﻿230 
  J. 
  "W. 
  JTJDD 
  ON 
  THE 
  SECONDAKY 
  KOCKS 
  0E 
  SCOTLAND. 
  

  

  place 
  the 
  extensive 
  workings 
  that 
  have 
  now 
  taken 
  place 
  have 
  

   exhibited 
  sections 
  which 
  leave 
  no 
  room 
  for 
  doubt 
  as 
  to 
  the 
  true 
  

   character 
  of 
  the 
  beds. 
  I 
  have 
  found 
  sections, 
  of 
  precisely 
  similar 
  

   deposits 
  on 
  the 
  shores 
  of 
  the 
  Sound 
  of 
  Ulva, 
  and 
  at 
  other 
  localities 
  

   in 
  the 
  Hebrides. 
  The 
  frequency 
  with 
  which 
  lakes, 
  shallow 
  lagoons, 
  

   and 
  pond-like 
  hollows 
  are 
  formed 
  in 
  a 
  district 
  subject 
  to 
  volcanic 
  

   eruptions 
  — 
  in 
  consequence 
  of 
  local 
  subsidences, 
  through 
  the 
  dam- 
  

   ming 
  up 
  of 
  riyer-conrses 
  by 
  lava-streams, 
  or 
  through 
  the 
  formation 
  

   of 
  " 
  pit-craters 
  " 
  by 
  explosion, 
  has 
  been 
  dwelt 
  upon 
  both 
  by 
  Mr. 
  

   Serope 
  and 
  Mr. 
  Dnrwin. 
  

  

  (6) 
  Local 
  masses 
  of 
  unstratiSed 
  volcanic 
  dust, 
  ashes, 
  scoria? 
  and 
  

   conglomerate 
  are, 
  by 
  no 
  means 
  rarely, 
  found 
  intercalated 
  with 
  the 
  

   great 
  streams 
  of 
  lava. 
  

  

  The 
  whole 
  of 
  these 
  facts 
  point 
  to 
  the 
  conclusion 
  that, 
  during 
  the 
  

   period 
  of 
  the 
  emission 
  of 
  the 
  great 
  lava 
  floods 
  which 
  form 
  the 
  

   enormous 
  plateaux 
  of 
  the 
  Hebrides 
  and 
  Antrim, 
  the 
  surfaces 
  over 
  

   which 
  they 
  flowed 
  were 
  above 
  the 
  sea-level, 
  and, 
  further, 
  that 
  

   intervals 
  of 
  sufficient 
  duration 
  occurred 
  between 
  the 
  outpourings 
  of 
  

   the 
  lava 
  streams 
  to 
  admit 
  of 
  the 
  formation 
  of 
  those 
  very 
  interesting 
  

   intercalated 
  deposits, 
  which 
  are 
  in 
  every 
  case 
  of 
  a 
  terrestrial, 
  fiuvia- 
  

   tile, 
  or 
  lacustrine 
  origin. 
  

  

  4. 
  Evidences 
  of 
  the 
  Former 
  Existence 
  of 
  great 
  Volcanic 
  mountains 
  in 
  

   the 
  district. 
  — 
  In 
  the 
  foregoing 
  paragraphs 
  we 
  have 
  shown 
  that 
  we 
  

   are 
  led 
  to 
  the 
  inevitable 
  conclusion, 
  not 
  only 
  that 
  the 
  great 
  pla- 
  

   teaux 
  of 
  the 
  Hebrides 
  and 
  the 
  north 
  of 
  Ireland 
  were 
  formed 
  by 
  the 
  

   gradual 
  accumulation 
  of 
  a 
  long 
  succession 
  of 
  lava-flows, 
  but 
  that, 
  

   like 
  the 
  similar 
  lavas 
  of 
  Sicily, 
  Auvergne, 
  and 
  Iceland, 
  they 
  were 
  

   actually 
  poured 
  out 
  on 
  the 
  surface 
  of 
  the 
  land. 
  

  

  Now, 
  in 
  all 
  the 
  cases 
  of 
  recently 
  formed 
  ]avas 
  (to 
  which 
  we 
  have 
  

   had 
  such 
  frequent 
  occasion 
  to 
  refer 
  as 
  exhibiting 
  so 
  perfect 
  an 
  

   identity 
  in 
  character 
  with 
  the 
  ancient 
  rocks 
  which 
  we 
  are 
  describing), 
  

   their 
  extrusion 
  has 
  be 
  en 
  accompanied 
  by 
  the 
  piling 
  up 
  of 
  ejected 
  

   fragmentary 
  and 
  semifluid 
  materials 
  around 
  the 
  vents, 
  so 
  as 
  to 
  

   form 
  those 
  more 
  or 
  less 
  conical 
  mountain-piles 
  which 
  we 
  call 
  " 
  vol- 
  

   canoes 
  ;" 
  and 
  it 
  is 
  from 
  the 
  summits 
  or 
  sides 
  of 
  these 
  that 
  the 
  

   great, 
  lava 
  streams 
  have 
  usually 
  flowed. 
  

  

  Can 
  we 
  then 
  hesitate 
  to 
  accept 
  the 
  conclusion 
  that 
  rocks 
  which 
  

   present 
  such 
  a 
  remarkable 
  identity 
  in 
  characters, 
  even 
  to 
  the 
  

   minutest 
  details, 
  must 
  have 
  had 
  a 
  similar 
  origin 
  ? 
  — 
  that 
  the 
  forma- 
  

   tion 
  of 
  these 
  ancient 
  subaerial 
  lavas 
  was 
  attended 
  with 
  the 
  produc- 
  

   tion 
  of 
  phenomena 
  similar 
  to 
  tho 
  c 
  e 
  which, 
  at 
  so 
  many 
  different 
  

   points 
  of 
  the 
  earth's 
  surface, 
  we 
  still 
  observe 
  to 
  be 
  the 
  constant 
  

   effects 
  of 
  volcanic 
  action 
  ?— 
  that, 
  in 
  short, 
  within 
  our 
  British 
  district 
  

   there 
  once 
  existed 
  volcanoes 
  from 
  which 
  those 
  great 
  volumes 
  of 
  

   igneous 
  materials 
  we 
  have 
  been 
  describing 
  were 
  ejected 
  ? 
  

  

  Further, 
  when 
  we 
  reflect 
  upon 
  the 
  fact 
  (one 
  patent 
  to 
  the 
  most 
  

   ■Casual 
  observer 
  of 
  the 
  geology 
  of 
  the 
  district) 
  that 
  the 
  masses 
  of 
  these 
  

   ancient 
  lavas 
  which 
  remain 
  for 
  our 
  study, 
  notwithstanding 
  their 
  

   vastness, 
  are 
  nevertheless 
  mere 
  isolated 
  relics, 
  which 
  have 
  escaped 
  

   destruction 
  by 
  denudation, 
  of 
  plateaux 
  which 
  must 
  originally 
  have 
  

  

  