﻿3. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OP 
  SCOTLAND. 
  249 
  

  

  Moreover 
  the 
  history 
  of 
  the 
  accumulation 
  of 
  its 
  materials, 
  the 
  

   plan 
  of 
  its 
  architecture, 
  and 
  the 
  succession 
  of 
  the 
  changes 
  which 
  

   have 
  taken 
  place 
  during 
  its 
  formation 
  may 
  be 
  distinctly 
  read 
  in 
  the 
  

   ruins 
  of 
  this 
  old 
  volcanic 
  cone. 
  

  

  At 
  the 
  commencement 
  of 
  the 
  Tertiary 
  Period 
  the 
  area 
  now 
  con- 
  

   stituting 
  the 
  island 
  of 
  Mull 
  formed 
  part 
  of 
  an 
  old 
  land-surface 
  com- 
  

   posed 
  of 
  many 
  different 
  rocks. 
  It 
  is 
  not 
  difficult 
  to 
  reconstruct 
  the 
  

   geological 
  features 
  of 
  the 
  district 
  at 
  that 
  period. 
  Strata 
  of 
  Lauren- 
  

   tian 
  age 
  were 
  covered 
  unconformahly 
  by 
  highly 
  metamorphosed 
  

   Lower 
  Silurian 
  rocks, 
  through 
  the 
  midst 
  of 
  which 
  arose 
  a 
  great 
  

   eruptive 
  mass 
  of 
  granite. 
  Lying 
  unconformably 
  upon 
  the 
  Older 
  

   Palaeozoic 
  rocks 
  were 
  great 
  masses 
  of 
  old 
  lavas 
  &c. 
  belonging 
  to 
  the 
  

   Newer 
  Palaeozoic 
  periods, 
  while 
  the 
  whole 
  was 
  unconforrnably 
  over- 
  

   lain 
  by 
  many 
  patches 
  of 
  Jurassic 
  strata. 
  Lastly, 
  overlapping 
  the 
  

   whole 
  of 
  the 
  rocks 
  already 
  noticed, 
  and 
  lying 
  upon 
  their 
  upturned 
  

   and 
  denuded 
  edges, 
  were 
  a 
  number 
  of 
  tracts 
  composed 
  of 
  Cretaceous 
  

   rocks 
  only 
  recently 
  upheaved 
  above 
  the 
  sea-level. 
  

  

  Such 
  were 
  the 
  geological 
  features 
  of 
  the 
  land 
  upon 
  which 
  the 
  

   grand 
  and 
  long-continued 
  series 
  of 
  volcanic 
  phenomena 
  which 
  we 
  

   have 
  to 
  describe 
  were 
  displayed. 
  This 
  volcanic 
  action 
  appears 
  to 
  

   have 
  commenced 
  by 
  the 
  opening 
  of 
  vents 
  from 
  which 
  violently 
  

   escaping 
  steam 
  hurled 
  aloft 
  great 
  volumes 
  of 
  felspathic 
  ashes, 
  lapilli, 
  

   and 
  scoriae, 
  mingled 
  with 
  numerous 
  fragments 
  torn 
  from 
  the 
  rocks 
  

   through 
  which 
  the 
  explosive 
  discharges 
  took 
  place. 
  These 
  eruptions 
  

   of 
  fragmentary 
  matters 
  were 
  accompanied, 
  at 
  intervals, 
  by 
  the 
  out- 
  

   flow 
  of 
  streams 
  of 
  acid 
  or 
  trachytic 
  lavas 
  now 
  forming 
  felstones. 
  

   And, 
  further, 
  the 
  rising 
  liquid 
  mass 
  from 
  which 
  these 
  streams 
  were 
  

   fed, 
  to 
  a 
  certain 
  extent 
  forced 
  upwards 
  the 
  rapidly 
  accumulating 
  

   deposits 
  of 
  ejected 
  materials, 
  injecting 
  at 
  the 
  same 
  time 
  with 
  its 
  

   substance 
  the 
  numerous 
  fissures 
  which 
  the 
  expansive 
  force 
  could 
  

   not 
  fail 
  to 
  create 
  in 
  the 
  superincumbent 
  and 
  surrounding 
  rocks*. 
  

   By 
  slow 
  consolidation 
  under 
  pressure 
  the 
  fluid 
  matter 
  gave 
  rise 
  to 
  

   the 
  formation 
  of 
  masses 
  and 
  veins 
  of 
  felsite 
  and 
  granite. 
  Thus 
  

   there 
  was 
  gradually 
  built 
  up 
  a 
  great 
  volcanic 
  pile, 
  composed 
  princi- 
  

   pally 
  of 
  igneous 
  materials 
  of 
  the 
  acid 
  variety. 
  

  

  This 
  period 
  of 
  the 
  ejection 
  of 
  acid 
  materials, 
  which 
  was 
  doubt- 
  

   less 
  one 
  of 
  vast 
  duration, 
  appears 
  to 
  have 
  been 
  followed 
  by 
  one 
  

   of 
  inactivity; 
  for 
  there 
  is 
  distinct 
  evidence 
  that 
  before 
  the 
  second 
  

   series 
  of 
  eruptions 
  took 
  place 
  the 
  first-formed 
  cone 
  had 
  undergone 
  

   great 
  decay 
  : 
  explosions 
  had 
  destroyed 
  its 
  symmetry 
  ; 
  subsidences 
  

   had 
  taken 
  place 
  in 
  portions 
  of 
  its 
  mass 
  ; 
  and 
  denudation 
  had 
  removed 
  

   to 
  a 
  large 
  extent 
  its 
  covering 
  of 
  agglomerates 
  and 
  lavas, 
  exposing 
  

   the 
  granites 
  and 
  felsites 
  of 
  its 
  central 
  mass. 
  

  

  The 
  second 
  period 
  of 
  eruption 
  was 
  marked 
  by 
  the 
  prevalence 
  of 
  

  

  * 
  Whether 
  some 
  of 
  the 
  granitic 
  and 
  felsitic 
  hills 
  of 
  Mull 
  are 
  the 
  centres 
  

   of 
  great 
  masses 
  of 
  acid 
  lavas, 
  extruded 
  in 
  a 
  bulky 
  form 
  at 
  the 
  surface, 
  like 
  the 
  

   " 
  domitic 
  puys 
  " 
  of 
  Auvergne, 
  the 
  central 
  trachytic 
  masses 
  of 
  Rocca 
  Monfina, 
  or 
  

   the 
  bosses 
  in 
  the 
  midst 
  of 
  the 
  crater 
  of 
  Astroni, 
  it 
  is 
  not 
  possible, 
  in 
  the 
  present 
  

   degraded 
  condition 
  of 
  the 
  Mull 
  volcano, 
  absolutely 
  to 
  determine. 
  From 
  the 
  

   analogy 
  of 
  the 
  cases 
  we 
  have 
  cited 
  and 
  also 
  many 
  similar 
  ones, 
  however, 
  we? 
  

   may 
  reasonably 
  infer 
  such 
  to 
  have 
  really 
  been 
  the 
  case. 
  

  

  