﻿J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  237 
  

  

  to 
  be 
  in 
  almost 
  every 
  case 
  labradorite, 
  of 
  white, 
  green, 
  or 
  purplish 
  tints, 
  

   and 
  often 
  belonging- 
  to 
  the 
  beautifully 
  glassy 
  varieties. 
  The 
  pyroxenic 
  

   ingredient 
  is 
  usually 
  diallage 
  ; 
  but 
  tbis 
  is 
  often 
  replaced 
  on 
  the 
  one 
  

   hand 
  by 
  hypersthene, 
  and 
  on 
  the 
  other 
  hand 
  by 
  augite, 
  in 
  the 
  

   same 
  manner 
  as 
  Streng 
  has 
  shown 
  to 
  be 
  the 
  case 
  with 
  the 
  gabbros 
  

   of 
  the 
  Hartz. 
  In 
  many 
  cases 
  tbe 
  augite 
  wholly 
  replaces 
  the 
  diallage; 
  

   and 
  this 
  is 
  found 
  to 
  be 
  always 
  the 
  case 
  as 
  we 
  pass 
  to 
  the 
  bigber 
  and 
  

   outer 
  portions 
  of 
  the 
  great 
  intrusive 
  masses 
  ; 
  while 
  occasionally, 
  

   instead 
  of 
  augite, 
  we 
  find 
  some 
  other 
  variety 
  of 
  pyroxene, 
  such 
  as 
  

   tbe 
  beautiful 
  green 
  coccolite. 
  These 
  brilliant 
  varieties 
  of 
  pyroxene, 
  

   with 
  the 
  glassy 
  labradorite 
  and 
  the 
  peculiar 
  forms 
  of 
  olivine, 
  con- 
  

   stitute 
  rocks 
  of 
  great 
  beauty. 
  The 
  olivine 
  is 
  usually 
  filled 
  with 
  

   innumerable 
  mhroliths 
  of 
  cbromic 
  or 
  titaniferous 
  iron, 
  by 
  which 
  its 
  

   ordinary 
  aspect 
  is 
  entirely 
  masked 
  ; 
  sometimes, 
  bowever, 
  it 
  presents 
  

   its 
  usual 
  clear 
  pale-green 
  tint, 
  and 
  at 
  others, 
  through 
  partial 
  de- 
  

   composition, 
  has 
  assumed 
  a 
  reddish-brown 
  colour. 
  The 
  gabbros 
  

   vary 
  in 
  structure 
  from 
  aggregates 
  of 
  crystals, 
  sometimes 
  2 
  inches 
  in 
  

   length, 
  down 
  to 
  finely 
  granular 
  rocks. 
  Among 
  the 
  adventitious 
  

   minerals 
  which 
  they 
  contain, 
  the 
  first 
  place 
  must 
  be 
  assigned 
  to 
  

   magnetic 
  iron, 
  which 
  is 
  often 
  present 
  in 
  large 
  quantities 
  ; 
  pyrites, 
  

   marcasite, 
  chalcopyrite, 
  biotite, 
  garnet, 
  apatite, 
  and 
  epidote 
  also 
  

   occur 
  in 
  it 
  ; 
  while 
  serpentine 
  and 
  chlorite 
  are 
  among 
  the 
  results 
  of 
  

   the 
  incipient 
  alteration 
  which 
  is 
  often 
  found 
  taking 
  place 
  in 
  its 
  mass. 
  

  

  When 
  the 
  gabbro 
  in 
  which 
  the 
  diallage 
  is 
  wholly 
  replaced 
  by 
  

   augite 
  assumes 
  a 
  granular 
  structure, 
  it 
  becomes 
  dolerite 
  ; 
  and 
  when 
  

   still 
  finer 
  grained, 
  it 
  passes 
  into 
  anamesite 
  and 
  basalt 
  ; 
  finally, 
  Avhen 
  

   the 
  basic 
  rock 
  becomes 
  glassy 
  in 
  structure, 
  it 
  is 
  known 
  as 
  tachylite. 
  

  

  Each 
  member 
  of 
  the 
  basic 
  series 
  of 
  rocks 
  can 
  be 
  seen 
  passing 
  into 
  

   the 
  others 
  by 
  the 
  most 
  insensible 
  gradations. 
  Thus 
  the 
  side 
  of 
  a 
  

   gabbro 
  vein 
  is 
  often 
  formed 
  by 
  a 
  layer 
  of 
  basalt 
  ; 
  that 
  of 
  a 
  basalt 
  

   vein 
  by 
  one 
  of 
  tachylite. 
  

  

  The 
  parallelism 
  between 
  the 
  different 
  varieties 
  of 
  the 
  acid 
  and 
  

   basic 
  series 
  of 
  igneous 
  rocks, 
  and 
  the 
  similarity 
  in 
  their 
  modes 
  of 
  

   occurrence, 
  is 
  very 
  striking. 
  There 
  is 
  one 
  important 
  distinction 
  

   between 
  them, 
  however, 
  which 
  it 
  is 
  necessary 
  to 
  bear 
  in 
  mind 
  : 
  the 
  

   crystalline 
  acid 
  rocks 
  (granites) 
  are 
  usually 
  among 
  the 
  most 
  stable 
  

   and 
  indestructible 
  of 
  the 
  materials 
  of 
  the 
  globe 
  ; 
  the 
  crystalline 
  

   basic 
  rocks 
  (gabbros) 
  are, 
  on 
  the 
  other 
  hand, 
  particularly 
  liable 
  to 
  

   decomposition, 
  the 
  diallage 
  and 
  oh 
  vine 
  giving 
  rise 
  to 
  the 
  formation 
  

   of 
  various 
  serpentinous 
  minerals. 
  

  

  I 
  have 
  not 
  attempted 
  to 
  describe 
  the 
  interesting 
  details 
  of 
  minute 
  

   structure 
  which 
  the 
  microscope 
  has 
  revealed 
  in 
  most 
  of 
  the 
  rocks 
  of 
  

   these 
  two 
  series, 
  but 
  must 
  refer 
  the 
  reader 
  for 
  information 
  on 
  this 
  

   subject 
  to 
  the 
  excellent 
  memoir 
  of 
  Prof. 
  Zirkel 
  before 
  quoted. 
  

  

  Besides 
  the 
  acid 
  and 
  basic 
  rocks 
  of 
  Tertiary 
  age 
  in 
  the 
  Hebrides, 
  

   there 
  occur 
  also 
  a 
  few 
  examples 
  of 
  the 
  intermediate 
  forms- 
  — 
  snch 
  as 
  

   diorites 
  and 
  syenites 
  among 
  the 
  intrusive 
  masses, 
  and 
  " 
  porphyrites 
  " 
  

   and 
  phonolites 
  among 
  the 
  lavas. 
  But 
  these 
  are 
  so 
  small 
  in 
  quantity, 
  

   and 
  so 
  local 
  and 
  exceptional 
  in 
  their 
  mode 
  of 
  occurrence, 
  as 
  not 
  to 
  

   demand 
  more 
  than 
  a 
  passing 
  notice 
  in 
  this 
  place. 
  

  

  