﻿246 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  

  

  scribed, 
  are 
  sheets 
  of 
  lava 
  of 
  the 
  highly 
  acid 
  variety 
  (felstones), 
  

   often 
  highly 
  vesicular 
  and 
  amygdaloidal 
  in 
  structure, 
  which 
  alternate 
  

   with 
  great 
  masses 
  of 
  ash, 
  lapilli, 
  and 
  scoriaceous 
  fragments. 
  The 
  

   very 
  rugged 
  forms 
  which 
  these 
  rocks 
  assume 
  in 
  weathering 
  causes 
  

   them 
  to 
  present 
  a 
  marked 
  contrast 
  with 
  the 
  rocks 
  on 
  which 
  they 
  

   repose. 
  They 
  have 
  evidently 
  been 
  thrust 
  upwards 
  to 
  a 
  certain 
  

   extent 
  by 
  the 
  great 
  intrusive 
  masses 
  below 
  chem, 
  and 
  are 
  seen 
  

   dipping 
  in 
  both 
  directions 
  from 
  them 
  ; 
  on 
  the 
  west 
  they 
  are 
  inter- 
  

   sected 
  by 
  the 
  shores 
  of 
  Loch-na-Kael, 
  where 
  their 
  characters 
  can 
  

   be 
  conveniently 
  studied. 
  

  

  b. 
  Beinn 
  Uaig. 
  — 
  The 
  relations 
  of 
  the 
  intrusive 
  granite 
  and 
  felsite 
  

   to 
  the 
  overlying 
  felstone 
  lavas 
  is, 
  owing 
  to 
  the 
  inaccessibility 
  of 
  many 
  

   parts 
  of 
  the 
  mountain, 
  not 
  conveniently 
  exposed 
  for 
  study 
  upon 
  Beinn 
  

   Greig. 
  But 
  in 
  Beinn 
  Uaig 
  we 
  have 
  a 
  precisely 
  similar 
  section 
  in 
  

   which 
  the 
  junction 
  in 
  question 
  is 
  beautifully 
  illustrated. 
  This 
  sec- 
  

   tion 
  is 
  represented 
  in 
  the 
  woodcut, 
  fig. 
  1, 
  The 
  granite 
  and 
  felsite 
  

  

  Fig. 
  1. 
  — 
  Rocks 
  forming 
  the 
  Summit 
  of 
  Beinn 
  Uaig, 
  Isle 
  of 
  Mull. 
  

  

  a. 
  Felstone 
  lavas, 
  with 
  agglomerates. 
  

  

  b. 
  Syenite-granite 
  graduating 
  into 
  felsite. 
  

  

  are 
  seen 
  to 
  give 
  off 
  great 
  veins 
  which 
  traverse 
  the 
  masses 
  of 
  felstone 
  

   lava 
  and 
  volcanic 
  agglomerate, 
  producing 
  a 
  very 
  sensible 
  degree 
  of 
  

   alteration 
  in 
  them 
  along 
  the 
  surfaces 
  of 
  contact 
  ; 
  and, 
  further, 
  these 
  

   veins 
  are 
  seen 
  to 
  include 
  masses 
  of 
  the 
  traversed 
  rock 
  which 
  have 
  

   been 
  caught 
  up 
  in 
  them. 
  In 
  all 
  these 
  features 
  we 
  recognize 
  the 
  

   characteristics 
  of 
  intrusive 
  granitic 
  masses, 
  which 
  have 
  been 
  so 
  ad- 
  

   mirably 
  illustrated 
  in 
  the 
  writings 
  of 
  Hutton, 
  Playfair, 
  Webb 
  Sey- 
  

   mour, 
  and 
  Macculloch. 
  

  

  c. 
  Craig 
  Craggen 
  (Section, 
  PI. 
  XXIII. 
  fig. 
  3).— 
  The 
  valley 
  of 
  the 
  

   Porsa 
  or 
  Pennygown 
  river 
  exposes 
  on 
  its 
  western 
  side 
  a 
  fine 
  section 
  

   of 
  the 
  mountain 
  called 
  Craig 
  Craggen, 
  which 
  rises 
  to 
  the 
  height 
  of 
  

   1885 
  feet. 
  We 
  here 
  find 
  the 
  granites 
  and 
  felsites 
  presenting 
  the 
  

   same 
  relations 
  to 
  the 
  overlying 
  felstone 
  lavas 
  as 
  in 
  the 
  two 
  examples 
  

   before 
  cited. 
  As 
  we 
  trace 
  the 
  lavas 
  up 
  the 
  slopes 
  of 
  the 
  mountain 
  

   the 
  intercalated 
  masses 
  of 
  agglomerate 
  are 
  found 
  to 
  become 
  gradually 
  

   thicker, 
  until 
  they 
  constitute 
  the 
  larger 
  portion 
  of 
  the 
  mass, 
  though 
  

   still 
  traversed 
  throughout 
  by 
  lava 
  sheets, 
  and 
  intersected 
  by 
  in- 
  

  

  