﻿23*4 
  

  

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

  

  stitute 
  a 
  very 
  large 
  proportion 
  of 
  the 
  mass, 
  while 
  the 
  quantity 
  of 
  

   soda 
  and 
  potash 
  is 
  comparatively 
  insignificant. 
  

  

  The 
  contrast 
  between 
  the 
  ultimate 
  composition 
  of 
  the 
  rocks 
  of 
  the 
  

   two 
  series 
  is 
  illustrated 
  in 
  the 
  following 
  Table 
  : 
  — 
  

  

  

  Acid 
  sei'ies. 
  

  

  Basic 
  series. 
  

  

  Max. 
  

  

  Average. 
  

  

  Min. 
  

  

  Max. 
  

  

  Average. 
  

  

  Min. 
  

  

  

  80 
  

  

  19 
  

  

  20 
  

  

  8 
  

  

  5 
  

  

  72 
  

   15 
  

  

  7 
  

  

  2 
  

  

  2-5 
  

  

  61 
  

  

  10 
  

  

  

  

  

  

  

  

  55 
  

  

  18 
  

  

  8 
  

  

  29 
  

  

  19 
  

  

  50 
  

   14-5 
  

   3-5 
  

   17 
  

  

  14 
  

  

  45 
  

   10 
  

   0-5 
  

  

  8 
  

  

  8 
  

  

  

  

  

  Oxides 
  of 
  iron 
  

   manganese 
  

  

  and! 
  

  

  In 
  the 
  acid 
  series 
  the 
  highly 
  crystalline 
  form 
  is 
  known 
  as 
  granite, 
  

   the 
  glass 
  as 
  pitch 
  stone 
  or 
  obsidian. 
  The 
  intermediate 
  terms 
  of 
  the 
  

   series 
  have 
  received 
  a 
  great 
  number 
  of 
  names, 
  only 
  a 
  few 
  of 
  which 
  

   it 
  wi]l 
  be 
  necessary 
  to 
  notice 
  here. 
  

  

  In 
  the 
  basic 
  series 
  the 
  most 
  highly 
  crystalline 
  form 
  is 
  known 
  as 
  

   gabbro, 
  the 
  glass 
  as 
  tachylite 
  ; 
  and 
  the 
  intermediate 
  forms 
  are 
  also, 
  

   as 
  in 
  the 
  former 
  case, 
  very 
  numerous. 
  

  

  In 
  the 
  accompanying 
  Table 
  (p. 
  23-5) 
  the 
  composition 
  of 
  the 
  princi- 
  

   pal 
  rocks 
  of 
  these 
  two 
  series 
  is 
  illustrated; 
  the 
  averages 
  are 
  for 
  the 
  

   most 
  part 
  those 
  calculated 
  by 
  Durocher 
  ; 
  in 
  the 
  instances 
  marked 
  by 
  

   an 
  asterisk 
  I 
  have 
  been 
  compelled 
  to 
  employ 
  averages 
  calculated 
  by 
  

   myself 
  from 
  published 
  analyses. 
  

  

  The 
  Tertiary 
  granites 
  consist 
  essentially 
  of 
  two 
  species 
  of 
  felspar 
  

   with 
  quartz, 
  and 
  either 
  mica 
  or 
  hornblende, 
  or 
  both 
  of 
  these 
  minerals 
  

   together. 
  These 
  granites 
  often, 
  but 
  by 
  no 
  means 
  uniformly, 
  present 
  

   a 
  somewhat 
  cavernous 
  structure, 
  in 
  which 
  case 
  the 
  constituent 
  

   minerals 
  are 
  found 
  with 
  perfectly 
  terminated 
  crystals 
  projecting 
  

   into 
  the 
  cavities 
  of 
  the 
  rock. 
  The 
  felspars 
  consist 
  of 
  orthoclase 
  

   and 
  oligoclase, 
  with 
  much 
  more 
  rarely 
  albite 
  also 
  ; 
  their 
  crystals 
  

   often 
  present 
  a 
  pale 
  brown 
  or 
  buff 
  colour, 
  owing 
  to 
  the 
  partial 
  

   decomposition 
  of 
  ferruginous 
  materials 
  entangled 
  in 
  them. 
  When, 
  

   however, 
  the 
  rock 
  is 
  quarried 
  at 
  some 
  depth 
  from 
  the 
  surface 
  the 
  

   felspars 
  are 
  usually 
  seen 
  to 
  he 
  of 
  a 
  beautifully 
  white 
  colour 
  ; 
  while 
  

   more 
  rarely 
  they 
  present 
  pink 
  and 
  reddish 
  tints. 
  The 
  quartz 
  is 
  

   usually 
  white, 
  but 
  not 
  unfrequently 
  smoky, 
  and 
  in 
  rare 
  cases 
  black 
  ; 
  

   it 
  sometimes 
  forms 
  perfect 
  crystals. 
  The 
  felspar 
  and 
  quartz 
  make 
  

   up 
  the 
  mass 
  of 
  the 
  rock, 
  which 
  is 
  always 
  a 
  highly 
  silicated 
  one 
  ; 
  

   and 
  the 
  magnesian 
  and 
  ferruginous 
  silicates 
  are 
  present 
  only 
  in 
  

   comparatively 
  small 
  quantities. 
  In 
  the 
  deeper 
  and 
  more 
  central 
  parts 
  

   of 
  the 
  great 
  granitic 
  masses 
  the 
  rock 
  is 
  micaceous 
  ; 
  but 
  as 
  we 
  pass 
  

  

  