﻿J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  285 
  

  

  a 
  lava 
  current. 
  The 
  dark-blue 
  highly 
  cleaved 
  slate 
  is 
  changed 
  to 
  a 
  

   soft 
  grey 
  rock, 
  divided 
  into 
  angular 
  fragments 
  by 
  numerous 
  joints, 
  

   while 
  the 
  brilliant 
  crystals 
  of 
  iron-pyrites 
  with 
  which 
  the 
  slate 
  is 
  

   studded 
  are 
  converted 
  into 
  a 
  black 
  amorphous 
  substance 
  still 
  filling 
  

   the 
  original 
  cavities 
  (see 
  woodcut, 
  fig. 
  9, 
  p. 
  281). 
  

  

  Many 
  examples 
  of 
  the 
  contact 
  of 
  a 
  lava 
  stream 
  with 
  a 
  bed 
  of 
  sand- 
  

   stone 
  are 
  exhibited 
  in 
  Lorn 
  and 
  the 
  adjoining 
  islands. 
  The 
  sandstone, 
  

   while 
  still 
  exhibiting 
  more 
  or 
  less 
  clearly, 
  under 
  the 
  lens, 
  its 
  original 
  

   granular 
  structure, 
  has 
  for 
  some 
  depth 
  from 
  its 
  surface 
  been 
  con- 
  

   verted 
  into 
  a 
  substance 
  of 
  intense 
  hardness, 
  and 
  the 
  mixture 
  of 
  fel- 
  

   spathie 
  and 
  siliceous 
  materials 
  of 
  which 
  it 
  is 
  composed 
  has 
  evidently 
  

   undergone 
  incipient 
  fusion. 
  

  

  "Where 
  the 
  lava 
  has 
  flowed 
  over 
  a 
  bed 
  of 
  conglomerate, 
  the 
  pebbles 
  

   of 
  the 
  latter 
  have 
  been 
  frequently 
  caught 
  up 
  and 
  enveloped 
  in 
  the 
  

   mass 
  of 
  vesicular 
  and 
  scoriaceous 
  rock 
  forming 
  the 
  base 
  of 
  the 
  lava 
  

   stream, 
  in 
  a 
  manner 
  which 
  has 
  often 
  been 
  described 
  as 
  taking 
  place 
  

   in 
  the 
  case 
  of 
  the 
  products 
  of 
  recent 
  volcanoes*. 
  

  

  In 
  a 
  road-cutting 
  in 
  the 
  vicinity 
  of 
  Oban 
  I 
  found 
  a 
  very 
  interesting 
  

   illustration 
  of 
  the 
  phenomena 
  presented 
  at 
  the 
  contact 
  of 
  some 
  of 
  the 
  

   lava 
  streams 
  with 
  the 
  beds 
  below. 
  The 
  section 
  was 
  as 
  follows 
  : 
  — 
  

  

  (a) 
  Compact 
  dark-coloured 
  felstone 
  lava, 
  which 
  has 
  apparently 
  undergone 
  con- 
  

  

  siderable 
  alteration 
  ; 
  here 
  and 
  there 
  a 
  hollow 
  vesicle 
  occurs 
  in 
  the 
  mass. 
  A 
  

   thickness 
  of 
  15 
  or 
  20 
  feet 
  of 
  this 
  rock 
  is 
  seen 
  in 
  the 
  section. 
  

  

  (b) 
  About 
  nine 
  inches 
  or 
  a 
  foot 
  from 
  its 
  base 
  this 
  mass 
  of 
  lava 
  becomes 
  highly 
  

  

  vesicular 
  or 
  scoriaceous, 
  the 
  cavities 
  being 
  strikingly 
  flattened 
  and 
  drawn 
  

   out. 
  Many 
  burnt-looking 
  fragments 
  (lapilli) 
  are 
  caught 
  up 
  in 
  this 
  part 
  of 
  

   the 
  lava 
  stream. 
  

