﻿J. 
  "W. 
  JtTDD 
  03T 
  THE 
  SECONDAEX 
  EOCKS 
  OF 
  SCOTLAND, 
  239 
  

  

  great 
  mountain 
  masses; 
  the 
  dolerites 
  and 
  the 
  felsites 
  occur 
  as 
  in- 
  

   trusive 
  sheets 
  or 
  bosses 
  ; 
  while 
  the 
  basalts 
  and 
  felstones 
  form 
  narrow 
  

   dykes 
  and 
  veins, 
  of 
  which 
  the 
  sides 
  and 
  the 
  smaller 
  offshoots 
  pass 
  

   into 
  tachyhte 
  and 
  pitchstone 
  respectively. 
  Sometimes 
  different 
  

   varieties 
  of 
  rock-texture 
  are 
  exhibited 
  in 
  the 
  same 
  mass 
  ; 
  thus 
  the 
  

   6ides 
  of 
  a 
  mass 
  of 
  granite 
  often 
  pass 
  into 
  felsite, 
  and 
  a 
  vein 
  of 
  

   gabbro 
  is 
  bounded 
  b} 
  r 
  surfaces 
  of 
  dolerite 
  or 
  basalt. 
  That 
  these 
  

   masses 
  were 
  actually 
  forced 
  through 
  older 
  rocks, 
  is 
  shown 
  by 
  the 
  

   manner 
  in 
  which 
  the 
  latter, 
  whether 
  of 
  aqueous 
  or 
  igneous 
  origin, 
  

   are 
  disturbed 
  in 
  the 
  neighbourhood 
  of 
  the 
  larger 
  masses 
  of 
  the 
  kind 
  ; 
  

   that 
  they 
  were, 
  at 
  the 
  time 
  of 
  their 
  eruption, 
  in 
  a 
  fluid 
  condition, 
  is 
  

   proved 
  by 
  the 
  manner 
  in 
  Avhich 
  they 
  have 
  occupied 
  even 
  the 
  

   minutest 
  fissures 
  of 
  the 
  disturbed 
  rocks 
  ; 
  and 
  that 
  this 
  fluidity 
  was 
  

   connected 
  with 
  a 
  great 
  development 
  of 
  heat, 
  is 
  indicated 
  by 
  the 
  

   changes 
  to 
  which 
  they 
  have 
  given 
  rise 
  in 
  the 
  rocks 
  with 
  which 
  they 
  

   have 
  come 
  into 
  contact. 
  

  

  The 
  intrusive 
  rucks 
  seldom 
  exhibit 
  the 
  vesicular 
  (resulting 
  in 
  the 
  

   amygdaloidal)- 
  structure 
  ; 
  but 
  they 
  are, 
  on 
  the 
  other 
  hand, 
  often 
  co- 
  

   lumnar. 
  In 
  the 
  case 
  of 
  the 
  larger 
  masses 
  the 
  columns 
  are 
  generally 
  

   of 
  large 
  proportions, 
  but 
  indistinct 
  character, 
  while 
  in 
  the 
  dykes 
  and 
  

   veins 
  the}' 
  are 
  usually 
  small 
  and 
  often 
  very 
  minute, 
  being 
  in 
  all 
  cases 
  

   arranged 
  at 
  right 
  angles 
  to 
  the 
  surfaces 
  of 
  the 
  intersected 
  rock. 
  

  

  c. 
  Volcanic 
  At 
  i 
  jhm 
  urates. 
  — 
  But 
  besides 
  the 
  two 
  series 
  of 
  rocks 
  

   which 
  we 
  have 
  described 
  as 
  constituting 
  by 
  far 
  the 
  greater 
  portion 
  

   of 
  the 
  volcanic 
  products 
  of 
  the 
  Hebrides, 
  namely 
  the 
  lavas 
  and 
  

   eruptive 
  masses, 
  there 
  are 
  others 
  of 
  a 
  very 
  remarkable 
  character,., 
  

   which 
  have 
  hitherto 
  almost 
  wholly 
  escaped 
  observation. 
  Like 
  the 
  

   two 
  classes 
  of 
  rocks 
  just 
  described, 
  these 
  rocks 
  have 
  also 
  been 
  con- 
  

   founded 
  under 
  the 
  general 
  name 
  of 
  " 
  trap." 
  They 
  are 
  always 
  found 
  

   associated 
  with 
  the 
  great 
  masses 
  of 
  eruptive 
  rock, 
  and 
  are 
  often 
  

   remarkable 
  for 
  the 
  fantastic 
  forms 
  to 
  which, 
  in 
  consequence 
  of 
  their 
  

   peculiar 
  modes 
  of 
  yielding 
  to 
  denudation, 
  they 
  gire 
  rise. 
  On 
  their 
  

   fresh 
  surfaces 
  of 
  fracture 
  the 
  rocks 
  of 
  this 
  class 
  often 
  present 
  no 
  

   peculiarity 
  which 
  would 
  enable 
  us 
  to 
  infer 
  their 
  mode 
  of 
  origin 
  ; 
  it 
  

   is 
  only 
  on 
  the 
  surfaces 
  of 
  masses 
  which 
  have 
  had 
  their 
  remarkable 
  

   structure 
  " 
  developed 
  " 
  by 
  weathering 
  operations, 
  that 
  these 
  striking 
  

   features 
  are 
  revealed 
  to 
  us. 
  In 
  the 
  same 
  manner 
  that 
  a 
  highly 
  

   crystalline 
  and 
  apparently 
  altogether 
  unfossiliferous 
  limestone 
  (like 
  

   that 
  of 
  Durness, 
  for 
  example) 
  often 
  exhibits 
  on 
  its 
  weathered 
  faces 
  

   proofs 
  that 
  it 
  was 
  originally 
  made 
  up 
  of 
  a 
  congeries 
  of 
  beautiful 
  

   organisms, 
  so 
  these 
  apparently 
  structureless 
  igneous 
  rocks 
  are 
  by 
  

   the 
  same 
  agency 
  made 
  to 
  reassume 
  to 
  a 
  certain 
  extent 
  their 
  original 
  

   form. 
  The 
  weathered 
  surfaces 
  of 
  the 
  blocks 
  show 
  that 
  the 
  rocks 
  in 
  

   question 
  are 
  really 
  agglomerates, 
  composed 
  of 
  angular 
  and 
  sub- 
  

   angulir 
  masses 
  of 
  rock 
  of 
  all 
  sizes, 
  from 
  blocks 
  several 
  tons 
  in 
  

   weight 
  down 
  to 
  lapilli, 
  volcanic 
  sand 
  and 
  dust. 
  Many 
  of 
  these 
  

   fragments 
  are 
  shown 
  by 
  the 
  same 
  weathering 
  process 
  to 
  be 
  highly 
  

   vesicular 
  and 
  scoriaceous, 
  and 
  among 
  them 
  not 
  a 
  few 
  present 
  the 
  

   peculiar 
  characteristics 
  of 
  entire 
  or 
  fragmentary 
  " 
  volcanic 
  bombs." 
  

   In 
  many 
  cases 
  crystals 
  of 
  various 
  volcanic 
  minerals, 
  either 
  fractured 
  

  

  