﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  301 
  

  

  insuperable 
  difficulties 
  in 
  attempting 
  to 
  record 
  and 
  correlate 
  his 
  observations 
  

   on 
  the 
  interior 
  portions 
  of 
  the 
  larger 
  islands, 
  from 
  the 
  want 
  of 
  even 
  tolerably 
  

   accurate 
  maps. 
  

  

  Although 
  I 
  have 
  endeavoured 
  to 
  make 
  the 
  best 
  possible 
  use 
  of 
  all 
  the 
  mate- 
  

   rials 
  which 
  were 
  available 
  as 
  a 
  basis 
  for 
  my 
  study 
  of 
  the 
  position 
  and 
  relations 
  

   of 
  the 
  rocks 
  of 
  Mull, 
  yet 
  nothing 
  more 
  could 
  be 
  attempted, 
  in 
  such 
  a 
  sketch 
  

   map 
  as 
  the 
  present, 
  than 
  to 
  convey 
  a 
  general 
  idea 
  of 
  the 
  structure 
  of 
  the 
  island. 
  

   It 
  must 
  be 
  remembered, 
  in 
  examining 
  this 
  map, 
  that 
  the 
  forms 
  of 
  the 
  outcrops 
  

   of 
  the 
  ramifying 
  masses 
  of 
  igneous 
  rock, 
  as 
  there 
  laid 
  down, 
  are 
  in 
  part 
  deter- 
  

   mined 
  by 
  the 
  surface-contours 
  in 
  these 
  highly 
  mountainous 
  districts. 
  Thus 
  the 
  

   outcrop 
  of 
  a 
  mass 
  of 
  rock 
  filling 
  a 
  rectilinear 
  fissure, 
  when 
  exhibited 
  on 
  a 
  moun- 
  

   tain-side, 
  forms 
  a 
  curved 
  line. 
  It 
  is 
  especially 
  necessary 
  to 
  bear 
  this 
  fact 
  in 
  mind 
  

   in 
  seeking 
  to 
  realize 
  from 
  the 
  map 
  the 
  actual 
  relations 
  of 
  the 
  great 
  solid 
  rock 
  

   masses. 
  Of 
  course 
  in 
  a 
  map 
  on 
  so 
  small 
  a 
  scale, 
  the 
  size 
  of 
  the 
  dykes 
  and 
  sheets 
  

   has 
  had 
  to 
  be 
  enormously 
  exaggerated, 
  while 
  only 
  the 
  most 
  imperfect 
  idea 
  is 
  

   conveyed 
  of 
  their 
  vast 
  numbers 
  and 
  complicated 
  relations. 
  

  

  By 
  employing 
  strongly 
  contrasted 
  colours 
  for 
  the 
  two 
  great 
  classes 
  of 
  igneous 
  

   rock, 
  I 
  have 
  endeavoured 
  to 
  make 
  their 
  relations 
  to 
  one 
  another 
  as 
  clear 
  as 
  

   possible 
  ; 
  while 
  by 
  varying 
  the 
  depth 
  of 
  tint 
  in 
  each 
  case 
  I 
  have 
  sought 
  to 
  illus- 
  

   trate 
  the 
  gradations 
  from 
  coarsely 
  crystalline 
  to 
  granular, 
  compact, 
  and 
  vitreous 
  

   structures, 
  in 
  rocks 
  of 
  the 
  same 
  ultimate 
  chemical 
  composition. 
  In 
  the 
  scale 
  of 
  

   colours 
  on 
  the 
  map 
  I 
  have 
  employed 
  the 
  same 
  petrological 
  nomenclature 
  as 
  is 
  

   used 
  in 
  the 
  memoir 
  ; 
  it 
  must 
  be 
  remembered, 
  however, 
  that 
  I 
  have 
  been 
  com- 
  

   pelled 
  to 
  use 
  some 
  terms 
  in 
  a 
  restricted 
  and, 
  to 
  a 
  certain 
  extent, 
  conventional 
  

   manner 
  — 
  as, 
  for 
  instance, 
  in 
  applying 
  the 
  name 
  " 
  felsite 
  " 
  to 
  rocks 
  of 
  granular 
  

   and 
  " 
  felstone 
  " 
  to 
  those 
  of 
  compact 
  texture. 
  The 
  index 
  of 
  colours 
  on 
  Plate 
  XXII. 
  

   serves 
  also 
  for 
  the 
  sections 
  on 
  Plate 
  XXIII. 
  On 
  the 
  latter, 
  however, 
  an 
  addi- 
  

   tional 
  tint 
  (pale 
  red 
  dotted 
  with 
  black) 
  has 
  been 
  employed 
  in 
  fig. 
  5 
  for 
  the 
  

   Cambrian, 
  which 
  does 
  not 
  occur 
  in 
  the 
  map. 
  

