﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECOXDARr 
  ROCKS 
  OF 
  SCOTLAND. 
  263 
  

  

  The 
  central 
  and 
  apical 
  portion 
  of 
  the 
  mountain 
  of 
  Beinn 
  Shiant 
  is 
  

   formed 
  of 
  rocks 
  of 
  similar 
  petrological 
  character, 
  which, 
  however, 
  are 
  

   apparently 
  of 
  intrusive 
  origin. 
  On 
  the 
  northern 
  side 
  of 
  the 
  moun- 
  

   tain 
  these 
  rocks 
  form 
  a 
  lofty 
  and 
  precipitous 
  ridge. 
  

  

  The 
  masses 
  of 
  columnar 
  rock 
  evidently 
  constitute 
  portions 
  of 
  a 
  

   number 
  of 
  lava 
  streams 
  which 
  appear 
  to 
  have 
  been 
  originally 
  in 
  

   connexion 
  with 
  the 
  central 
  mass, 
  and 
  to 
  have 
  diverged 
  in 
  a 
  radial 
  

   manner 
  from 
  it. 
  Some 
  of 
  these 
  lava 
  streams 
  can 
  be 
  traced 
  to 
  very 
  

   considerable 
  distances. 
  

  

  "Where 
  the 
  grassy 
  slopes 
  below 
  the 
  lava 
  cappings 
  of 
  the 
  spurs 
  are 
  

   cut 
  through 
  by 
  streams, 
  they 
  are 
  seen 
  to 
  be 
  composed 
  of 
  volcanic 
  ag- 
  

   glomerates 
  and 
  breccias. 
  These 
  contain 
  numerous 
  angular 
  frag- 
  

   ments 
  of 
  both 
  the 
  felspathic 
  and 
  basaltic 
  lavas 
  of 
  earlier 
  Tertiary 
  

   periods 
  with 
  some 
  of 
  the 
  underlying 
  Lower 
  Silurian 
  gneiss, 
  such 
  

   being 
  the 
  rocks 
  upon 
  which 
  the 
  mass 
  of 
  Beinn 
  Shiant 
  is 
  built 
  up 
  ; 
  

   mingled 
  with 
  these 
  are 
  numerous 
  fragments 
  of 
  felspathic 
  scoriae 
  and 
  

   ashes, 
  with 
  many 
  fractured 
  and 
  worn 
  crystals 
  of 
  glassy 
  felspar 
  in- 
  

   cluded 
  in 
  the 
  mass*. 
  

  

  Fortunately 
  for 
  the 
  geologist, 
  one 
  of 
  the 
  spurs 
  of 
  Beinn 
  Shiant 
  is 
  

   cut 
  across 
  in 
  a 
  sea-cliff 
  forming 
  the 
  headland 
  of 
  Srone 
  More 
  or 
  

   Maclean's 
  Nose 
  ; 
  and 
  here 
  we 
  have 
  a 
  magnificent 
  display 
  of 
  the 
  

   wonderfully 
  interesting 
  features 
  in 
  the 
  rock-masses 
  composing 
  this 
  

   remarkable 
  mountain. 
  The 
  upper 
  part 
  of 
  Srone 
  More, 
  which 
  rises 
  

   to 
  a 
  height 
  of 
  1050 
  feet, 
  is 
  composed 
  of 
  the 
  columnar 
  lavas 
  already 
  

   described 
  ; 
  but 
  the 
  almost 
  precipitous 
  cliffs 
  below 
  are 
  seen 
  to 
  be 
  

   made 
  up 
  of 
  nearly 
  angular 
  blocks 
  of 
  all 
  sizes, 
  up 
  to 
  6 
  or 
  8 
  feet 
  in 
  

   diameter, 
  heaped 
  together 
  in 
  the 
  wildest 
  confusion, 
  and 
  presenting 
  

   no 
  appearance 
  whatever 
  of 
  stratification 
  or 
  of 
  the 
  sorting 
  of 
  the 
  ma- 
  

   terials 
  according 
  to 
  their 
  specific 
  gravity. 
  

  

  Lastly, 
  it 
  is 
  necessary 
  to 
  notice 
  that 
  the 
  great 
  masses 
  of 
  agglome- 
  

   rate 
  which 
  constitute 
  so 
  large 
  a 
  portion 
  of 
  Beinn 
  Shiant, 
  together 
  

   with 
  the 
  older 
  lavas 
  and 
  gneiss 
  on 
  which 
  they 
  are 
  piled, 
  are 
  tra- 
  

   versed 
  in 
  every 
  direction 
  by 
  dykes 
  and 
  veins 
  of 
  every 
  size, 
  composed 
  

   of 
  similar 
  rocks 
  to 
  those 
  forming 
  the 
  lava 
  streams 
  — 
  namely, 
  trachyte 
  

   and 
  " 
  pitchstone-porphyry." 
  

  

  To 
  any 
  one 
  familiar 
  with 
  the 
  characters 
  presented 
  by 
  Arthur's 
  

  

  of 
  Beinn 
  Shiant, 
  and 
  like 
  them 
  graduate, 
  in 
  consequence 
  of 
  the 
  matrix 
  in 
  which 
  

   the 
  felspar 
  crystals 
  are 
  imbedded 
  acquiring 
  a 
  glassy 
  texture, 
  into 
  " 
  pitchstone- 
  

   porphyry." 
  In 
  speaking 
  of 
  the 
  Eocene 
  acid 
  lavas 
  of 
  the 
  Hebrides, 
  I 
  have 
  

   preferred 
  to 
  apply 
  to 
  them 
  the 
  term 
  "felstone," 
  although 
  it 
  must 
  be 
  re- 
  

   membered 
  that 
  they 
  present 
  no 
  essential 
  points 
  of 
  distinction 
  from 
  recent 
  

   quartz-trachytes, 
  either 
  in 
  chemical 
  composition 
  or 
  mineralogical 
  structure. 
  

   The 
  later 
  lavas 
  of 
  Beinn 
  Shiant 
  and 
  the 
  Sciir 
  of 
  Eigg 
  present 
  still 
  fewer 
  points 
  

   of 
  difference 
  from 
  those 
  of 
  active 
  volcanoes 
  ; 
  and 
  it 
  would 
  perhaps 
  be 
  more 
  

   correct 
  to 
  speak 
  of 
  them 
  as 
  "trachytes" 
  than 
  as 
  " 
  porphyrites." 
  

  

  * 
  The 
  frequency 
  with 
  which 
  crystals, 
  often 
  more 
  or 
  less 
  worn 
  and 
  fractured, 
  

   of 
  various 
  volcanic 
  minerals, 
  such 
  as 
  augite, 
  biotite, 
  the 
  felspars, 
  &c, 
  are 
  thrown 
  

   out 
  from 
  the 
  vents 
  of 
  volcanoes 
  has 
  been 
  noticed 
  by 
  Mr. 
  Scrope. 
  Such 
  crystals 
  

   are 
  found 
  alike 
  in 
  the 
  recent 
  ejections 
  of 
  Stromboli, 
  and 
  imbedded 
  in 
  the 
  tuffs 
  

   connected 
  with 
  extinct 
  volcanoes 
  at 
  Vulcano, 
  Albano, 
  Bracciano, 
  Bocca 
  Mon- 
  

   fina, 
  and 
  some 
  of 
  the 
  puys 
  of 
  Auvergne. 
  

  

  v2 
  

  

  