  

  (c) 
  For 
  a 
  depth 
  of 
  about 
  2 
  feet 
  below 
  we 
  have 
  a 
  confused 
  mass 
  of 
  vesicular 
  lava, 
  

  

  rounded 
  blocks 
  of 
  lava, 
  pebbles 
  of 
  quartz, 
  &c, 
  all 
  imbedded 
  in 
  a 
  matrix 
  of 
  

   highly 
  scoriaceous 
  rock. 
  The 
  lower 
  part 
  of 
  this 
  mass 
  passes 
  in 
  places 
  

   into 
  a 
  sandy 
  conglomerate 
  ; 
  the 
  ordinary 
  coarse 
  sand 
  consisting 
  of 
  lava 
  de- 
  

   tritus 
  constituting 
  the 
  matrix. 
  

  

  (d) 
  Conglomerates, 
  composed 
  of 
  pebbles 
  of 
  trap 
  and 
  of 
  the 
  Lower 
  Silurian 
  rocks, 
  

   with 
  some 
  unmistakable 
  volcanic 
  bombs, 
  and 
  with 
  a 
  matrix 
  of 
  very 
  coarse 
  

   sand, 
  2 
  feet 
  thick. 
  In 
  this 
  bed 
  are 
  lenticular 
  patches 
  of 
  a 
  very 
  fine-grained 
  

   sedimentary 
  material, 
  probably 
  stratified 
  volcanic 
  dust. 
  

  

  (e) 
  Beds 
  of 
  well-stratified 
  and 
  finely 
  laminated 
  red 
  sandstone, 
  very 
  similar 
  in 
  

  

  character 
  to 
  the 
  typical 
  rocks 
  of 
  the 
  Old 
  .Red 
  Sandstone. 
  

  

  The 
  stream 
  of 
  lava 
  of 
  which 
  (a) 
  and 
  (b) 
  form 
  the 
  lower 
  portion 
  

   appears 
  to 
  be 
  40 
  or 
  50 
  feet 
  in 
  thickness, 
  to 
  be 
  highly 
  vesicular 
  in 
  its 
  

   upper 
  part, 
  and 
  to 
  be 
  covered 
  directly 
  by 
  another 
  lava 
  stream. 
  The 
  

   appearance 
  presented 
  by 
  (6) 
  and 
  (o) 
  is 
  exactly 
  such 
  as 
  would 
  be 
  

   produced 
  by 
  a 
  lava 
  stream 
  with 
  a 
  cindery 
  crust 
  which 
  it 
  rolls 
  over 
  

   as 
  it 
  flows 
  along, 
  in 
  the 
  manner 
  so 
  graphically 
  described 
  by 
  Mr. 
  

   Scrope, 
  and 
  passing 
  over 
  a 
  mass 
  of 
  shallow 
  -water 
  deposits, 
  the 
  mate- 
  

   rials 
  of 
  which 
  it 
  entangles 
  in 
  its 
  course. 
  

  

  The 
  sandstones 
  and 
  conglomerates, 
  which 
  in 
  places 
  attain 
  to 
  a 
  

   thickness 
  of 
  several 
  hundreds 
  of 
  feet, 
  appear 
  to 
  be 
  wholly 
  confined 
  to 
  

   the 
  lower 
  part 
  of 
  the 
  volcanic 
  series 
  of 
  Lorn. 
  In 
  its 
  upper 
  part 
  

  

  * 
  In 
  the 
  island 
  of 
  Lipari 
  there 
  occur 
  beautiful 
  instances 
  of 
  lava 
  streams 
  of 
  

   glassy 
  character 
  having 
  entangled 
  in 
  their 
  mass 
  many 
  fragments 
  of 
  the 
  older 
  

   rocks, 
  that 
  have 
  evidently 
  strewed 
  the 
  surfaces 
  over 
  which 
  they 
  flowed. 
  

  

  