﻿278 
  

  

  J. 
  W. 
  JUDD 
  OX 
  TUU 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  of 
  Glen 
  Lonnan 
  (woodcut, 
  fig. 
  7). 
  When 
  compared 
  with 
  the 
  great 
  

   plateaux 
  of 
  the 
  Tertiary 
  lavas 
  which 
  have 
  heen 
  already 
  described, 
  a 
  

   number 
  of 
  points 
  of 
  difference, 
  however, 
  will 
  be 
  remarked. 
  The 
  

   wonderful 
  regularity 
  of 
  the 
  terraced 
  features, 
  so 
  strikingly 
  displayed 
  

   by 
  the 
  basaltic 
  plateaux 
  of 
  the 
  Tertiary 
  period, 
  are 
  to 
  a 
  certain 
  

  

  Fig. 
  7 
  .—Outline 
  Sketch 
  of 
  a 
  Mountain 
  near 
  Glen 
  Lonnan, 
  illustrating 
  

   the 
  Mode 
  of 
  Weathering 
  of 
  the 
  Porphyrite 
  and 
  Felstone 
  Lavas 
  of 
  

   Lorn. 
  (The 
  granitic 
  peaks 
  of 
  Beinn 
  Cruachan 
  are 
  seen 
  in 
  the 
  

   background.) 
  

  

  extent 
  wanting 
  in 
  the 
  piles 
  of 
  felspathic 
  lavas 
  which 
  were 
  poured 
  

   out 
  before 
  the 
  deposition 
  of 
  the 
  Secondary 
  rocks 
  ; 
  the 
  cause 
  of 
  this 
  

   difference 
  is 
  of 
  course 
  to 
  be 
  sought 
  in 
  the 
  usually 
  greater 
  fluidity 
  and 
  

   consequent 
  even 
  diffusion 
  of 
  the 
  lavas 
  of 
  basic 
  as 
  compared 
  with 
  those 
  

   of 
  acid 
  and 
  intermediate 
  composition. 
  

  

  2. 
  Characters 
  of 
  the 
  Volcanic 
  Rocks 
  of 
  Lorn. 
  — 
  As 
  already 
  inti- 
  

   mated, 
  the 
  great 
  mass 
  of 
  these 
  lavas 
  is 
  of 
  felspathic 
  or 
  acid 
  compo- 
  

   sition, 
  and 
  includes 
  innumerable 
  varieties 
  of 
  the 
  rocks 
  which 
  were 
  

   formerly 
  known 
  by 
  the 
  names 
  of 
  felspar 
  -porphyry, 
  compact 
  felspar, 
  

   clinkstone, 
  claystone, 
  &c. 
  For 
  the 
  most 
  part 
  they 
  may 
  be 
  regarded 
  

   either 
  as 
  varieties 
  of 
  the 
  rocks 
  included 
  in 
  this 
  country 
  under 
  the 
  

   name 
  of 
  " 
  felstone," 
  or 
  of 
  the 
  extensive 
  class 
  called 
  " 
  porphyrites 
  " 
  

   upon 
  the 
  Continent 
  — 
  the 
  former 
  representing 
  the 
  quartz-trachytes, 
  

   and 
  the 
  latter 
  the 
  ordinary 
  trachytes 
  of 
  modern 
  volcanoes. 
  

  

  The 
  changes 
  which 
  have 
  taken 
  place 
  in 
  many 
  of 
  these 
  rocks 
  sub- 
  

   sequently 
  to 
  their 
  deposition 
  have 
  been 
  such, 
  as 
  in 
  many 
  cases 
  to 
  

   mask 
  their 
  real 
  origin 
  as 
  lava 
  streams 
  ; 
  in 
  this 
  respect, 
  however, 
  

   we 
  find 
  the 
  greatest 
  variations 
  in 
  the 
  same 
  mass 
  of 
  rock. 
  A 
  close 
  

   examination 
  of 
  the 
  structure 
  and 
  relations 
  of 
  these 
  rocks 
  will, 
  in 
  

   almost 
  every 
  instance, 
  furnish 
  many 
  interesting 
  points 
  of 
  evidence 
  

   bearing 
  upon 
  their 
  mode 
  of 
  origin. 
  

  

  The 
  columnar 
  structure 
  is 
  by 
  no 
  means 
  so 
  common 
  among 
  the 
  fel- 
  

   spathic 
  lavas 
  as 
  among 
  those 
  of 
  the 
  basaltic 
  class 
  ; 
  nevertheless 
  some 
  

   interesting 
  examples 
  of 
  it 
  are 
  afforded 
  by 
  the 
  rocks 
  of 
  Lorn. 
  The 
  

   columnar 
  forms 
  assumed 
  by 
  the 
  acid 
  lavas 
  present 
  distinctive 
  charac- 
  

   teristics 
  of 
  their 
  own, 
  as 
  compared 
  with 
  those 
  of 
  the 
  basalts. 
  In 
  the 
  

   former 
  class 
  of 
  rocks 
  the 
  columns 
  are 
  usually 
  of 
  smaller 
  diameter 
  and 
  

  

  