﻿J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCK.S 
  OF 
  SCOTLAND. 
  269 
  

  

  ditions, 
  spread 
  in 
  vast 
  sheets, 
  which 
  insert 
  themselves 
  along 
  a 
  plane 
  

   of 
  weakness 
  between 
  two 
  sets 
  of 
  beds, 
  and 
  often 
  proceed 
  to 
  enormous 
  

   distances 
  from 
  the 
  centres 
  of 
  eruption. 
  

  

  Very 
  wonderful 
  and 
  striking 
  are 
  the 
  relations 
  between 
  the 
  great 
  

   intrusive 
  sheets 
  of 
  dolerite 
  and 
  basalt, 
  connected 
  with 
  the 
  great 
  

   centres 
  of 
  volcanic 
  action, 
  and 
  tbe 
  stratified 
  rocks 
  which 
  they 
  tra- 
  

   verse. 
  These 
  relations 
  are 
  exhibited 
  in 
  a 
  very 
  beautiful 
  manner 
  

   along 
  the 
  magnificent 
  range 
  of 
  cliffs 
  that 
  forms 
  the 
  eastern 
  boundary 
  

   of 
  the 
  peninsula 
  of 
  Trotternish 
  in 
  Skye, 
  and 
  also 
  in 
  Baasay, 
  Apple- 
  

   cross, 
  and 
  the 
  south 
  of 
  Mull. 
  So 
  remarkable 
  is 
  the 
  regularity 
  with 
  

   which 
  many 
  of 
  these 
  sheets 
  of 
  molten 
  rock 
  have 
  flowed 
  between 
  the 
  

   same 
  two 
  strata 
  for 
  great 
  distances, 
  that 
  it 
  is 
  not 
  surprising 
  they 
  

   were 
  for 
  a 
  long 
  period 
  regarded 
  as 
  being 
  lava 
  streams 
  contempora- 
  

   neous 
  in 
  date 
  with 
  the 
  sediments 
  among 
  which 
  they 
  he. 
  A 
  close 
  

   examination, 
  however, 
  shows 
  that 
  these 
  sheets 
  of 
  igneous 
  material 
  

   alter 
  alike 
  the 
  rocks 
  lying 
  above 
  and 
  below 
  them, 
  and 
  that 
  they 
  are 
  

   altogether 
  destitute 
  of 
  the 
  vesicular 
  character 
  leading 
  to 
  the 
  produc- 
  

   tion 
  of 
  amygdaloids, 
  as 
  well 
  as 
  other 
  features 
  always 
  presented 
  by 
  true 
  

   lava 
  streams. 
  When, 
  too, 
  their 
  courses 
  are 
  followed 
  over 
  considerable 
  

   distances, 
  they 
  are 
  found, 
  in 
  places, 
  either 
  bifurcating 
  into 
  separate 
  

   sheets 
  which 
  enclose 
  masses 
  of 
  the 
  stratified 
  rocks, 
  or 
  entangling 
  

   fragments 
  of 
  these 
  in 
  their 
  midst, 
  or 
  cutting 
  for 
  a 
  time 
  across 
  the 
  

   beds, 
  or 
  sending 
  off 
  processes 
  and 
  veins, 
  or 
  terminating 
  abruptly 
  in 
  

   wedge-shaped 
  masses, 
  or 
  breaking 
  suddenly 
  through 
  the 
  superin- 
  

   cumbent 
  strata, 
  and 
  so 
  reaching 
  the 
  surface. 
  

  

  The 
  rocks 
  of 
  wbich 
  the 
  great 
  intrusive 
  felspathic 
  masses 
  are 
  usually 
  

   composed, 
  include 
  many 
  different 
  varieties 
  of 
  felsite, 
  usually 
  more 
  

   or 
  less 
  quartziferous, 
  and 
  often 
  beautifully 
  porphyritic 
  in 
  struc- 
  

   ture 
  ; 
  these, 
  in 
  many 
  places, 
  by 
  the 
  appearance 
  of 
  scattered 
  crystals 
  

   of 
  hornblende 
  in 
  their 
  mass 
  pass 
  into 
  a 
  fine-grained 
  syenite-granite. 
  

   The 
  intrusive 
  sheets 
  of 
  basic 
  rock 
  consist 
  in 
  almost 
  all 
  cases 
  of 
  dole- 
  

   rite, 
  often 
  containing 
  much 
  olivine, 
  and 
  passing 
  on 
  the 
  one 
  hand 
  

   into 
  fine-grained 
  gabbro, 
  and 
  on 
  the 
  other 
  into 
  many 
  varieties 
  of 
  

   basalt. 
  

  

  Besides 
  these 
  larger 
  masses, 
  the 
  igneous 
  intrusions 
  of 
  all 
  ages 
  give 
  

   rise 
  to 
  the 
  formation 
  of 
  dykes 
  and 
  veins 
  in 
  prodigious 
  numbers. 
  

   Those 
  of 
  felspathic 
  composition 
  appear 
  to 
  be 
  for 
  the 
  most 
  part 
  con- 
  

   fined 
  to 
  within 
  comparatively 
  moderate 
  distances 
  from 
  the 
  eruptive 
  

   centres 
  ; 
  those 
  of 
  basaltic 
  composition, 
  on 
  the 
  other 
  hand, 
  are 
  found 
  

   proceeding 
  to 
  extraordinary 
  distances 
  from 
  them. 
  Prof. 
  Geikie 
  has 
  

   even 
  speculated, 
  with 
  much 
  show 
  of 
  probability, 
  on 
  the 
  existence 
  of 
  a 
  

   connexion 
  between 
  the 
  great 
  basaltic 
  dykes 
  which 
  traverse 
  the 
  whole 
  

   of 
  the 
  rocks 
  in 
  the 
  north 
  of 
  England 
  and 
  the 
  great 
  focus 
  of 
  igneous 
  ac- 
  

   tivity 
  in 
  the 
  Hebrides. 
  Basaltic 
  dykes 
  with 
  such 
  a 
  connexion 
  certainly 
  

   traverse 
  all 
  the 
  rocks 
  in 
  the 
  west 
  of 
  Scotland 
  in 
  prodigious 
  numbers 
  ; 
  

   and 
  the 
  manner 
  in 
  which 
  they 
  sometimes, 
  through 
  greater 
  relative 
  

   capacity 
  for 
  resisting 
  denuding 
  forces, 
  stand 
  up 
  like 
  immense 
  walls, 
  

   and 
  at 
  others, 
  by 
  their 
  more 
  rapid 
  decay, 
  originate 
  vast 
  chasms, 
  are 
  

   facts 
  which 
  must 
  have 
  been 
  observed 
  by 
  all 
  who 
  have 
  travelled 
  in 
  

   the 
  western 
  Highlands. 
  In 
  the 
  immediate 
  neighbourhoods 
  of 
  the 
  

  

  