﻿ANNIVERSARY 
  ADDRESS 
  OP 
  THE 
  PRESIDENT. 
  67 
  

  

  by 
  petrological 
  characters 
  irrespective 
  of 
  their 
  date 
  of 
  origin. 
  No 
  

   doubt, 
  in 
  the 
  map 
  of 
  Europe, 
  a 
  petrological 
  distinction 
  is 
  made 
  which 
  

   is 
  supposed 
  to 
  be 
  connected 
  with 
  geological 
  age, 
  when 
  diabase 
  and 
  

   melaphyre 
  are 
  distinguished 
  from 
  basalts, 
  and 
  porphyry 
  from 
  tra- 
  

   chyte 
  and 
  its 
  allies 
  ; 
  but 
  the 
  supposed 
  connexion 
  between 
  geolo- 
  

   gical 
  antiquity 
  and 
  mineral 
  characters 
  in 
  the 
  case 
  of 
  these 
  rocks 
  

   is 
  held 
  by 
  the 
  best 
  English 
  petrologists, 
  and 
  on 
  sound 
  evidence, 
  

   to 
  be 
  erroneous. 
  It 
  is 
  difficult 
  to 
  see 
  why 
  the 
  igneous 
  rocks 
  of 
  

   which 
  the 
  period 
  is 
  known 
  cannot 
  be 
  represented 
  by 
  the 
  colour 
  

   of 
  the 
  period 
  to 
  which 
  they 
  belong, 
  but 
  deep 
  and 
  opaque, 
  the 
  

   petrological 
  character, 
  if 
  it 
  is 
  considered 
  essential, 
  being 
  represented 
  

   by 
  the 
  monogram 
  ; 
  whilst 
  some 
  simple 
  convention 
  might 
  be 
  used 
  for 
  

   outbursts 
  of 
  unknown 
  period. 
  Or 
  all 
  igneous 
  rocks 
  might 
  be 
  re- 
  

   presented 
  by 
  one 
  conspicuous 
  colour, 
  the 
  monogram 
  serving 
  to 
  

   distinguish 
  the 
  kind 
  of 
  rock. 
  It 
  is 
  difficult 
  to 
  understand 
  why 
  in 
  a 
  

   map 
  in 
  which 
  no 
  colour-distinction 
  is 
  drawn 
  between 
  limestone 
  and 
  

   sandstone, 
  chalk 
  and 
  slate, 
  rock-salt 
  and 
  coal, 
  clay 
  and 
  iron-ore, 
  

   quartzite 
  and 
  gneiss, 
  except 
  that 
  due 
  to 
  geological 
  age, 
  it 
  is 
  essential 
  

   to 
  distinguish 
  by 
  colour 
  between 
  granite 
  and 
  porphyry, 
  melaphyre 
  

   and 
  basalts, 
  peridotite 
  and 
  serpentine, 
  although 
  several 
  of 
  these 
  

   are 
  so 
  inextricably 
  mixed 
  in 
  places 
  that 
  their 
  outcrops 
  could 
  not 
  

   be 
  disentangled 
  on 
  a 
  map 
  with 
  a 
  scale 
  100 
  times 
  larger 
  than 
  

   that 
  of 
  the 
  map 
  of 
  Europe. 
  It 
  is 
  reasonable 
  to 
  suppose 
  that 
  the 
  

   assignment 
  of 
  one 
  colour 
  to 
  the 
  lavas 
  of 
  active 
  or 
  extinct 
  volcanoes, 
  

   another 
  to 
  the 
  ashes 
  and 
  scorise, 
  and 
  a 
  third 
  to 
  " 
  stratified 
  tuffs," 
  

   was 
  proposed 
  by 
  geologists 
  who 
  had 
  not 
  much 
  personal 
  experience 
  

   of 
  volcanoes 
  of 
  either 
  kind. 
  Really 
  the 
  proposal 
  to 
  map 
  the 
  three 
  

   apart 
  on 
  a 
  scale 
  of 
  24 
  miles 
  to 
  the 
  inch 
  sounds 
  like 
  a 
  joke. 
  

  

  But 
  even 
  accepting 
  the 
  fact 
  that 
  what 
  has 
  been 
  done 
  hitherto 
  

   must 
  be 
  done 
  in 
  the 
  future, 
  and 
  that 
  the 
  geologists 
  of 
  Western 
  

   Europe 
  insist 
  on 
  colouring 
  the 
  igneous 
  rocks 
  on 
  a 
  system 
  radically 
  

   distinct 
  from 
  that 
  employed 
  for 
  sedimentary 
  formations, 
  it 
  is 
  still 
  

   difficult 
  to 
  understand 
  the 
  advantages 
  of 
  so 
  many 
  subdivisions 
  as 
  

   have 
  been 
  adopted 
  in 
  the 
  map 
  of 
  Europe, 
  or 
  to 
  understand 
  how 
  it 
  

   will 
  be 
  possible 
  to 
  colour 
  the 
  numerous 
  outcrops 
  of 
  igneous 
  rocks 
  

   that 
  are 
  intermediate 
  between 
  two 
  of 
  the 
  ill-defined 
  groups 
  estab- 
  

   lished, 
  or 
  composed 
  of 
  two 
  kinds 
  belonging 
  each 
  to 
  a 
  separate 
  

   group. 
  Surely 
  if 
  petrological 
  characters 
  must 
  be 
  the 
  guide 
  in 
  

   coloration, 
  some 
  simple 
  division, 
  as 
  into 
  basic 
  and 
  acid 
  rocks, 
  would 
  

   suffice. 
  There 
  would 
  be 
  difficulty 
  even 
  in 
  this, 
  as 
  when 
  trachytic 
  

   lavas 
  are 
  interstratified 
  with 
  basaltic, 
  or 
  when 
  a 
  rock 
  occurs 
  con- 
  

  

  