﻿242 
  J. 
  W. 
  JXJDD 
  ON 
  THE 
  SECONDAET 
  BOCKS 
  OF 
  SC0TI4KP. 
  

  

  Secondary 
  strata, 
  fragments 
  of 
  the 
  whole 
  of 
  which 
  occur 
  in 
  the 
  vol- 
  

   canic 
  breccias 
  ; 
  while 
  at 
  Beinn 
  Shiant, 
  a 
  volcanic 
  vent 
  having 
  been 
  

   opened 
  in 
  a 
  tract 
  made 
  up 
  of 
  older 
  lavas, 
  easily 
  recognizable 
  por- 
  

   tions 
  of 
  these 
  occur 
  in 
  the 
  great 
  masses 
  of 
  agglomerate 
  which 
  con- 
  

   stitute 
  a 
  portion 
  of 
  the 
  relics 
  of 
  the 
  volcanic 
  cone. 
  

  

  In 
  many 
  cases 
  such 
  ejected 
  blocks 
  exhibit 
  evidence 
  of 
  having 
  

   been 
  submitted 
  to 
  a 
  certain 
  amount 
  of 
  heat. 
  Being 
  often 
  composed 
  

   of 
  materials 
  of 
  much 
  more 
  indestructible 
  character 
  than 
  the 
  consoli- 
  

   dated 
  ashes 
  &c. 
  in 
  which 
  they 
  are 
  imbedded, 
  they 
  are 
  frequen 
  tly 
  left 
  

   lying, 
  like 
  the 
  included 
  blocks 
  of 
  masses 
  of 
  Boulder-clay 
  which 
  have 
  

   been 
  destroyed 
  by 
  denudation, 
  upon 
  the 
  surfaces 
  of 
  the 
  mountains 
  ; 
  

   and 
  for 
  these, 
  indeed, 
  they 
  may, 
  by 
  a 
  casual 
  observer, 
  be 
  easily 
  mis- 
  

   taken 
  in 
  some 
  instances. 
  Such 
  an 
  idea, 
  however, 
  is 
  at 
  once 
  removed 
  

   by 
  an 
  attentive 
  study 
  of 
  the 
  whole 
  of 
  the 
  phenomena 
  and 
  conditions 
  

   of 
  the 
  case. 
  

  

  3. 
  Relations 
  of 
  the 
  Volcanic 
  MocJcs 
  to 
  one 
  another 
  and 
  to 
  the 
  Older 
  

   Deposits 
  in 
  the 
  Island 
  of 
  Midi. 
  — 
  We 
  now 
  turn 
  to 
  the 
  consideration 
  

   of 
  the 
  very 
  interesting 
  question 
  of 
  the 
  manner 
  in 
  which 
  the 
  several 
  

   kinds 
  of 
  volcanic 
  products, 
  which 
  we 
  have 
  described 
  in 
  the 
  preceding 
  

   pages, 
  are 
  associated 
  with 
  one 
  another, 
  and 
  of 
  the 
  positions 
  which 
  

   they 
  occupy 
  with 
  respect 
  to 
  the 
  strata 
  of 
  the 
  Palaeozoic 
  and 
  Mesozoie 
  

   epochs. 
  Owing 
  to 
  a 
  peculiar 
  combination 
  of 
  favourable 
  circum- 
  

   stances, 
  which 
  will 
  be 
  more 
  particularly 
  described 
  in 
  the 
  sequel, 
  the 
  

   island 
  of 
  Mull 
  furnishes 
  us 
  with 
  a 
  most 
  complete 
  and 
  beautiful 
  illus- 
  

   tration 
  of 
  the 
  relations 
  which 
  the 
  volcanic 
  rocks 
  bear 
  to 
  one 
  another; 
  

   and 
  we 
  therefore 
  propose, 
  in 
  the 
  first 
  instance, 
  to 
  describe 
  the 
  struc- 
  

   ture 
  of 
  this 
  district. 
  

  

  The 
  islaud 
  of 
  Mull, 
  with 
  the 
  adjacent 
  peninsula 
  of 
  Morvern 
  and 
  

   various 
  small 
  surrounding 
  islands, 
  form 
  portions 
  of 
  a 
  great 
  plateau, 
  

   composed 
  of 
  sheets 
  of 
  basaltic 
  lava 
  piled 
  upon 
  one 
  another 
  to 
  the 
  

   depth 
  of 
  nearly 
  2000 
  feet. 
  This 
  plateau, 
  which 
  is 
  now 
  broken 
  up 
  

   by 
  denuding 
  agencies 
  and 
  traversed 
  by 
  numerous 
  fiords 
  and 
  

   sounds, 
  is, 
  on 
  its 
  southern 
  side, 
  brought 
  into 
  abrupt 
  contact 
  with 
  

   the 
  Palaeozoic 
  rocks 
  through 
  the 
  agency 
  of 
  a 
  great 
  fault, 
  which 
  has 
  

   a 
  throw 
  of 
  at 
  least 
  1600 
  feet. 
  

  

  In 
  the 
  centre 
  of 
  this 
  basaltic 
  plateau 
  rises 
  a 
  group 
  of 
  lofty 
  moun- 
  

   tains, 
  about 
  twelve 
  miles 
  in 
  diameter, 
  many 
  of 
  the 
  peaks 
  of 
  which 
  rise 
  

   to 
  heights 
  of 
  from 
  2000 
  to 
  3000 
  feet 
  above 
  the 
  sea-level, 
  while 
  one 
  of 
  

   them, 
  Beinn 
  More, 
  attains 
  an 
  altitude 
  of 
  no 
  less 
  than 
  3172 
  feet. 
  

  

  Nothing 
  can 
  be 
  more 
  striking 
  than 
  the 
  contrast 
  presented 
  by 
  the 
  

   mountain-peaks 
  of 
  this 
  central 
  group 
  of 
  mountains 
  and 
  the 
  emi- 
  

   nences 
  of 
  the 
  surrounding 
  plateau. 
  While 
  the 
  latter 
  are 
  every- 
  

   where 
  characterized 
  by 
  those 
  remarkable 
  tabular 
  and 
  terraced 
  forms 
  

   which 
  have 
  suggested 
  the 
  name 
  of 
  " 
  trap 
  " 
  (forms 
  which, 
  by 
  con- 
  

   stant 
  repetition 
  in 
  these 
  districts, 
  tend 
  to 
  become 
  monotonous), 
  the 
  

   former 
  exhibit 
  the 
  most 
  wonderful 
  diversity 
  of 
  outlines 
  — 
  lofty, 
  

   smooth, 
  pyramidal 
  masses, 
  like 
  Beinn 
  Tsalla, 
  being 
  mingled 
  with 
  

   wildly 
  irregular, 
  and 
  fantastically 
  shaped 
  peaks, 
  like 
  Beinn 
  Buy, 
  

   Creach 
  Beinn, 
  Beinn 
  Varnach, 
  Dun-da-gu, 
  and 
  Craig 
  Craggen. 
  

  

  A 
  very 
  slight 
  examination 
  of 
  the 
  rocks 
  of 
  Mull 
  is 
  sufficient 
  to 
  

  

  