﻿266 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  HOCKS 
  OF 
  SCOTLAND. 
  

  

  sequence 
  of 
  their 
  greater 
  hardness, 
  an 
  almost 
  entire 
  belt 
  of 
  the 
  

   altered 
  and 
  indurated 
  basalts 
  is 
  found 
  surrounding 
  the 
  crystalline 
  

   mass 
  on 
  all 
  sides. 
  Prom 
  the 
  central 
  mass 
  a 
  number 
  of 
  dykes 
  and 
  

   veins 
  can 
  be 
  traced 
  intersecting 
  the 
  surrounding 
  older 
  lavas. 
  

  

  That, 
  subsequently 
  to 
  the 
  eruption 
  of 
  the 
  great 
  sheets 
  of 
  basaltic 
  

   lavas, 
  a 
  mass 
  of 
  basic 
  igneous 
  rock 
  was 
  upheaved 
  in 
  their 
  midst 
  at 
  

   Sarsta 
  Beinn 
  is 
  evident. 
  And 
  that 
  this 
  eruptive 
  mass 
  was 
  surmounted 
  

   by 
  a 
  volcanic 
  cone 
  from 
  which 
  probably 
  lava 
  streams 
  flowed, 
  no 
  one 
  

   who 
  has 
  studied 
  the 
  example 
  of 
  Beinn 
  Shiant 
  will, 
  I 
  think, 
  be 
  disposed 
  

   to 
  doubt. 
  Indeed 
  fragments 
  of 
  these 
  lava-streams 
  may 
  still 
  remain, 
  

   though 
  now 
  forming 
  undistinguishable 
  portions 
  of 
  the 
  great 
  basaltic 
  

   plateau. 
  When 
  we 
  remember 
  the 
  observation 
  of 
  Mr. 
  Darwin, 
  that 
  

   both 
  in 
  the 
  Cape-Yerde 
  Islands 
  and 
  the 
  Galapagos 
  archipelago 
  he 
  

   found 
  it 
  impossible 
  to 
  trace 
  the 
  boundaries 
  of 
  recently 
  erupted 
  lava 
  

   streams, 
  " 
  except 
  by 
  the 
  size 
  of 
  the 
  bushes 
  growing 
  upon 
  them, 
  or 
  

   by 
  the 
  comparative 
  glossiness 
  of 
  their 
  surfaces, 
  characters 
  which 
  a 
  

   short 
  lapse 
  of 
  time 
  would 
  be 
  sufficient 
  to 
  obscure," 
  it 
  will 
  not 
  be 
  sur- 
  

   prising 
  that 
  we 
  are 
  not, 
  as 
  a 
  rule, 
  able 
  to 
  trace 
  the 
  separate 
  lava 
  

   streams 
  of 
  the 
  Hebrides. 
  The 
  fact 
  that 
  in 
  the 
  case 
  of 
  the 
  lavas 
  of 
  

   Beinn 
  Shiant 
  we 
  are 
  able 
  so 
  to 
  do, 
  is 
  due 
  solely 
  to 
  the 
  fact 
  that 
  in 
  

   them 
  a 
  great 
  capacity 
  for 
  resisting 
  degrading 
  influences 
  is 
  combined 
  

   with 
  remarkably 
  distinctive 
  penological 
  characters. 
  

  

  I 
  have 
  described 
  in 
  some 
  detail 
  these 
  two 
  examples 
  of 
  Beinn 
  

   Shiant 
  and 
  Sarsta 
  Beinn 
  on 
  account 
  of 
  their 
  great 
  size 
  and 
  typical 
  

   character 
  ; 
  but 
  there 
  is 
  the 
  clearest 
  evidence 
  that 
  at 
  a 
  very 
  great 
  num- 
  

   ber 
  of 
  points 
  the 
  basalts 
  of 
  the 
  great 
  plateau 
  were 
  broken 
  through 
  

   by 
  similar 
  eruptive 
  masses, 
  sometimes, 
  as 
  in 
  the 
  case 
  of 
  Sarsta 
  

   Beinn, 
  composed 
  of 
  basic 
  rocks, 
  at 
  others, 
  as 
  in 
  Beinn 
  Shiant, 
  of 
  more 
  

   felspathic 
  materials, 
  and 
  in 
  others, 
  again, 
  of 
  both 
  these 
  varieties. 
  Few 
  

   of 
  these 
  later 
  eruptions 
  appear 
  to 
  have 
  been 
  on 
  the 
  same 
  grand 
  scale 
  

   as 
  those 
  which 
  we 
  have 
  selected 
  as 
  types 
  ; 
  and 
  many 
  of 
  the 
  intruded 
  

   rock 
  masses, 
  which 
  now 
  alone 
  remain 
  to 
  indicate 
  their 
  points 
  of 
  

   origin, 
  are 
  of 
  quite 
  insignificant 
  proportions. 
  Thus 
  in 
  the 
  little 
  

   island 
  of 
  Eigg 
  two 
  such 
  masses 
  were 
  detected 
  by 
  Professor 
  Geikie, 
  

   and 
  I 
  have 
  found 
  a 
  third 
  of 
  similar 
  character 
  ; 
  iu 
  the 
  other 
  islands 
  

   also 
  such 
  masses 
  are 
  found 
  scattered 
  in 
  all 
  directions. 
  Until, 
  how- 
  

   ever, 
  the 
  whole 
  district 
  has 
  been 
  mapped 
  in 
  the 
  most 
  careful 
  detail, 
  

   it 
  will 
  be 
  impossible 
  to 
  give 
  a 
  distinct 
  view 
  of 
  the 
  distribution 
  

   of 
  these 
  latest 
  points 
  of 
  eruption 
  in 
  the 
  district, 
  and 
  to 
  show 
  their 
  

   exact 
  relations 
  to 
  the 
  great 
  volcanoes. 
  Even 
  after 
  such 
  survey, 
  

   indeed, 
  many 
  of 
  the 
  smaller 
  intrusive 
  masses 
  may 
  escape 
  the 
  obser- 
  

   vation 
  of 
  the 
  geologist 
  through 
  being 
  concealed 
  by 
  peat-mosses 
  or 
  the 
  

   vegetable 
  covering. 
  

  

  Having 
  pointed 
  out 
  that, 
  even 
  in 
  the 
  great 
  mass 
  of 
  Beinn 
  Shiant, 
  

   the 
  preservation 
  of 
  the 
  ruins 
  of 
  the 
  volcanic 
  cone 
  are 
  due 
  to 
  very 
  re- 
  

   markable 
  and 
  exceptional 
  circumstances, 
  it 
  will 
  be 
  almost 
  unneces- 
  

   sary 
  to 
  add 
  that 
  the 
  small 
  cones 
  which 
  doubtless 
  once 
  surmounted 
  

   most, 
  if 
  not 
  all, 
  of 
  these 
  eruptive 
  masses, 
  have 
  in 
  almost 
  every 
  instance 
  

   been 
  wholly 
  swept 
  away 
  by 
  denudation, 
  and 
  that 
  the 
  lavas 
  which 
  

   flowed 
  from 
  them 
  have 
  also 
  disappeared 
  or 
  are 
  now 
  undistinguishable. 
  

  

  