﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  283 
  

  

  usually 
  subangular 
  or 
  well-rounded, 
  of 
  " 
  trappean 
  " 
  materials 
  

   mingled 
  with 
  others 
  derived 
  from 
  the 
  metamorphic 
  rocks. 
  These 
  

   blocks 
  are 
  so 
  firmly 
  cemented 
  together 
  by 
  a 
  sandy 
  matrix, 
  derived 
  

   from 
  the 
  same 
  materials, 
  that 
  the 
  great 
  joints 
  which 
  traverse 
  the 
  

   rock 
  in 
  all 
  directions, 
  and 
  give 
  rise 
  to 
  the 
  bold 
  and 
  fantastic 
  forms 
  

   which 
  it 
  assumes 
  under 
  denudation, 
  frequently 
  cleanly 
  divide 
  the 
  sepa- 
  

   rate 
  pebbles 
  of 
  which 
  the 
  rock 
  is 
  made 
  up, 
  even 
  when 
  they 
  are 
  com- 
  

   posed 
  of 
  the 
  hardest 
  known 
  materials. 
  These 
  conglomerates, 
  which 
  

   attain 
  to 
  thicknesses 
  of 
  hundreds 
  of 
  feet, 
  exhibit 
  great 
  variations 
  in 
  

   the 
  size 
  of 
  their 
  materials, 
  sometimes 
  passing 
  into 
  coarse 
  sandstone, 
  

   which 
  usually 
  occupies 
  lenticular 
  patches 
  in 
  their 
  midst, 
  and 
  at 
  others 
  

   containing 
  blocks 
  of 
  very 
  large 
  dimensions. 
  In 
  the 
  remarkable 
  na- 
  

   turally 
  isolated 
  pillar 
  of 
  this 
  conglomerate 
  near 
  Dunolly 
  Castle, 
  which, 
  

   is 
  known 
  by 
  the 
  name 
  of 
  Clach-a-choin, 
  there 
  is 
  a 
  single 
  block 
  of 
  

   felstone, 
  8 
  feet 
  long, 
  6 
  feet 
  broad, 
  and 
  5 
  feet 
  thick; 
  and 
  many 
  similar 
  

   blocks 
  of 
  almost 
  equal 
  dimensions 
  are 
  seen 
  at 
  other 
  points. 
  

  

  The 
  sandstones 
  of 
  the 
  Lorn 
  series 
  vary 
  from 
  very 
  coarse 
  grits, 
  

   almost 
  wholly 
  made 
  up 
  of 
  felspathic 
  materials, 
  to 
  fine 
  micaceous 
  

   sandstones, 
  exactly 
  resembling 
  those 
  so 
  abundant 
  in, 
  but 
  by 
  no 
  means 
  

   peculiar 
  to 
  the 
  Old 
  Red 
  Sandstone. 
  These 
  sandstones 
  are 
  usually 
  of 
  

   a 
  grey 
  colour, 
  but 
  sometimes 
  brownish-red. 
  They 
  frequently 
  exhibit 
  

   much 
  false-bedding, 
  and 
  sometimes 
  contain 
  fragments 
  of 
  rock 
  of 
  con- 
  

   siderable 
  size 
  but 
  of 
  flat 
  form, 
  such 
  as 
  can 
  easily 
  be 
  borne 
  along 
  by 
  

   currents 
  of 
  water. 
  In 
  manj* 
  places 
  they 
  include 
  bands 
  of 
  pebbles 
  of 
  

   very 
  various 
  size, 
  and 
  thus 
  graduate 
  into 
  the 
  conglomerates. 
  

  

  With 
  the 
  conglomerates, 
  sandstones, 
  and 
  " 
  traps 
  " 
  are 
  interstra- 
  

   tified 
  a 
  number 
  of 
  subordinate 
  beds 
  of 
  peculiar 
  character 
  ; 
  of 
  these 
  

   some 
  are 
  composed 
  of 
  well-stratified 
  materials 
  of 
  excessive 
  fineness, 
  

   which 
  appear 
  to 
  occupy 
  pond-like 
  cavities 
  in 
  the 
  midst 
  of 
  the 
  other 
  

   beds 
  ; 
  and 
  these 
  seem 
  to 
  be 
  made 
  up 
  of 
  the 
  fine 
  volcanic 
  dust 
  so 
  

   frequently 
  ejected 
  in 
  great 
  quantities 
  from 
  volcanic 
  vents. 
  In 
  other 
  

   cases 
  we 
  find 
  well-stratified 
  beds 
  of 
  small 
  angular 
  fragments 
  of 
  

   igneous 
  material 
  (lapilli 
  &c), 
  constituting 
  " 
  tuffs 
  " 
  precisely 
  similar 
  

   to 
  those 
  of 
  modern 
  volcanoes. 
  

  

  As 
  might 
  be 
  anticipated 
  from 
  the 
  characters 
  of 
  these 
  rocks, 
  there 
  is 
  

   no 
  appearance 
  of 
  a 
  regular 
  order 
  of 
  succession 
  among 
  them 
  ; 
  but 
  on 
  

   the 
  contrary 
  the 
  deposits 
  are 
  of 
  the 
  most 
  local 
  description. 
  In 
  order 
  

   to 
  illustrate 
  their 
  mode 
  of 
  occurrence 
  I 
  give 
  a 
  section 
  taken 
  along 
  

   the 
  south 
  end 
  of 
  the 
  island 
  of 
  Kerrera, 
  where 
  these 
  strata 
  are 
  

   especially 
  well 
  exposed 
  (woodcut, 
  fig. 
  10, 
  p. 
  284). 
  

  

  From 
  the 
  descriptions 
  already 
  given 
  of 
  the 
  relations 
  of 
  the 
  series 
  

   of 
  rocks 
  in 
  Lorn, 
  it 
  is 
  clear 
  that 
  the 
  lava 
  streams 
  must 
  sometimes 
  be 
  

   found 
  in 
  contact 
  with 
  the 
  slates, 
  at 
  other 
  times 
  with 
  the 
  conglome- 
  

   rates, 
  and 
  at 
  others, 
  again, 
  with 
  the 
  sandstones. 
  The 
  appearance 
  

   presented 
  by 
  each 
  of 
  these 
  junctions 
  I 
  propose 
  to 
  describe 
  in 
  illus- 
  

   trative 
  examples. 
  

  

  In 
  the 
  island 
  of 
  Kerrera 
  we 
  find 
  a 
  remarkably 
  interesting 
  example 
  

   of 
  the 
  contact 
  of 
  one 
  of 
  the 
  lava 
  streams 
  with 
  the 
  slate. 
  Here 
  a 
  pin- 
  

   nacle 
  of 
  slate 
  rock 
  protruding 
  above 
  thick 
  masses 
  of 
  sandstone 
  and 
  

   conglomerate, 
  in 
  the 
  manner 
  already 
  described 
  has 
  been 
  involved 
  in 
  

  

  