﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  261 
  

  

  which 
  they 
  are 
  known 
  in 
  the 
  district 
  rendered 
  classical 
  by 
  the 
  ad- 
  

   mirable 
  researches 
  and 
  descriptions 
  of 
  Mr. 
  Scrope. 
  I 
  shall 
  now 
  

   proceed 
  to 
  describe 
  the 
  remarkably 
  interesting 
  evidences 
  which 
  re- 
  

   main 
  of 
  the 
  former 
  existence 
  of 
  such 
  " 
  puys 
  " 
  in 
  connexion 
  with 
  the 
  

   great 
  volcanoes 
  of 
  the 
  Hebrides. 
  

  

  In 
  the 
  peninsula 
  of 
  Ardnamurchan 
  the 
  highest 
  mountain, 
  Beinn 
  

   Shiant, 
  rises 
  to 
  the 
  height 
  of 
  1759 
  feet. 
  Its 
  very 
  peculiar 
  outlines 
  (see 
  

   woodcuts 
  figs. 
  2 
  & 
  3, 
  p. 
  262), 
  exhibiting 
  neither 
  the 
  terraced 
  slopes 
  

   of 
  the 
  masses 
  composed 
  of 
  superposed 
  lava 
  streams, 
  nor 
  the 
  pecu- 
  

   liar 
  contours 
  of 
  either 
  of 
  the 
  two 
  classes 
  of 
  eruptive 
  rocks, 
  are 
  alone 
  

   sufficient 
  to 
  arrest 
  the 
  attention 
  of 
  the 
  geologist 
  and 
  to 
  lead 
  him 
  to 
  

   its 
  careful 
  examination. 
  And 
  such 
  study 
  is 
  amply 
  rewarded. 
  The 
  

   mountain, 
  which 
  exhibits 
  a 
  series 
  of 
  grassy 
  slopes 
  reaching 
  almost 
  

   to 
  its 
  summit, 
  is 
  remarkable 
  for 
  the 
  far-stretching 
  and 
  boldly 
  

   shaped 
  spurs 
  which 
  on 
  all 
  sides 
  branch 
  out 
  from 
  a 
  common 
  centre. 
  

   On 
  a 
  nearer 
  approach 
  we 
  find 
  the 
  cause 
  of 
  these 
  striking 
  features 
  to 
  

   be 
  that 
  each 
  of 
  the 
  spurs 
  is 
  capped 
  by 
  masses 
  of 
  lava 
  usually 
  exhi- 
  

   biting 
  beautifully 
  columnar 
  forms, 
  while 
  the 
  green 
  slopes 
  below 
  are 
  

   composed 
  of 
  softer 
  and 
  more 
  easily 
  weathered 
  rocks. 
  The 
  moun- 
  

   tain 
  rises 
  near 
  the 
  line 
  of 
  junction 
  of 
  the 
  older 
  felspathic 
  and 
  

   basaltic 
  lavas 
  before 
  described. 
  

  

  The 
  lavas 
  of 
  Beinn 
  Shiant 
  are 
  intermediate 
  in 
  character 
  to 
  the 
  

   acid 
  and 
  basic 
  rocks 
  which 
  constitute 
  the 
  masses 
  of 
  the 
  older 
  and 
  great 
  

   central 
  volcanoes 
  ; 
  they 
  present 
  distinctive 
  features 
  by 
  which 
  they 
  are 
  

   easily 
  recognizable, 
  and 
  are 
  remarkable 
  alike 
  for 
  their 
  great 
  hard- 
  

   ness 
  and 
  their 
  power 
  of 
  resisting 
  atmospheric 
  denudation. 
  They 
  

   constitute 
  rocks 
  of 
  great 
  beauty, 
  varying 
  from 
  aggregates 
  of 
  crystals 
  

   of 
  glassy 
  felspar, 
  through 
  numerous 
  porphyritic 
  and 
  highly 
  crystal- 
  

   line 
  varieties 
  to 
  compact 
  felstones, 
  which 
  finally 
  pass 
  into 
  pitch- 
  

   stones 
  of 
  more 
  or 
  less 
  porphyritic 
  character, 
  these 
  last 
  being 
  some- 
  

   times 
  identical 
  in 
  their 
  mineralogical 
  features 
  and 
  the 
  peculiarities 
  

   of 
  columnar 
  structure 
  with 
  the 
  rock 
  of 
  the 
  well-known 
  Scur 
  of 
  

   Eigg. 
  These 
  lavas 
  appear 
  to 
  be 
  very 
  similar 
  in 
  character 
  to 
  a 
  

   dark-coloured 
  porphyritic 
  trachyte 
  which 
  occurs 
  at 
  Mont 
  Dore 
  les 
  

   Bains 
  in 
  Auvergne*. 
  

  

  generally 
  as 
  follows 
  : 
  — 
  By 
  constant 
  ejections 
  from 
  its 
  summit 
  a 
  volcano 
  is 
  con- 
  

   tinually 
  increasing 
  its 
  height 
  and 
  adding 
  to 
  the 
  length 
  of 
  the 
  column 
  through 
  

   which 
  fluid 
  materials 
  must 
  be 
  raised 
  in 
  order 
  to 
  produce 
  a 
  central 
  eruption. 
  

   In 
  the 
  course 
  of 
  this 
  action 
  a 
  point 
  will 
  certainly 
  be 
  arrived 
  at 
  when 
  it 
  becomes 
  

   easier 
  for 
  the 
  subterranean 
  forces 
  to 
  rend 
  new 
  fissures 
  in 
  the 
  flanks 
  of 
  the 
  

   mountain 
  than 
  to 
  raise 
  a 
  column 
  of 
  lava 
  to 
  its 
  elevated 
  summit. 
  At 
  this 
  stage 
  

   of 
  its 
  history 
  Etna 
  now 
  appears 
  to 
  have 
  arrived, 
  the 
  formation 
  of 
  new 
  para- 
  

   sitical 
  cones 
  on 
  its 
  flanks 
  alternating 
  with 
  comparatively 
  feeble 
  ejections 
  from 
  

   its 
  summit. 
  But, 
  as 
  the 
  bulk 
  and 
  solidity 
  of 
  a 
  volcanic 
  cone 
  are 
  continually 
  

   added 
  to 
  by 
  these 
  operations, 
  the 
  subterranean 
  forces 
  will 
  be 
  gradually 
  forced 
  

   to 
  seek 
  new 
  vents 
  at 
  lower 
  levels 
  in 
  the 
  plains 
  around 
  the 
  mountain, 
  and 
  

   thus 
  give 
  rise 
  to 
  the 
  formation 
  of 
  " 
  puys." 
  In 
  Ischia, 
  the 
  Lipari 
  Islands, 
  and 
  

   many 
  other 
  volcanic 
  districts, 
  we 
  have 
  admirable 
  opportunities 
  afforded 
  to 
  us 
  

   for 
  tracing 
  the 
  sequence 
  of 
  this 
  very 
  interesting 
  series 
  of 
  operations. 
  

  

  * 
  In 
  the 
  islands 
  of 
  Lipari 
  and 
  Salina 
  we 
  have 
  many 
  beautiful 
  illustrations 
  of 
  

   lava 
  streams, 
  composed 
  of 
  dark-coloured, 
  often 
  nearly 
  black, 
  trachyte, 
  with 
  

   disseminated 
  crystals 
  of 
  sanidine. 
  These 
  rocks 
  are 
  precisely 
  similar 
  to 
  those 
  

  

  Q.J. 
  G.S. 
  No. 
  119. 
  v 
  

  

  