﻿236 
  J. 
  W. 
  JXJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OP 
  SCOTLAND.' 
  

  

  towards 
  the 
  higher 
  and 
  outer 
  parts 
  the 
  mica 
  is 
  gradually 
  replaced 
  

   by 
  hornblende, 
  and 
  the 
  rock 
  passes 
  through 
  the 
  varieties 
  of 
  horn- 
  

   blendic 
  granite 
  into 
  syenite-granite. 
  The 
  adventitious 
  minerals 
  in 
  

   these 
  granites 
  are 
  pyrites, 
  marcasite, 
  chalcopyrite, 
  garnet, 
  apatite, 
  

   and 
  epidote. 
  The 
  texture 
  of 
  the 
  rock 
  varies 
  greatly, 
  from 
  coarse 
  

   and 
  sometimes 
  porphyritic 
  varieties 
  to 
  others 
  so 
  finely 
  granular 
  that 
  

   they 
  might 
  be 
  mistaken 
  at 
  first 
  sight 
  for 
  sandstone. 
  

  

  By 
  the 
  disappearance 
  of 
  the 
  crystals 
  of 
  hornblende 
  (which 
  appear 
  

   to 
  be 
  replaced 
  in 
  many 
  instances 
  by 
  some 
  easily 
  decomposable 
  

   mineral, 
  lining 
  the 
  cavities 
  of 
  the 
  rock) 
  the 
  syenite-granites 
  pass 
  

   into 
  felsites. 
  These 
  exhibit 
  every 
  variety, 
  from 
  the 
  most 
  granitic 
  

   forms, 
  like 
  eurite, 
  to 
  others 
  which 
  have 
  been 
  called 
  quartz-trachyte, 
  

   from 
  which 
  volcanic 
  rock, 
  indeed, 
  they 
  are 
  in 
  no 
  waj 
  r 
  distinguishable. 
  

   These 
  felsites 
  are 
  usually 
  quartziferous, 
  and 
  sometimes 
  very 
  highly 
  

   so 
  ; 
  at 
  other 
  times 
  they 
  are 
  almost 
  wholly 
  made 
  up 
  of 
  crystals 
  of 
  

   felspar, 
  imbedded 
  in 
  an 
  amorphous 
  paste; 
  and 
  not 
  unfrequently 
  

   they 
  exhibit 
  the 
  porphyritic 
  structure. 
  

  

  "When 
  the 
  felspatbic 
  rock 
  becomes 
  compact, 
  as 
  it 
  usually 
  does 
  in 
  

   the 
  lava 
  streams, 
  it 
  may 
  be 
  called 
  felstone. 
  The 
  felstones 
  of 
  the 
  

   Hebrides 
  vary 
  in 
  colour 
  from 
  black, 
  through 
  various 
  shades 
  of 
  green 
  

   and 
  grey, 
  to 
  white 
  ; 
  but 
  in 
  almost 
  all 
  cases 
  their 
  surfaces 
  acquire 
  a 
  

   white 
  crust 
  as 
  the 
  consequence 
  of 
  weathering 
  action. 
  By 
  this 
  cir- 
  

   cumstance, 
  and 
  also 
  by 
  the 
  manner 
  in 
  which 
  they 
  withstand 
  denuding 
  

   influences, 
  preserving 
  everywhere 
  the 
  striae 
  and 
  roche-moutonnee 
  

   forms 
  impressed 
  upon 
  them 
  during 
  the 
  glacial 
  epoch, 
  the 
  felstone- 
  

   lava 
  streams 
  are 
  strikingly 
  distinguished 
  from 
  the 
  basaltic. 
  Both, 
  

   however, 
  exhibit 
  the 
  columnar 
  structure 
  (though 
  with 
  characteristic 
  

   peculiarities 
  in 
  either 
  case), 
  the 
  globular 
  forms 
  developed 
  by 
  wea- 
  

   thering, 
  and 
  the 
  amygdaloidal 
  structure, 
  which 
  in 
  both 
  is 
  developed 
  

   to 
  the 
  greatest 
  extent 
  at 
  the 
  upper 
  and 
  under 
  surfaces 
  of 
  each 
  lava 
  

   stream. 
  The 
  felstones 
  appear 
  to 
  be 
  highly 
  siliceous 
  trachytic 
  lavas 
  

   which 
  have 
  undergone 
  more 
  or 
  less 
  alteration 
  ; 
  and 
  occasionally, 
  by 
  

   weathering 
  action, 
  the 
  obliterated 
  banded 
  and 
  sphserulitic 
  structures 
  

   so 
  commonly 
  found 
  in, 
  and 
  so 
  characteristic 
  of, 
  quartz-trachytes*, 
  

   are 
  restored 
  in 
  their 
  ancient 
  representatives, 
  the 
  felstones. 
  

  

  When 
  the 
  same 
  rock 
  becomes 
  glassy 
  in 
  structure 
  it 
  is 
  pitchstone 
  

   or 
  obsidian, 
  the 
  former 
  name 
  being 
  properly 
  applied 
  to 
  the 
  varieties 
  

   with 
  a 
  resinous, 
  and 
  the 
  latter 
  to 
  those 
  with 
  a 
  vitreous 
  lustre. 
  

  

  In 
  the 
  various 
  intrusive 
  masses 
  and 
  veins 
  the 
  several 
  forms 
  of 
  

   highly 
  felspathic 
  rock 
  are 
  found 
  passing 
  into 
  one 
  another 
  by 
  the 
  

   most 
  insensible 
  gradations. 
  

  

  In 
  the 
  basic 
  series 
  of 
  rocks 
  the 
  place 
  of 
  granite 
  is 
  occupied 
  by 
  

   gabbro, 
  a 
  rock 
  which, 
  from 
  the 
  similarity 
  of 
  its 
  mode 
  of 
  occurrence 
  

   and 
  behaviour 
  to 
  that 
  of 
  the 
  former, 
  has 
  been 
  called 
  "granitone" 
  and 
  

   "granito-di-gabbro." 
  It 
  consists 
  essentially 
  of 
  a 
  plagioclase 
  felspar, 
  

   one 
  or 
  more 
  pyroxenic 
  minerals, 
  and 
  olivine-— 
  the 
  latter 
  mineral 
  being 
  

   so 
  uniformly 
  present 
  that, 
  according 
  to 
  Zirkel, 
  it 
  must 
  be 
  regarded 
  as 
  

   an 
  essential 
  ingredient 
  of 
  the 
  rock. 
  The 
  felspar 
  of 
  this 
  rock 
  appears 
  

  

  * 
  See 
  Mr. 
  Scrope 
  " 
  On 
  the 
  Geology 
  of 
  the 
  Ponza 
  Islands," 
  Trans. 
  Geol. 
  Soc. 
  

   2nd 
  ser. 
  vol. 
  ii. 
  p. 
  195. 
  

  

  