 
  the 
  several 
  rocks 
  in 
  

   these 
  cases. 
  

  

  The 
  rocks 
  on 
  which 
  the 
  volcanic 
  series 
  of 
  Lorn 
  usually 
  rests 
  are 
  

   black 
  clay-slates, 
  abounding 
  in 
  fine 
  cubical 
  crystals 
  of 
  pyrites 
  and 
  

   often 
  traversed 
  by 
  numerous 
  veins 
  of 
  quartz. 
  This 
  formation 
  is 
  

   perhaps 
  tbe 
  highest 
  member 
  preserved 
  to 
  us 
  of 
  that 
  great 
  series 
  of 
  

   Lower 
  Silurian 
  rocks 
  which, 
  bent 
  into 
  endless 
  folds 
  aud 
  greatly 
  me- 
  

   tamorphosed, 
  occupies 
  so 
  large 
  a 
  portion 
  of 
  the 
  Scottish 
  Highlands. 
  

   At 
  Seil 
  and 
  Easdale 
  it 
  is 
  extensively 
  worked 
  for 
  roofing-slates 
  ; 
  as 
  

   yet, 
  unfortunately, 
  it 
  has 
  not 
  yielded 
  any 
  fossils 
  ; 
  but 
  it 
  exhibits 
  in 
  

   the 
  island 
  of 
  Seil 
  interbedded 
  igneous 
  rocks, 
  apparently 
  of 
  contem- 
  

   poraneous 
  character. 
  

  

  The 
  only 
  point 
  at 
  which 
  the 
  series 
  of 
  rocks 
  so 
  well 
  displayed 
  in 
  

   Lorn 
  is 
  seen 
  in 
  juxtaposition 
  with 
  the 
  Mesozoic 
  sedimentary 
  rocks 
  

   and 
  the 
  Tertiary 
  lavas 
  is 
  in 
  the 
  south-eastern 
  part 
  of 
  Mull. 
  Here, 
  

   unfortunately, 
  among 
  the 
  wonderfully 
  disturbed 
  and 
  greatly 
  meta- 
  

   morphosed 
  rocks, 
  which 
  are 
  exposed 
  only 
  in 
  precipitous 
  and 
  altogether 
  

   inaccessible 
  cliffs, 
  I 
  have 
  sought 
  in 
  vain 
  for 
  any 
  simple 
  section 
  illus- 
  

   trating 
  the 
  relations 
  of 
  these 
  three 
  series 
  of 
  deposits. 
  Nevertheless, 
  

   after 
  carefully 
  tracing 
  the 
  positions 
  of 
  such 
  masses 
  as 
  can 
  be 
  reached 
  

   and 
  studied, 
  there 
  appears 
  to 
  be 
  no 
  room 
  for 
  doubt 
  that 
  the 
  various 
  

   members 
  of 
  the 
  Mesozoic 
  series 
  rest 
  indifferently 
  upon 
  the 
  denuded 
  

   lavas 
  of 
  Lorn 
  and 
  the 
  older 
  rocks, 
  and 
  that 
  they 
  are 
  themselves 
  

   covered 
  unconformably 
  by 
  the 
  Tertiary 
  volcanic 
  rocks. 
  

  

  4. 
  Succession 
  of 
  Rocks 
  in 
  Lorn. 
  — 
  The 
  series 
  of 
  rocks 
  which 
  we 
  have 
  

   been 
  describing 
  has 
  usually 
  been 
  represented 
  as 
  consisting 
  of 
  beds 
  of 
  

   conglomerate 
  and 
  sandstone 
  at 
  the 
  base, 
  overlain 
  by 
  a 
  great 
  mass 
  of 
  

   " 
  trap 
  " 
  rocks. 
  A 
  careful 
  examination 
  of 
  the 
  district, 
  however, 
  

   proves 
  that 
  its 
  structure 
  is 
  by 
  no 
  means 
  so 
  simple 
  as 
  this 
  statement 
  

   would 
  imply. 
  The 
  outpouring 
  of 
  the 
  great 
  lava 
  streams 
  was, 
  in 
  part 
  

   at 
  least, 
  contemporaneous 
  with, 
  as 
  well 
  as 
  subsequent 
  to, 
  the 
  depo- 
  

   sition 
  of 
  the 
  conglomerates 
  and 
  sandstones. 
  This 
  is 
  proved 
  by 
  the 
  

   alternation 
  of 
  the 
  " 
  trap 
  " 
  rocks 
  with 
  the 
  conglomerates 
  and 
  sand- 
  

   stones, 
  and 
  by 
  the 
  fact 
  that 
  the 
  materials 
  of 
  the 
  latter 
  are 
  to 
  a 
  

   great 
  extent 
  derived 
  from 
  the 
  former. 
  Good 
  examples 
  of 
  the 
  alter- 
  

   nations 
  of 
  the 
  traps 
  with 
  the 
  sandstones 
  and 
  conglomerates 
  are 
  to 
  be 
  

   seen 
  near 
  Dunolly. 
  

  

  At 
  the 
  base 
  of 
  the 
  whole 
  series 
  of 
  the 
  Lorn 
  rocks, 
  and 
  in 
  imme- 
  

   diate 
  contact 
  with 
  the 
  subjacent 
  slates, 
  is 
  often 
  found 
  a 
  breccia 
  of 
  

   very 
  interesting 
  character. 
  It 
  is 
  wholly 
  composed 
  of 
  perfectly 
  an- 
  

   gular 
  fragments, 
  sometimes 
  of 
  considerable 
  size, 
  of 
  the 
  slate 
  and 
  

   quartz 
  rocks 
  on 
  which 
  it 
  rests 
  ; 
  this 
  breccia 
  exhibits 
  no 
  trace 
  of 
  

   stratification 
  or 
  of 
  its 
  materials 
  having 
  been 
  sorted 
  or 
  acted 
  upon 
  

   by 
  water. 
  It 
  sometimes 
  forms 
  masses 
  of 
  considerable 
  thickness, 
  

   which 
  appear 
  to 
  have 
  been 
  accumulated 
  upon 
  old 
  terrestrial 
  surfaces 
  

   of 
  the 
  slate 
  rocks 
  by 
  purely 
  subaerial 
  agencies. 
  

  

  The 
  conglomerates 
  of 
  the 
  Lorn 
  series 
  present 
  very 
  remarkable 
  

   characters, 
  which 
  are 
  familiar 
  to 
  all 
  who 
  have 
  examined 
  the 
  pictur- 
  

   esque 
  cliffs 
  on 
  either 
  side 
  of 
  the 
  beautiful 
  Bay 
  of 
  Oban. 
  They 
  are 
  

   made 
  up 
  of 
  blocks 
  of 
  very 
  various 
  sizes, 
  occasionally 
  angular 
  but 
  

  

  