﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  245 
  

  

  jointed 
  condition, 
  some 
  of 
  the 
  mountains 
  in 
  the 
  island 
  present 
  

   splendid 
  natural 
  sections, 
  which 
  afford 
  the 
  geologist 
  the 
  clearest 
  

   insight 
  into 
  the 
  relations 
  of 
  the 
  different 
  rocks 
  which 
  constitute 
  

   them. 
  By 
  selecting 
  several 
  of 
  these 
  fine 
  natural 
  sections 
  as 
  types, 
  

   we 
  shall 
  best 
  be 
  able 
  to 
  illustrate 
  clearly 
  the 
  somewhat 
  compli- 
  

   cated 
  relations 
  of 
  the 
  different 
  volcanic 
  rocks 
  of 
  the 
  island 
  to 
  one 
  

   another. 
  

  

  a. 
  Beinn 
  Greig 
  (see 
  Section, 
  PI. 
  XXIII. 
  fig. 
  2) 
  is 
  a 
  mountain 
  which 
  

   rises 
  near 
  the 
  western 
  end 
  of 
  Loch 
  Bah 
  to 
  the 
  height 
  of 
  1941 
  feet. 
  

   The 
  appearance 
  of 
  this 
  mountain, 
  as 
  viewed 
  from 
  the 
  north, 
  is 
  very 
  

   striking, 
  the 
  much-jointed 
  granite 
  and 
  felsite 
  which 
  constitute 
  the 
  

   greater 
  part 
  of 
  its 
  mass 
  having 
  crumbled 
  down, 
  leaving 
  an 
  almost 
  

   precipitous 
  face. 
  The 
  lower 
  and 
  internal 
  portion 
  of 
  the 
  mountain, 
  

   as 
  is 
  well 
  seen 
  on 
  its 
  south 
  side, 
  in 
  the 
  deep 
  ravines 
  which 
  divide 
  it 
  

   from 
  Beinn-y-chat 
  and 
  Beinn-a-Gobhar, 
  is 
  composed 
  of 
  a 
  well- 
  

   formed 
  typical 
  granite, 
  for 
  the 
  most 
  part 
  of 
  the 
  hornblendic 
  kind. 
  

   As 
  we 
  ascend 
  the 
  mountain 
  this 
  granite 
  is 
  found 
  to 
  pass 
  by 
  in- 
  

   sensible 
  gradations 
  into 
  a 
  quartziferous 
  felsite, 
  the 
  hornblende 
  being 
  

   replaced 
  by 
  easily 
  decomposable 
  minerals, 
  the 
  decay 
  of 
  which 
  greatly 
  

   facilitates 
  the 
  disintegration 
  of 
  the 
  rock. 
  Still 
  higher 
  the 
  rock 
  

   becomes 
  finely 
  crystalline 
  or 
  granular, 
  the 
  magnesian 
  and 
  ferruginous 
  

   materials 
  being 
  apparently 
  no 
  longer 
  separately 
  crystallized, 
  but 
  dif- 
  

   fused 
  through 
  the 
  mass 
  as 
  colouring-matters. 
  The 
  porphyritic 
  struc- 
  

   ture 
  is 
  locally 
  displayed 
  by 
  all 
  portions 
  of 
  the 
  mass 
  alike, 
  from 
  the 
  

   coarsest 
  granite 
  to 
  the 
  finest-grained 
  felsite. 
  

  

  When 
  the 
  northern 
  face 
  of 
  this 
  mountain 
  is 
  viewed 
  from 
  a 
  little 
  

   distance, 
  the 
  whole 
  mass 
  presents 
  the 
  appearance 
  of 
  being 
  made 
  up 
  

   of 
  a 
  number 
  of 
  concentrically 
  curved 
  beds. 
  This 
  pseudo-stratified 
  

   appearance 
  is, 
  as 
  all 
  geologists 
  are 
  aware, 
  very 
  frequently 
  presented 
  

   by 
  masses 
  of 
  undoubted 
  igneous 
  origin*; 
  whether 
  in 
  the 
  present 
  

   example 
  it 
  is 
  to 
  be 
  regarded 
  as 
  due 
  to 
  the 
  manner 
  in 
  which 
  the 
  

   fluid 
  rock 
  was 
  extruded, 
  or 
  to 
  changes 
  which 
  have 
  taken 
  place 
  in 
  the 
  

   mass 
  during 
  the 
  process 
  of 
  cooling, 
  I 
  shall 
  not 
  here 
  attempt 
  to 
  

   determine. 
  

  

  The 
  granite 
  and 
  felsite 
  of 
  Beinn 
  Greig 
  are 
  traversed 
  by 
  in- 
  

   numerable 
  veins. 
  These 
  all 
  appear 
  to 
  be 
  of 
  the 
  " 
  contemporaneous 
  " 
  

   class, 
  and 
  to 
  be 
  composed 
  of 
  similar 
  materials 
  to 
  those 
  of 
  the 
  mass 
  

   itself, 
  differing 
  for 
  the 
  most 
  part 
  only 
  in 
  the 
  degree 
  of 
  fineness 
  of 
  

   grain, 
  colour, 
  &c. 
  Usually 
  the 
  rock 
  of 
  the 
  veins 
  is 
  of 
  much 
  finer 
  

   grain 
  than 
  that 
  which 
  it 
  traverses 
  ; 
  and 
  in 
  some 
  analogous 
  cases 
  the 
  

   acid 
  rock 
  actually 
  passes 
  into 
  the 
  glassy 
  condition, 
  and 
  exists 
  as 
  pitch- 
  

   stone 
  veins 
  traversing 
  granite. 
  In 
  a 
  few 
  instances 
  thin 
  veins 
  of 
  almost 
  

   pure 
  quartz 
  and 
  others 
  made 
  up 
  of 
  crystallized 
  felspar 
  are 
  found. 
  

  

  Lying 
  upon 
  the 
  summit 
  and 
  flanks 
  of 
  the 
  eruptive 
  rocks 
  just 
  de- 
  

  

  * 
  I 
  may 
  especially 
  cite 
  the 
  granite 
  of 
  Arran, 
  whore 
  similar 
  features 
  to 
  those 
  

   described 
  as 
  occurring 
  in 
  Mull 
  were 
  noticed 
  by 
  Macculloch. 
  This 
  concentric 
  

   structure 
  appears 
  to 
  be 
  similar 
  to 
  that 
  referred 
  to 
  by 
  Mr. 
  Scrope 
  as 
  occurring 
  in 
  

   certain 
  of 
  the 
  domitic 
  puys 
  of 
  Auvergne, 
  such 
  as 
  Clierson 
  and 
  Le 
  Grand 
  Sarcoui 
  

   (see 
  ' 
  The 
  Geology 
  and 
  Extinct 
  Volcanos 
  of 
  Central 
  France,' 
  2nd 
  edition, 
  1858, 
  

   p. 
  68). 
  

  

  Q.J. 
  G. 
  S. 
  No. 
  119. 
  t 
  

  

  