﻿248 
  J. 
  W. 
  JT7DD 
  ON 
  THE 
  SECONDARY 
  KOCiCS 
  OP 
  SCOTLAND. 
  

  

  piled 
  on 
  one 
  another 
  to 
  the 
  depth 
  of 
  more 
  than 
  1600 
  feet. 
  It 
  is 
  evi- 
  

   dent 
  that 
  the 
  beds 
  forming 
  the 
  summit 
  of 
  Benin 
  More, 
  which 
  are 
  com- 
  

   posed 
  of 
  alternations 
  of 
  lavas 
  and 
  agglomerates, 
  the 
  latter 
  exhibiting 
  

   in 
  the 
  fissures 
  of 
  ejected 
  blocks 
  so 
  many 
  beautiful 
  minerals 
  of 
  the 
  

   same 
  kind 
  as 
  are 
  found 
  in 
  similar 
  positions 
  in 
  connexion 
  with 
  ex- 
  

   isting 
  volcanoes, 
  constitute 
  . 
  the 
  last 
  vestige 
  of 
  a 
  volcanic 
  cone 
  

   formed 
  during 
  the 
  period 
  at 
  which 
  the 
  basaltic 
  lavas 
  were 
  ejected. 
  

   The 
  entire 
  absence 
  of 
  ejected 
  blocks 
  of 
  the 
  stratified 
  rocks 
  in 
  these 
  

   later 
  agglomerates 
  becomes 
  a 
  very 
  significant 
  fact 
  when 
  we 
  remem- 
  

   ber 
  that, 
  while 
  the 
  earlier 
  eruptions 
  of 
  acid 
  rocks 
  broke 
  through 
  

   masses 
  of 
  the 
  older 
  strata, 
  the 
  later 
  basaltic 
  masses 
  forced 
  their 
  way 
  

   through 
  the 
  midst 
  of 
  the 
  former. 
  

  

  Although 
  the 
  great 
  mass 
  of 
  the 
  lavas 
  constituting 
  the 
  great 
  

   plateaux 
  are 
  of 
  basaltic 
  composition, 
  yet 
  they 
  vary 
  greatly 
  among 
  

   themselves 
  in 
  many 
  minor 
  features 
  ; 
  and 
  among 
  them 
  are 
  occasionally 
  

   found 
  sheets 
  of 
  clinkstone*, 
  a 
  rock 
  of 
  a 
  more 
  or 
  less 
  acid 
  or 
  inter- 
  

   mediate 
  composition. 
  The 
  number 
  of 
  these 
  exceptional 
  lavas 
  is 
  

   found 
  to 
  greatly 
  increase 
  as 
  we 
  approach 
  their 
  points 
  of 
  origin, 
  as 
  in 
  

   passing 
  up 
  Beinn 
  More. 
  This 
  circumstance 
  (one 
  presented 
  in 
  many 
  

   existing 
  volcanoes) 
  is 
  easily 
  explained 
  by 
  the 
  well-known 
  fact 
  that 
  

   the 
  more 
  acid 
  lavas 
  as 
  a 
  general 
  rule 
  exhibit 
  a 
  much 
  less 
  perfect 
  

   fluidity 
  than 
  those 
  of 
  the 
  basic 
  class. 
  

  

  5. 
  Proofs 
  that 
  the 
  Central 
  Mountain-group 
  in 
  Mull 
  constitutes 
  the 
  

   Relic 
  of 
  a 
  great 
  Volcano. 
  — 
  No 
  one 
  familiar 
  with 
  the 
  phenomena 
  of 
  

   active 
  or 
  recently 
  extinct 
  volcanoes, 
  illustrated 
  as 
  these 
  have 
  been 
  

   by 
  the 
  researches 
  of 
  Lyell, 
  Darwin, 
  Von 
  Waltershausen, 
  Humboldt, 
  

   Yon 
  Buch, 
  and 
  many 
  other 
  investigators, 
  but 
  more 
  especially 
  by 
  

   Scrope, 
  in 
  his 
  classical 
  work 
  on 
  ' 
  The 
  Geology 
  and 
  Extinct 
  Volcanos 
  

   of 
  Central 
  France,' 
  can 
  for 
  one 
  moment 
  hesitate 
  as 
  to 
  the 
  interpreta- 
  

   tion 
  to 
  be 
  given 
  to 
  the 
  phenomena 
  which 
  I 
  have 
  described 
  as 
  being 
  

   presented 
  by 
  the 
  rocks 
  of 
  the 
  island 
  cf 
  Mull. 
  

  

  The 
  group 
  of 
  mountains 
  occupying 
  the 
  central 
  portion 
  of 
  the 
  

   island 
  is 
  clearly 
  the 
  greatly 
  denuded 
  core 
  of 
  an 
  immense 
  volcanic 
  

   pile, 
  the 
  great 
  accumulations 
  of 
  scoria? 
  and 
  lavas 
  which 
  formed 
  the 
  

   bulk 
  of 
  this 
  mountain-mass 
  having 
  been 
  to 
  a 
  great 
  extent 
  removed, 
  

   and 
  what 
  now 
  remains 
  to 
  our 
  study 
  being 
  little 
  more 
  than 
  the 
  skele- 
  

   ton 
  or 
  framework 
  of 
  the 
  vast 
  pile, 
  formed 
  by 
  the 
  consolidation 
  of 
  the 
  

   springs 
  of 
  liquefied 
  rock 
  which 
  rose 
  through 
  its 
  mass. 
  " 
  If," 
  wrote 
  

   the 
  late 
  Professor 
  Jukes, 
  " 
  we 
  could 
  follow 
  any 
  stream 
  of 
  lava 
  to 
  its 
  

   source 
  in 
  the 
  bowels 
  of 
  the 
  earth, 
  we 
  should 
  probably 
  find 
  it 
  

   changing, 
  under 
  varying 
  circumstances 
  of 
  depth 
  and 
  pressure, 
  from 
  

   scoriae 
  or 
  pumice 
  to 
  granite" 
  f. 
  By 
  the 
  study 
  of 
  the 
  relations 
  of 
  the 
  

   rocks 
  of 
  Mull 
  the 
  geologist 
  is 
  able 
  to 
  supply 
  the 
  most 
  complete 
  

   verification 
  of 
  this 
  conclusion, 
  and 
  indeed 
  to 
  illustrate 
  every 
  stage 
  

   of 
  the 
  transformation. 
  

  

  * 
  The 
  term 
  "porphyrite" 
  has 
  been 
  sometimes 
  applied 
  to 
  these 
  rooks; 
  but 
  

   they 
  agree 
  in 
  every 
  essential 
  respect 
  with 
  many 
  of 
  the 
  phonolitic 
  lavas 
  of 
  recent 
  

   volcanoes. 
  Rocks 
  precisely 
  resembling 
  those 
  of 
  Mull 
  occur 
  in 
  the 
  central 
  masses 
  

   of 
  Mont 
  Dore 
  and 
  the 
  Mezen 
  in 
  Central 
  France. 
  

  

  t 
  Encyclopaedia 
  Britannica, 
  Art. 
  " 
  Mineralogy 
  and 
  Geology." 
  

  

  