  

  In 
  Plate 
  XXIII. 
  all 
  the 
  sections 
  are 
  constructed, 
  as 
  far 
  as 
  possible, 
  on 
  a 
  true 
  

   scale, 
  the 
  proportions, 
  both 
  vertical 
  and 
  horizontal, 
  being 
  the 
  same 
  in 
  each 
  

   instance. 
  But 
  this 
  scale 
  is 
  in 
  each 
  section 
  perfectly 
  independent 
  of 
  that 
  of 
  the 
  

   others 
  ; 
  consequently 
  the 
  sections 
  bear 
  no 
  relation 
  to 
  one 
  another. 
  

  

  In 
  the 
  section 
  across 
  the 
  island 
  of 
  Mull 
  (fig. 
  1) 
  a 
  dotted 
  line 
  is 
  added 
  to 
  show 
  

   the 
  least 
  possible 
  elevation 
  of 
  the 
  volcano 
  before 
  it 
  had 
  undergone 
  central 
  sub- 
  

   sidence 
  and 
  extensive 
  denudation. 
  The 
  asterisks 
  show 
  the 
  difference 
  between 
  

   the 
  original 
  and 
  the 
  present 
  positions 
  of 
  what 
  is 
  now 
  the 
  summit 
  of 
  Beinn 
  

   More, 
  as 
  explained 
  on 
  page 
  259. 
  

  

  The 
  publication 
  of 
  the 
  general 
  Map 
  of 
  the 
  Western 
  Isles 
  is 
  postponed 
  till 
  the 
  

   appearance 
  of 
  the 
  next 
  paper 
  of 
  the 
  series, 
  as 
  it 
  will 
  serve 
  at 
  the 
  same 
  time 
  to 
  

   illustrate 
  the 
  relations 
  of 
  the 
  volcanic 
  rocks 
  to 
  one 
  another 
  and 
  the 
  positions 
  of 
  

   the 
  scattered 
  fragments 
  of 
  sedimentary 
  deposits, 
  from 
  which 
  the 
  history 
  of 
  the 
  

   Mesozoic 
  periods 
  in 
  this 
  district 
  has 
  to 
  be 
  reconstructed. 
  

  

  Discussion. 
  

  

  Mr. 
  Campbell 
  thanked 
  the 
  author 
  for 
  having 
  taught 
  him 
  so 
  much 
  

   concerning 
  the 
  country 
  in 
  which 
  he 
  had 
  been 
  born 
  and 
  bred, 
  and 
  

   invited 
  him 
  to 
  inspect 
  models 
  which 
  are 
  described 
  in 
  ' 
  Frost 
  and 
  

   Fire.' 
  

  

  Mr. 
  D. 
  Forbes 
  was 
  gratified 
  to 
  find 
  the 
  subject 
  of 
  the 
  igneous 
  

   rocks 
  of 
  Britain 
  taken 
  up 
  in 
  so 
  able 
  a 
  manner, 
  and 
  the 
  subject 
  not 
  

   left 
  entirely 
  to 
  continental 
  geologists. 
  The 
  author 
  was 
  fortunate 
  in 
  

   the 
  fact 
  that 
  he 
  had 
  occupied 
  the 
  same 
  ground 
  as 
  that 
  which 
  had 
  

   already 
  been 
  explored 
  by 
  Prof. 
  Zirkel, 
  whose 
  work 
  had 
  been 
  thus 
  

   supplemented. 
  He 
  agreed 
  with 
  the 
  author 
  in 
  regarding 
  Yolcanic 
  

   and 
  Plutonic 
  rocks, 
  from 
  granite 
  down 
  to 
  the 
  most 
  recent 
  lavas, 
  as 
  

  

  