﻿238 
  J. 
  W. 
  JtTDD 
  OIT 
  THE 
  SECOISTDAET 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  2. 
  Nature 
  and 
  Origin 
  of 
  the 
  Great 
  Volcanic 
  Rock 
  Masses. 
  — 
  Con- 
  

   founded 
  under 
  the 
  common 
  name 
  of 
  "trap," 
  several 
  very 
  important 
  

   distinctions 
  have 
  been, 
  in 
  the 
  case 
  of 
  these 
  igneous 
  rocks, 
  to 
  a 
  great 
  

   extent 
  lost 
  sight 
  of 
  ; 
  these 
  it 
  will 
  therefore 
  be 
  necessary 
  to 
  refer 
  to 
  

   particularly. 
  

  

  a. 
  Lavas. 
  — 
  These, 
  as 
  already 
  noticed, 
  constitute 
  the 
  great 
  mass 
  

   of 
  the 
  plateaux, 
  which 
  have 
  been 
  cut 
  up 
  by 
  denudation 
  into 
  those 
  

   terraced 
  hills 
  so 
  characteristic 
  of 
  the 
  district. 
  They 
  are 
  usually 
  

   composed 
  of 
  compact 
  or 
  crypto-crystalline 
  varieties 
  of 
  rocks, 
  more 
  

   rarely 
  of 
  the 
  glassy 
  forms 
  (in 
  the 
  case 
  of 
  the 
  acid 
  and 
  intermediate 
  

   varieties), 
  but 
  never 
  of 
  the 
  largely 
  crystalline 
  or 
  granitic 
  forms. 
  

   In 
  very 
  many 
  cases 
  they 
  are 
  distinguished 
  by 
  their 
  araygdaloidal 
  

   character 
  ; 
  but 
  the 
  absence 
  of 
  this 
  feature 
  must 
  not 
  be 
  regarded 
  as 
  

   an 
  evidence 
  that 
  any 
  particular 
  rock 
  is 
  not 
  to 
  be 
  considered 
  a 
  lava 
  ; 
  

   and, 
  indeed, 
  in 
  thick 
  lava 
  streams 
  the 
  amygdaloidal 
  (or 
  altered 
  vesi- 
  

   cular) 
  varieties 
  are 
  almost 
  wholly 
  confined 
  to 
  the 
  upper 
  and 
  lower 
  

   portions 
  of 
  the 
  mass*. 
  The 
  amygdaloidal 
  lavas 
  often 
  exhibit 
  evidence 
  

   of 
  their 
  vesicles 
  having 
  been 
  drawn 
  out 
  by 
  the 
  flowing 
  of 
  the 
  mass, 
  

   but 
  in 
  very 
  various 
  degrees 
  in 
  different 
  cases. 
  Both 
  the 
  basic 
  and 
  the 
  

   acid 
  lavas 
  at 
  times 
  assume 
  columnar 
  characters 
  ; 
  but 
  in 
  those 
  of 
  the 
  

   former 
  class 
  these 
  are 
  much 
  more 
  frequently 
  displayed, 
  and 
  in 
  a 
  more 
  

   striking 
  form. 
  The 
  basaltic 
  lavas 
  often 
  exhibit 
  the 
  thick 
  regular 
  

   columns 
  with 
  equidistant 
  joints, 
  cup-and-socket 
  articulations, 
  and 
  an- 
  

   gular 
  processes 
  so 
  well 
  known 
  as 
  occurring 
  at 
  Staffa 
  and 
  the 
  Giant's 
  

   Causeway; 
  in 
  the 
  felspalhie 
  lavas, 
  on 
  the 
  other 
  hand, 
  the 
  columns, 
  

   when 
  present, 
  are 
  usually 
  of 
  smaller 
  diameter 
  and 
  less 
  regular 
  form, 
  

   while 
  they 
  are 
  often 
  of 
  great 
  length, 
  and 
  never 
  exhibit 
  joints 
  at 
  

   regular 
  intervals. 
  The 
  masses 
  of 
  felstone 
  lava 
  are 
  seldom 
  found 
  

   extending 
  to 
  greater 
  distances 
  than 
  ten 
  miles 
  from 
  their 
  centres 
  of 
  

   eruption 
  ; 
  the 
  basaltic 
  lavas, 
  however, 
  have 
  spread 
  in 
  vast 
  sheets 
  to 
  

   distances 
  of 
  fifty 
  or 
  sixty 
  miles 
  from 
  those 
  centres. 
  

  

  b. 
  Intrusive 
  Masses. 
  — 
  These 
  vary 
  in 
  dimension, 
  from 
  the 
  thinnest 
  

   veins 
  or 
  strings, 
  to 
  vast 
  bosses 
  constituting 
  great 
  mountain-groups 
  

   like 
  the 
  Cuchullin 
  Hills 
  of 
  Skye, 
  or 
  the 
  deer-forest 
  of 
  Hum 
  and 
  

   Ardnamurchan. 
  In 
  their 
  ultimate 
  chemical 
  composition 
  they 
  coin- 
  

   cide 
  perfectly 
  with 
  the 
  rocks 
  composing 
  the 
  lavas 
  ; 
  but 
  in 
  the 
  varieties 
  

   of 
  their 
  texture 
  and 
  mineralogical 
  constitution 
  they 
  exhibit 
  a 
  much 
  

   wider 
  range. 
  Thus, 
  while 
  we 
  find 
  veins 
  of 
  basalt 
  in 
  which 
  the 
  

   rock- 
  structure 
  is 
  identical 
  with 
  that 
  of 
  many 
  of 
  the 
  lavas, 
  we 
  find 
  

   also 
  others 
  in 
  which 
  the 
  same 
  rock 
  passes 
  into 
  a 
  glass, 
  tachylite 
  ; 
  

   while 
  others, 
  again, 
  are 
  composed 
  of 
  the 
  highly 
  crystalline 
  or 
  granitic 
  

   gabbro 
  rocks. 
  Similarly, 
  felstone 
  veins 
  are 
  related 
  to 
  those 
  of 
  pitch- 
  

   stone 
  and 
  obsidian 
  on 
  the 
  one 
  hand, 
  and 
  to 
  masses 
  of 
  felsite, 
  syenite- 
  

   granite, 
  and 
  granite 
  on 
  the 
  other. 
  As 
  a 
  general 
  rule 
  it 
  may 
  be 
  

   stated 
  that 
  the 
  largest 
  intrusive 
  masses 
  are 
  composed 
  of 
  the 
  most 
  

   highly 
  crystalline 
  or 
  granitic 
  rocks. 
  Thus 
  the 
  gabbros 
  on 
  the 
  one 
  

   hand, 
  and 
  the 
  granites 
  on 
  the 
  other, 
  constitute, 
  for 
  the 
  most 
  part, 
  

  

  * 
  The 
  manner 
  in 
  which 
  this 
  character 
  is 
  displayed 
  by 
  almost 
  every 
  recent 
  

   lava 
  stream 
  is 
  familiar 
  to 
  all 
  who 
  have 
  visited 
  volcanic 
  districts, 
  and 
  has 
  been 
  

   alluded 
  to 
  by 
  many 
  authors.. 
  

  

  