﻿J. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OP 
  SCOTLAND. 
  233 
  

  

  II. 
  The 
  Tertiary 
  Volcanoes. 
  

  

  1. 
  Classification 
  of 
  the 
  Tertiary 
  Volcanic 
  Rocks. 
  — 
  In 
  approaching 
  

   the 
  consideration 
  of 
  this 
  part 
  of 
  my 
  subject, 
  I 
  hasten 
  to 
  acknowledge 
  

   the 
  important 
  assistance 
  which 
  I 
  have 
  received 
  from 
  the 
  very 
  

   valuable 
  memoir 
  of 
  Prof. 
  Zirkel, 
  of 
  Leipzig, 
  entitled 
  " 
  Geologische 
  

   Skizzen 
  von 
  der 
  Westkiiste 
  Schottlands 
  " 
  *, 
  which 
  made 
  its 
  appear- 
  

   ance 
  in 
  1871. 
  This 
  author, 
  who 
  is 
  so 
  well 
  known 
  for 
  his 
  im- 
  

   portant 
  contributions 
  to 
  petrology, 
  made 
  a 
  tour 
  in 
  the 
  Hebrides 
  in 
  

   1868, 
  and 
  in 
  the 
  memoir 
  referred 
  to 
  has 
  given 
  an 
  admirable 
  sketch 
  

   of 
  all 
  that 
  had 
  hitherto 
  been 
  done 
  in 
  the 
  elucidation 
  of 
  the 
  geolo- 
  

   gical 
  structure 
  of 
  the 
  district, 
  accompanied 
  by 
  the 
  details 
  of 
  his 
  

   microscopic 
  studies 
  of 
  the 
  various 
  specimens 
  of 
  rocks 
  collected 
  by 
  him. 
  

  

  Bearing 
  in 
  mind 
  the 
  backward 
  state 
  of 
  petrological 
  investigations 
  in 
  

   this 
  country, 
  I 
  have 
  in 
  the 
  present 
  memoir 
  almost 
  uniformly 
  followed 
  

   the 
  classification 
  and 
  adopted 
  the 
  nomenclature 
  of 
  Prof. 
  Zirkel. 
  The 
  

   few 
  instances 
  in 
  which 
  I 
  have 
  departed 
  from 
  his 
  terminology, 
  such 
  

   as, 
  for 
  example, 
  in 
  avoiding 
  the 
  use 
  of 
  the 
  terms 
  " 
  porphyry 
  " 
  

   and 
  "amygdaloid," 
  and 
  employing 
  their 
  adjective 
  derivatives 
  instead, 
  

   will, 
  I 
  hope, 
  commend 
  themselves 
  to 
  all 
  geologists. 
  In 
  the 
  case 
  

   of 
  rocks 
  not 
  noticed 
  by 
  Prof. 
  Zirkel 
  in 
  the 
  memoir 
  cited, 
  I 
  have 
  

   endeavoured 
  as 
  far 
  as 
  possible 
  to 
  make 
  my 
  use 
  of 
  terms 
  correspond 
  

   with 
  the 
  definitions 
  given 
  by 
  the 
  same 
  author 
  in 
  his 
  very 
  admirable 
  

   ' 
  Lehrbuch 
  der 
  Petrographie,' 
  published 
  in 
  1866. 
  

  

  In 
  studying 
  these 
  rocks 
  and 
  minerals 
  I 
  have 
  also 
  to 
  acknowledge 
  

   the 
  valuable 
  assistance 
  of 
  Mr. 
  David 
  Porbes, 
  F.R.S.; 
  of 
  Mr. 
  Thomas 
  

   Davies, 
  of 
  the 
  British 
  Museum 
  ; 
  and 
  of 
  Mr. 
  Rudler, 
  of 
  Jermyn 
  Street. 
  

  

  The 
  Tertiary 
  volcanic 
  rocks 
  constitute 
  two 
  well-marked 
  parallel 
  

   series. 
  Each 
  of 
  these 
  series 
  has 
  at 
  one 
  end 
  of 
  it 
  a 
  highly 
  crystalline 
  

   rock, 
  and 
  at 
  the 
  other 
  a 
  perfect 
  glass 
  ; 
  while 
  the 
  intermediate 
  terms 
  

   constitute 
  the 
  most 
  complete 
  and 
  insensible 
  gradation 
  between 
  these 
  

   two 
  extremes. 
  The 
  ultimate 
  chemical 
  composition 
  of 
  the 
  rocks 
  of 
  

   either 
  series 
  is 
  identical 
  in 
  all 
  the 
  members 
  of 
  it, 
  or 
  rather 
  varies 
  

   between 
  the 
  same 
  comparatively 
  narrow 
  limits. 
  But 
  the 
  average 
  

   composition 
  of 
  the 
  rocks 
  of 
  the 
  two 
  series 
  presents 
  the 
  most 
  marked 
  

   contrast 
  when 
  compared 
  with 
  one 
  another. 
  

  

  These 
  two 
  series 
  of 
  igneous 
  rocks 
  have 
  long 
  been 
  recognized 
  by 
  

   petrologists 
  as 
  occurring 
  both 
  in 
  recent 
  volcanoes 
  and 
  in 
  association 
  

   with 
  the 
  strata 
  of 
  every 
  geological 
  period. 
  From 
  the 
  marked 
  differ- 
  

   ence 
  in 
  the 
  quantity 
  of 
  silica 
  which 
  they 
  contain 
  they 
  have 
  been 
  

   designated 
  the 
  acid 
  and 
  basic 
  series 
  : 
  the 
  former 
  is 
  sometimes 
  known 
  

   as 
  the 
  felspathic, 
  orthoclastic, 
  or 
  traehytic 
  series 
  ; 
  and 
  the 
  latter 
  as 
  

   the 
  augitic, 
  plagioclastic, 
  or 
  basaltic. 
  

  

  In 
  the 
  members 
  of 
  the 
  acid 
  series 
  of 
  igneous 
  rocks 
  the 
  percentage 
  

   of 
  silica 
  varies 
  from 
  60 
  to 
  80, 
  averaging 
  about 
  70 
  ; 
  while 
  in 
  the 
  

   basic 
  its 
  limits 
  are 
  45 
  and 
  55, 
  and 
  its 
  average 
  50. 
  In 
  the 
  former 
  

   class 
  of 
  rocks 
  the 
  proportion 
  of 
  alkalies 
  is 
  comparatively 
  large, 
  and 
  

   that 
  of 
  the 
  alkaline 
  earths 
  and 
  the 
  oxides 
  of 
  iron 
  and 
  manganese 
  

   small 
  ; 
  but 
  in 
  the 
  latter, 
  lime, 
  magnesia, 
  and 
  oxide 
  of 
  iron 
  con- 
  

   * 
  ' 
  Zeitschr. 
  d. 
  deutschen 
  geologischen 
  Gesellschaft,' 
  Jahrg. 
  1871, 
  pp. 
  1-124. 
  

  

  