﻿J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  253 
  

  

  passing 
  by 
  every 
  gradation 
  into 
  felsite. 
  These 
  rocks 
  are 
  identical 
  

   in 
  all 
  their 
  characters 
  with 
  the 
  acid 
  rocks 
  of 
  Mull. 
  They 
  occur 
  also 
  

   in 
  the 
  south-east 
  of 
  the 
  island 
  of 
  Rum, 
  being 
  there, 
  however, 
  much 
  

   broken 
  up 
  by 
  numerous 
  sheets 
  and 
  veins 
  of 
  the 
  crystalline 
  basic 
  

   rocks. 
  

  

  The 
  north-eastern 
  mountains 
  of 
  the 
  central 
  group 
  rise 
  into 
  

   singularly 
  wild 
  and 
  rugged 
  peaks, 
  among 
  which 
  are 
  Haskeval 
  

   (2667 
  feet), 
  Halival 
  (2367 
  feet), 
  Scur-na-gilean 
  (2553 
  feet), 
  and 
  

   Beinn 
  More 
  (2505 
  feet). 
  They 
  are 
  composed 
  of 
  the 
  varieties 
  of 
  

   gabbro 
  in 
  which 
  the 
  pyroxenic 
  constituent 
  consists 
  of 
  several 
  

   varieties 
  of 
  the 
  augite 
  group, 
  but 
  in 
  which 
  diallage 
  and 
  hypersthene 
  

   only 
  rarely 
  occur. 
  Precisely 
  similar 
  rocks 
  are 
  found, 
  as 
  we 
  have 
  

   seen, 
  forming 
  the 
  outer 
  portions 
  of 
  the 
  great 
  basic 
  intrusive 
  masses 
  

   and 
  the 
  veins 
  which 
  proceed 
  from 
  them, 
  both 
  in 
  Mull 
  and 
  Ardna- 
  

   murchan. 
  The 
  great 
  masses 
  of 
  gabbro 
  in 
  Rum 
  often 
  exhibit 
  that 
  

   pseudo-stratification 
  so 
  often 
  observed 
  in 
  igneous 
  rocks, 
  and 
  of 
  

   which 
  we 
  have 
  noticed 
  such 
  a 
  remarkable 
  example 
  in 
  the 
  granites 
  

   and 
  felsites 
  of 
  Mull. 
  They 
  are 
  also 
  penetrated 
  by 
  numerous 
  " 
  con- 
  

   temporaneous 
  veins." 
  

  

  The 
  relations 
  between 
  the 
  eruptive 
  masses 
  of 
  the 
  acid 
  and 
  basic 
  

   classes 
  respectively 
  are 
  scarcely 
  less 
  strikingly 
  displayed 
  in 
  Bum 
  

   than 
  in 
  Mull 
  ; 
  we 
  find 
  numerous 
  offshoots 
  and 
  veins 
  proceeding 
  from 
  

   the 
  gabbros 
  and 
  traversing 
  the 
  granites 
  and 
  felsites, 
  as 
  well 
  as 
  the 
  

   surrounding 
  older 
  strata 
  and 
  the 
  agglomerates 
  and 
  lavas. 
  

  

  Thus 
  the 
  island 
  of 
  Bum 
  is 
  for 
  the 
  most 
  part 
  made 
  up 
  of 
  great 
  

   intrusive 
  masses, 
  and 
  of 
  the 
  more 
  or 
  less 
  disturbed 
  and 
  altered 
  sedi- 
  

   mentary 
  deposits 
  through 
  which 
  these 
  have 
  been 
  thrust. 
  Of 
  the 
  

   overlying 
  ejected 
  materials 
  (lavas, 
  scoriae, 
  ashes, 
  and 
  breccias) 
  only 
  

   comparatively 
  small 
  and 
  isolated 
  vestiges 
  remain 
  ; 
  owing, 
  however, 
  

   to 
  the 
  clear 
  manner 
  in 
  which 
  the 
  relations 
  of 
  these 
  to 
  the 
  other 
  

   rocks 
  are 
  displayed, 
  they 
  are 
  well 
  worthy 
  of 
  attentive 
  study 
  by 
  the 
  

   geologist. 
  

  

  On 
  the 
  northern 
  flanks 
  of 
  the 
  mountain 
  of 
  Halival 
  we 
  find 
  a 
  great 
  

   mass 
  of 
  felstone 
  lavas 
  intermingled 
  with 
  agglomerates. 
  But 
  asso- 
  

   ciated 
  with 
  these 
  are 
  deposits 
  of 
  enormous 
  thickness, 
  consisting 
  of 
  

   blocks 
  of 
  Cambrian 
  sandstone 
  of 
  all 
  sizes 
  imbedded 
  in 
  a 
  matrix 
  of 
  

   felspathic 
  ash. 
  The 
  included 
  blocks 
  of 
  sandstone 
  in 
  this 
  volcanic 
  

   breccia 
  vary 
  greatly 
  in 
  size, 
  from 
  masses 
  several 
  feet 
  in 
  diameter 
  

   down 
  to 
  the 
  smallest 
  fragments. 
  As 
  we 
  approach 
  the 
  gabbro 
  masses 
  

   which 
  form 
  the 
  bulk 
  of 
  the 
  mountain, 
  the 
  felstone 
  lavas 
  are 
  seen 
  to 
  

   undergo 
  analogous 
  changes 
  to 
  those 
  manifested 
  in 
  similar 
  positions 
  

   by 
  the 
  same 
  rocks 
  in 
  Ardnamurchan 
  and 
  Mull. 
  

  

  Other 
  patches 
  of 
  felstone 
  lavas 
  with 
  accompanying 
  agglomerates, 
  

   which 
  have 
  escaped 
  destruction 
  by 
  denudation, 
  occur 
  at 
  several 
  

   points 
  on 
  the 
  south-eastern 
  side 
  of 
  the 
  island. 
  On 
  the 
  western 
  side, 
  

   however, 
  there 
  are 
  several 
  small 
  outliers 
  of 
  the 
  ordinary 
  columnar 
  

   and 
  basaltic 
  lavas. 
  These 
  exhibit 
  all 
  the 
  usual 
  characters 
  of 
  the 
  

   sheets 
  forming 
  the 
  great 
  plateau, 
  and 
  were 
  doubtless 
  once 
  continuous 
  

   with 
  the 
  other 
  and 
  larger 
  outliers 
  of 
  the 
  same 
  rocks, 
  which 
  con- 
  

   stitute 
  the 
  islands 
  of 
  Canna, 
  Eigg, 
  and 
  Muck. 
  At 
  Scur 
  More, 
  or 
  

  

  