﻿244 
  J. 
  AY. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND, 
  

  

  Owing 
  to 
  the 
  very 
  different 
  modes 
  of 
  weathering 
  of 
  the 
  igneous 
  

   rocks 
  of 
  the 
  acid 
  and 
  basic 
  series 
  respectively, 
  it 
  is 
  comparatively 
  

   easy 
  for 
  the 
  geologist 
  to 
  study 
  the 
  relations 
  which 
  they 
  bear 
  to 
  one 
  

   another. 
  This 
  study 
  is 
  conclusive 
  as 
  to 
  their 
  relative 
  age 
  and 
  position. 
  

   The 
  rocks 
  of 
  the 
  acid 
  series 
  were 
  first 
  extruded 
  and 
  consolidated 
  ; 
  

   and 
  subsequently 
  there 
  was 
  forced 
  through 
  their 
  midst 
  a 
  fluid 
  mass 
  

   of 
  basic 
  materials 
  penetrating 
  the 
  innumerable 
  fissures 
  of 
  every 
  

   size 
  which, 
  by 
  the 
  upheaving 
  force, 
  were 
  rent 
  alike 
  in 
  the 
  earlier- 
  

   formed 
  igneous 
  rocks 
  and 
  the 
  surrounding 
  strata. 
  

  

  Overlapping 
  the 
  edges 
  of 
  the 
  felstone-lava 
  series 
  we 
  find 
  the 
  basaltic 
  

   lavas 
  of 
  the 
  great 
  surrounding 
  plateaux 
  ; 
  and 
  that 
  these 
  once 
  entirely 
  

   buried 
  the 
  former 
  is 
  shown 
  by 
  the 
  small 
  outljung 
  portions 
  of 
  basalt 
  

   which 
  at 
  some 
  points 
  still 
  remain 
  capping 
  the 
  felstones. 
  

  

  Finally, 
  we 
  must 
  notice 
  that 
  all 
  round 
  the 
  central 
  masses 
  of 
  

   eruptive 
  rocks 
  there 
  occur 
  patches 
  of 
  volcanic 
  agglomerate 
  and 
  

   breccias, 
  alternating 
  with 
  lava-sheets 
  and 
  traversed 
  by 
  innumerable 
  

   veins 
  and 
  dykes. 
  These 
  patches 
  constitute 
  the 
  last 
  remaining 
  ves- 
  

   tiges 
  of 
  the 
  great 
  conical 
  piles 
  of 
  fragmentary 
  materials 
  which 
  so 
  

   frequently 
  surmount 
  volcanic 
  vents. 
  As 
  will 
  be 
  hereafter 
  shown, 
  

   they 
  belong 
  to 
  two 
  different 
  periods 
  of 
  eruption. 
  

  

  Of 
  the 
  rocks 
  upon 
  which 
  the 
  streams 
  of 
  lava 
  which 
  constitute 
  so 
  

   large 
  a 
  portion 
  of 
  Mull 
  were 
  poured 
  out, 
  and 
  through 
  which 
  the 
  great 
  

   eruptive 
  masses 
  of 
  the 
  central 
  portion 
  of 
  the 
  island 
  have 
  forced 
  a 
  

   passage 
  for 
  themselves, 
  we 
  are 
  not 
  left 
  in 
  doubt. 
  At 
  the 
  south- 
  

   western 
  extremity 
  of 
  the 
  island 
  the 
  peninsula 
  of 
  the 
  Ross 
  of 
  Mull, 
  

   consisting 
  of 
  Lower 
  Silurian 
  gneiss 
  and 
  mica-schist, 
  with 
  an 
  eruptive 
  

   mass 
  of 
  granite 
  (which 
  will 
  be 
  shown 
  hereafter 
  to 
  be 
  of 
  later 
  

   Palaeozoic 
  age), 
  stretches 
  beyond 
  the 
  lavas 
  of 
  the 
  great 
  plateau, 
  and 
  

   is 
  continued 
  in 
  the 
  proximate 
  island 
  of 
  Iona, 
  composed 
  of 
  rocks 
  

   supposed 
  to 
  be 
  of 
  Laurentian 
  age. 
  In 
  the 
  south-eastern 
  part 
  of 
  

   Mull 
  a 
  series 
  of 
  old 
  volcanic 
  rocks, 
  alternating 
  with 
  breccias, 
  con- 
  

   glomerates, 
  and 
  sandstones 
  (all 
  of 
  later 
  Palaeozoic 
  age), 
  is 
  exposed 
  

   by 
  the 
  agency 
  of 
  great 
  faults 
  ; 
  while, 
  at 
  a 
  great 
  number 
  of 
  different 
  

   points 
  around 
  the 
  coast, 
  strata 
  of 
  Middle 
  and 
  Lower 
  Lias 
  age 
  are 
  

   exposed 
  beneath 
  the 
  lava 
  streams, 
  and 
  are 
  seen 
  to 
  rest 
  uncon- 
  

   formably 
  upon 
  the 
  several 
  palaeozoic 
  rocks 
  ; 
  all 
  these 
  older 
  formations 
  

   are 
  covered 
  unconformably 
  at 
  some 
  points 
  by 
  Cretaceous 
  rocks 
  re- 
  

   presenting 
  the 
  Upper 
  Greensand 
  and 
  Chalk. 
  

  

  "Wherever 
  we 
  approach 
  the 
  great 
  central 
  eruptive 
  masses, 
  whether 
  

   of 
  granite 
  or 
  gabbro, 
  the 
  whole 
  of 
  the 
  stratified 
  rocks, 
  both 
  of 
  Pri- 
  

   mary 
  and 
  Secondary 
  age, 
  are 
  found 
  to 
  have 
  suffered 
  great 
  disturbance 
  

   and 
  intense 
  metamorphism 
  — 
  the 
  limestones 
  passing 
  into 
  saceharoid 
  

   marble, 
  the 
  sandstones 
  into 
  quartzites, 
  and 
  the 
  shales 
  into 
  indurated 
  

   slaty 
  rocks 
  or 
  the 
  " 
  Lydian 
  stone 
  " 
  of 
  many 
  old 
  authors. 
  

  

  Lastly, 
  as 
  already 
  noticed, 
  fragments 
  of 
  nearly 
  the 
  whole 
  of 
  the 
  

   underlying 
  rocks 
  of 
  the 
  island 
  occur 
  imbedded 
  among 
  the 
  ashes, 
  la- 
  

   pilli, 
  and 
  scoriae 
  forming 
  the 
  volcanic 
  agglomerates 
  already 
  noticed. 
  

  

  4. 
  Sections 
  illustrating 
  the 
  Structure 
  of 
  the 
  Island 
  of 
  Mull.— 
  Tn 
  

   consequence 
  of 
  the 
  facility 
  with 
  which 
  certain 
  of 
  the 
  volcanic 
  rocks 
  

   of 
  Mull 
  yield 
  to 
  disintegrating 
  forces, 
  owing 
  to 
  their 
  extremely 
  

  

  