﻿240 
  J. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDARY 
  KOCKS 
  OF 
  SCOTLAND; 
  

  

  or 
  with 
  worn 
  angles 
  or 
  faces, 
  such 
  as 
  commonly 
  occur 
  among 
  the 
  

   materials 
  ejected 
  from 
  volcanic 
  vents, 
  are 
  found 
  imbedded 
  in 
  these 
  

   agglomerates. 
  

  

  That 
  these 
  peculiar 
  rocks 
  are 
  really 
  portions 
  of 
  old 
  volcanic 
  

   piles, 
  and 
  that 
  they 
  are 
  made 
  up 
  of 
  the 
  fragments 
  actually 
  ejected 
  

   from 
  volcanic 
  vents, 
  no 
  one 
  who 
  has 
  had 
  an 
  opportunity 
  of 
  studying 
  

   the 
  appearances 
  presented 
  by 
  their 
  weathered 
  surfaces 
  can 
  for 
  one 
  mo- 
  

   ment 
  doubt. 
  But 
  in 
  the 
  same 
  manner 
  that 
  loose 
  masses 
  of 
  shells 
  and 
  

   corals 
  have 
  been 
  converted, 
  through 
  the 
  chemical 
  reactions 
  set 
  up 
  by 
  

   the 
  infiltration 
  of 
  water, 
  into 
  crystalline 
  and 
  apparently 
  structureless 
  

   limestone, 
  so 
  these 
  old 
  heaps 
  of 
  volcanic 
  cinders 
  have 
  been 
  transformed 
  

   by 
  similar 
  agencies 
  into 
  rock-masses 
  of 
  great 
  solidity 
  and 
  hardness. 
  

  

  But 
  lest 
  any 
  doubt 
  should 
  still 
  remain 
  as 
  to 
  the 
  real 
  origin 
  of 
  

   these 
  remarkable 
  rocks, 
  I 
  am 
  fortunately 
  able 
  to 
  adduce 
  a 
  kind 
  of 
  

   evidence 
  of 
  the 
  same 
  convincing 
  character 
  as 
  that 
  of 
  fossils 
  in 
  the 
  

   sedimentary 
  rocks. 
  He 
  who 
  would 
  hesitate 
  to 
  accept 
  as 
  evidence 
  

   of 
  aqueous 
  origin 
  in 
  a 
  rock 
  such 
  physical 
  characters 
  as 
  stratifi- 
  

   cation, 
  oblique 
  lamination, 
  and 
  ripple-marks, 
  could 
  " 
  scarcely 
  con- 
  

   tinue 
  to 
  doubt, 
  if 
  shown 
  in 
  its 
  mass, 
  groups 
  of 
  oysters, 
  bored 
  by 
  

   sponges 
  and 
  annelids, 
  and 
  overgrown 
  by 
  serpulse 
  and 
  polyzoa. 
  

  

  In 
  the 
  case 
  of 
  the 
  volcanic 
  rocks, 
  though 
  we 
  cannot 
  adduce 
  the 
  

   evidence 
  of 
  fossil 
  animals 
  and 
  plants, 
  we 
  can 
  nevertheless 
  point 
  to 
  the 
  

   characteristic 
  species 
  and 
  varieties 
  of 
  minerals 
  which 
  they 
  contain. 
  

   Every 
  one 
  is 
  aware 
  that 
  the 
  vents 
  and 
  cones 
  of 
  volcanoes 
  constitute 
  

   natural 
  chemical 
  laboratories 
  in 
  which 
  many 
  beautifully 
  crystallized 
  

   minerals 
  are 
  formed 
  which 
  are 
  seldom 
  or 
  never 
  found 
  except 
  in 
  the 
  ac- 
  

   tual 
  vicinity 
  of 
  eruptive 
  centres. 
  Hence 
  such 
  species 
  and 
  varieties 
  are 
  

   usually 
  classed 
  by 
  mineralogists 
  as 
  " 
  volcanic 
  minerals." 
  These 
  mi- 
  

   nerals, 
  as 
  is 
  so 
  well 
  known, 
  are 
  constantly 
  found 
  filling 
  the 
  fissures 
  and 
  

   vesicular 
  cavities 
  which 
  occur 
  in 
  the 
  various 
  blocks 
  ejected 
  from 
  the 
  

   volcanic 
  vent; 
  and 
  as 
  has 
  been 
  remarked 
  by 
  Daubeny, 
  Sorby, 
  and 
  

   other 
  authors, 
  they 
  usually 
  belong 
  to 
  different 
  species 
  and 
  varieties 
  

   from 
  those 
  which 
  characterize 
  the 
  lava 
  streams 
  which 
  have 
  solidified 
  

   at 
  some 
  distance 
  from 
  the 
  volcanic 
  vent. 
  The 
  agents 
  concerned 
  in 
  

   the 
  production 
  of 
  these 
  volcanic 
  minerals 
  appear 
  to 
  be 
  intense 
  heat, 
  

   combined 
  with 
  the 
  infiltration 
  of 
  water 
  under 
  immense 
  pressure, 
  and 
  

   the 
  penetration 
  of 
  various 
  acid 
  gases 
  and 
  volatile 
  materials. 
  

  

  Many 
  of 
  the 
  peculiar 
  volcanic 
  minerals 
  are 
  of 
  remarkably 
  unstable 
  

   character 
  ; 
  and 
  of 
  these 
  it 
  would 
  be 
  as 
  unreasonable 
  to 
  expect 
  the 
  pre- 
  

   servation 
  as 
  that 
  jelly-fish, 
  lobworms, 
  and 
  the 
  soft 
  tissues 
  of 
  animals 
  

   and 
  plants 
  should 
  be 
  found 
  in 
  sedimentary 
  rocks. 
  But 
  the 
  more 
  

   stable 
  species 
  of 
  volcanic 
  minerals 
  actually 
  do 
  occur 
  in 
  a 
  "fossil 
  state" 
  

   in 
  the 
  midst 
  of 
  the 
  volcanic 
  agglomerates 
  of 
  the 
  Hebrides 
  ; 
  and 
  the 
  

   preservation, 
  under 
  favourable 
  conditions, 
  of 
  their 
  often 
  delicate 
  and 
  

   beautiful 
  crystals 
  is 
  the 
  result 
  of 
  a 
  remarkable 
  series 
  of 
  operations. 
  

   In 
  the 
  first 
  place, 
  by 
  infiltration, 
  the 
  masses 
  of 
  crystals 
  are 
  surrounded 
  

   by, 
  and 
  imbedded 
  in, 
  a 
  mass 
  of 
  zeolitic 
  minerals 
  which 
  is 
  formed 
  by 
  

   the 
  decomposition 
  of 
  the 
  felspars 
  of 
  the 
  lava; 
  and 
  thus 
  carefully 
  packed 
  

   they 
  escape 
  further 
  injury. 
  Subsequently, 
  on 
  the 
  exposure 
  by 
  denu- 
  

   dation 
  of 
  the 
  rock-masses, 
  the 
  " 
  cotton-wool 
  " 
  of 
  zeolitic 
  materials 
  is 
  

  

  