﻿66 
  PROCEEDINGS 
  OE 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  faction 
  in 
  America, 
  nor, 
  so 
  far 
  as 
  is 
  known, 
  elsewhere. 
  It 
  is 
  to 
  be 
  

   regretted 
  that 
  violet 
  was 
  not 
  kept 
  for 
  Silurian, 
  as 
  proposed 
  by 
  

   several 
  of 
  the 
  National 
  Committees, 
  and 
  brick-red 
  for 
  Trias, 
  as 
  

   advocated 
  by 
  almost 
  all 
  ; 
  and 
  no 
  doubt 
  the 
  change 
  in 
  this 
  respect 
  

   has 
  done 
  something 
  to 
  render 
  the 
  system 
  adopted 
  unsatisfactory. 
  

   Yiolet 
  between 
  grey 
  (Carboniferous) 
  and 
  blue 
  (Jurassic) 
  is 
  open 
  to 
  

   the 
  objection 
  that 
  the 
  paler 
  shades 
  are 
  not 
  sufficiently 
  easy 
  to 
  distin- 
  

   guish, 
  whilst 
  brick-red 
  (impure 
  orange) 
  is 
  distinct 
  in 
  all 
  shades. 
  

  

  It 
  may, 
  indeed, 
  be 
  doubted 
  whether 
  any 
  scheme 
  of 
  coloration 
  

   could 
  be 
  devised 
  that 
  is 
  free 
  from 
  objection. 
  Much 
  has 
  been 
  said 
  

   of 
  the 
  advantage 
  of 
  a 
  system 
  based 
  upon 
  the 
  solar 
  spectrum, 
  and, 
  

   indeed, 
  in 
  the 
  plan 
  adopted 
  there 
  are 
  some 
  lingering 
  traces 
  of 
  a 
  

   spectral 
  system, 
  blue, 
  green, 
  and 
  yellow 
  succeeding 
  each 
  other 
  in 
  

   the 
  Upper 
  ILesozoic 
  and 
  Csenozoic 
  groups. 
  Prof. 
  Gilbert 
  has 
  sug- 
  

   gested 
  a 
  very 
  ingenious 
  and 
  apparently 
  simple 
  plan 
  of 
  dividing 
  the 
  

   spectrum 
  into 
  hues 
  denned 
  by 
  their 
  wave-lengths 
  or, 
  which 
  comes 
  

   to 
  the 
  same, 
  by 
  their 
  position 
  on 
  the 
  Kirchhoff 
  scale. 
  He 
  is 
  of 
  

   opinion 
  that 
  from 
  fifteen 
  to 
  twenty 
  sufficiently 
  distinct 
  hues 
  can 
  be 
  

   thus 
  selected, 
  and 
  these, 
  with 
  the 
  purples 
  which 
  are 
  not 
  contained 
  

   in 
  the 
  spectrum, 
  would 
  suffice 
  for 
  the 
  different 
  periods. 
  He 
  would 
  

   then 
  allot 
  brown 
  to 
  igneous 
  rocks. 
  

  

  This 
  plan 
  has, 
  I 
  believe, 
  one 
  fatal 
  objection. 
  If 
  the 
  adjacent 
  

   spectral 
  hues 
  are 
  in 
  contact 
  on 
  the 
  map, 
  even 
  though 
  the 
  full 
  tones 
  or 
  

   shades 
  be 
  distinct, 
  the 
  paler 
  tones 
  will 
  not 
  be 
  so. 
  The 
  only 
  remedy 
  

   would 
  be 
  to 
  alternate 
  colours 
  from 
  different 
  parts 
  of 
  the 
  spectrum, 
  

   and 
  then 
  the 
  orderliness 
  of 
  the 
  plan, 
  its 
  principal 
  recommendation, 
  

   would 
  disappear. 
  

  

  The 
  fact 
  is 
  that 
  the 
  distinctness 
  of 
  colours 
  adjacent 
  to 
  each 
  other 
  

   is 
  an 
  essential 
  requirement 
  of 
  any 
  efficient 
  system, 
  though 
  but 
  few 
  

   of 
  those 
  who 
  have 
  treated 
  of 
  map- 
  coloration 
  in 
  connexion 
  with 
  

   the 
  Congress 
  have 
  referred 
  to 
  this 
  important 
  point. 
  It 
  must 
  be 
  

   remembered 
  that 
  all 
  hues 
  in 
  contact 
  should 
  remain 
  sufficiently 
  

   distinct 
  after 
  a 
  certain 
  amount 
  of 
  fading 
  from 
  exposure, 
  and 
  that 
  all 
  

   should 
  be 
  easily 
  distinguished 
  by 
  artificial 
  light. 
  

  

  The 
  classification 
  and 
  coloration 
  of 
  the 
  igneous 
  rocks 
  on 
  the 
  map 
  

   of 
  Europe 
  is 
  very 
  unsatisfactory, 
  and 
  I 
  shall 
  probably 
  have 
  the 
  

   hearty 
  concurrence 
  of 
  English 
  geologists 
  in 
  general 
  in 
  condemning 
  

   the 
  system 
  proposed 
  as 
  impracticable. 
  In 
  the 
  first 
  place, 
  as 
  was 
  

   pointed 
  out 
  by 
  Prof. 
  Eenevier 
  some 
  years 
  ago, 
  it 
  is 
  illogical 
  to 
  

   arrange 
  the 
  sedimentary 
  rocks 
  entirely 
  by 
  their 
  geological 
  date, 
  

   irrespective 
  of 
  petrological 
  character, 
  and 
  the 
  igneous 
  rocks 
  entirely 
  

  

  