﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECOND 
  ARY 
  ROCKS 
  OE 
  SCOTXAND. 
  297 
  

  

  A 
  comparison 
  of 
  the 
  products 
  of 
  the 
  volcanoes 
  of 
  the 
  Newer 
  

   Palaeozoic 
  and 
  Tertiary 
  periods 
  respectively, 
  leads 
  us 
  to 
  the 
  following 
  

   conclusions. 
  

  

  During 
  both 
  of 
  these 
  periods 
  of 
  igneous 
  activity, 
  as 
  in 
  the 
  case 
  

   of 
  volcanic 
  outbursts 
  of 
  more 
  recent 
  times, 
  the 
  extrusion 
  of 
  lavas 
  of 
  

   highly 
  felspathic 
  composition 
  has, 
  as 
  a 
  general 
  rule, 
  preceded 
  those 
  

   of 
  the 
  basaltic 
  varieties. 
  

  

  But, 
  while 
  in 
  the 
  later 
  of 
  these 
  epochs, 
  the 
  Tertiary, 
  we 
  have 
  a 
  

   period 
  characterized 
  by 
  the 
  ejection 
  of 
  lavas 
  and 
  the 
  intrusion 
  of 
  

   subterranean 
  masses 
  of 
  highly 
  silicic 
  composition, 
  followed 
  after 
  a 
  

   long 
  interval 
  by 
  the 
  outburst 
  from 
  the 
  same 
  vents 
  of 
  highly 
  basic 
  

   igneous 
  rocks, 
  in 
  the 
  earlier 
  of 
  the 
  two 
  epochs 
  (the 
  Newer 
  Palaeo- 
  

   zoic) 
  we 
  witness, 
  apparently, 
  a 
  gradual 
  transition 
  during 
  the 
  enor- 
  

   mous 
  periods 
  through 
  which 
  the 
  eruptions 
  continued, 
  from 
  rocks 
  of 
  

   a 
  moderately 
  acid 
  to 
  others 
  of 
  a 
  moderately 
  basic 
  character. 
  

  

  The 
  last 
  efforts 
  of 
  volcanic 
  activity 
  in 
  each 
  of 
  these 
  epochs, 
  as 
  in 
  

   the 
  case 
  of 
  many, 
  if 
  not 
  all, 
  recent 
  exhibitions 
  of 
  volcanic 
  phenomena, 
  

   have 
  been 
  of 
  a 
  sporadic 
  character, 
  and 
  have 
  resulted 
  in 
  the 
  for- 
  

   mation 
  of 
  numerous, 
  but 
  comparatively 
  small, 
  volcanic 
  cones 
  or 
  

   " 
  puys." 
  

  

  At 
  the 
  earlier 
  epoch 
  the 
  range 
  of 
  great 
  volcanoes 
  was 
  thrown 
  

   up 
  along 
  a 
  line 
  ranging 
  N.E. 
  and 
  S.W., 
  coincident 
  with 
  the 
  

   direction 
  of 
  the 
  subterranean 
  forces 
  which, 
  both 
  long 
  before 
  and 
  

   subsequent 
  to 
  their 
  upheaval, 
  appear 
  to 
  have 
  determined 
  the 
  

   elevations, 
  foldings, 
  and 
  great 
  fractures 
  of 
  the 
  rocks 
  of 
  the 
  district. 
  

   But 
  at 
  the 
  later 
  epoch 
  the 
  volcanoes 
  were 
  thrown 
  up 
  along 
  quite 
  a 
  

   different 
  line, 
  one 
  ranging 
  1ST. 
  and 
  S.; 
  and 
  there 
  is 
  evidence 
  that 
  at 
  

   the 
  close 
  of 
  the 
  Mesozoic 
  era 
  new 
  axes 
  of 
  upheaval 
  were 
  originated, 
  

   differing 
  in 
  direction 
  from 
  those 
  which 
  had 
  prevailed 
  during 
  the 
  

   whole 
  of 
  the 
  periods 
  from 
  the 
  Silurian 
  to 
  the 
  Cretaceous. 
  

  

  But 
  to 
  the 
  consideration 
  of 
  these 
  general 
  conclusions 
  concerning 
  the 
  

   subterranean 
  forces, 
  and 
  their 
  influence 
  in 
  determining 
  the 
  charac- 
  

   ters 
  and 
  position 
  of 
  the 
  strata, 
  I 
  shall 
  have 
  to 
  return 
  in 
  the 
  con- 
  

   cluding 
  division 
  of 
  this 
  memoir. 
  

  

  2. 
  Influence 
  of 
  Volcanic 
  Action 
  in 
  determining 
  the 
  Characters 
  and 
  

   Relations 
  of 
  the 
  Secondary 
  Mocks 
  of 
  Scotland. 
  — 
  The 
  importance 
  of 
  a 
  

   clear 
  conception 
  of 
  the 
  history 
  of 
  the 
  two 
  great 
  periods 
  of 
  volcanic 
  

   activity, 
  which 
  respectively 
  preceded 
  and 
  followed 
  the 
  deposition 
  of 
  

   the 
  Mesozoic 
  strata, 
  to 
  a 
  complete 
  understanding 
  of 
  the 
  nature 
  and 
  

   significance 
  of 
  the 
  features 
  presented 
  to 
  us 
  by 
  the 
  latter, 
  is 
  shown 
  

   by 
  the 
  following 
  considerations. 
  

  

  (1) 
  The 
  main 
  features 
  of 
  the 
  physical 
  geography 
  of 
  the 
  area 
  in 
  

   which 
  the 
  Secondary 
  rocks 
  which 
  we 
  are 
  studying 
  were 
  deposited, 
  

   were 
  to 
  a 
  very 
  great 
  extent 
  originated 
  by 
  that 
  grand 
  exhibition 
  of 
  

   volcanic 
  activity 
  which 
  had 
  only 
  just 
  come 
  to 
  a 
  close 
  when 
  the 
  

   formation 
  of 
  the 
  latter 
  commenced. 
  Hence 
  this 
  pre-Mesozoic 
  vol- 
  

   canic 
  action 
  had 
  much 
  to 
  do 
  in 
  determining 
  the 
  nature 
  of 
  the 
  rocks 
  

   which 
  supplied 
  the 
  materials 
  of 
  the 
  Secondary 
  sediments, 
  and 
  also 
  

   the 
  conditions 
  under 
  which 
  they 
  were 
  accumulated. 
  

  

  (2) 
  Constituting, 
  as 
  we 
  have 
  seen 
  it 
  did, 
  a 
  period 
  of 
  rest 
  

  

  