﻿J. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  227 
  

  

  ally 
  constituted 
  a 
  mass 
  as 
  porous 
  as 
  a 
  sponge, 
  though 
  subsequently 
  

   converted, 
  by 
  the 
  chemical 
  action 
  of 
  infiltrating 
  water, 
  into 
  solid 
  

   amygdaloidal 
  rocks 
  *. 
  

  

  In 
  the 
  second 
  place 
  I 
  would 
  direct 
  attention 
  to 
  the 
  nature 
  of 
  the 
  

   surfaces 
  over 
  which 
  the 
  first 
  emitted 
  lavas 
  have 
  flowed. 
  Thus, 
  in 
  

   the 
  North 
  of 
  Ireland, 
  where 
  the 
  lavas 
  rest 
  directly 
  upon 
  the 
  chalk, 
  

   the 
  old 
  surface 
  of 
  the 
  latter 
  was 
  not 
  only 
  one 
  of 
  great 
  irregularity, 
  

   abounding 
  in 
  hills 
  and 
  valleys, 
  but 
  it 
  is 
  almost 
  everywhere 
  covered 
  

   with 
  an 
  accumulation 
  of 
  perfectly 
  angular 
  chalk-flints 
  ; 
  these 
  super- 
  

   ficial 
  deposits, 
  too, 
  often 
  fill 
  " 
  sand-pipes," 
  which 
  penetrate 
  for 
  con- 
  

   siderable 
  distances 
  into 
  the 
  mass 
  of 
  the 
  chalk 
  rock. 
  No 
  one 
  familiar 
  

   with 
  the 
  characteristic 
  modes 
  of 
  weathering 
  of 
  the 
  chalk 
  can 
  fail 
  to 
  

   recognize 
  in 
  these 
  appearances 
  the 
  evidences 
  of 
  subaerial 
  action. 
  At 
  

   other 
  points, 
  as 
  indicated 
  by 
  Portlock 
  in 
  the 
  case 
  of 
  Slieve 
  Gallion 
  

   and 
  Eathlin 
  Island, 
  there 
  are 
  found, 
  interposed 
  between 
  the 
  old 
  

   chalk 
  soils 
  and 
  the 
  superincumbent 
  lavas, 
  beds 
  of 
  lignite 
  containing 
  

   wood 
  and 
  amber, 
  which 
  are 
  unquestionably 
  the 
  remains 
  of 
  old 
  

   forests 
  belonging 
  to 
  the 
  period 
  immediately 
  preceding 
  that 
  of 
  the 
  

   emission 
  of 
  the 
  lavas. 
  In 
  other 
  cases, 
  again, 
  as 
  in 
  Morvern, 
  we 
  find, 
  

   in 
  a 
  similar 
  position, 
  thick 
  beds 
  of 
  unstratified 
  ash 
  or 
  volcanic 
  dustf 
  . 
  

   But 
  in 
  no 
  instance, 
  so 
  far 
  as 
  I 
  am 
  aware, 
  do 
  the 
  surfaces 
  of 
  the 
  

   rocks 
  immediately 
  underlying 
  the 
  lavas 
  exhibit 
  the 
  slightest 
  indica- 
  

   tion 
  of 
  having 
  been 
  subjected 
  to 
  the 
  action 
  of 
  marine 
  denudation.. 
  

  

  In 
  the 
  third 
  place, 
  when 
  we 
  examine 
  the 
  deposits 
  interposed 
  

   between 
  the 
  different 
  lava 
  sheets, 
  and 
  which 
  were 
  evidently 
  formed 
  

   during 
  the 
  intervals 
  between 
  their 
  eruption, 
  we 
  find 
  numerous 
  indi- 
  

   cations 
  of 
  the 
  prevalence 
  of 
  terrestrial, 
  fluviatile, 
  and 
  lacustrine 
  con- 
  

   ditions, 
  but 
  never 
  in 
  any 
  instance 
  of 
  those 
  of 
  a 
  marine 
  character. 
  

  

  (1) 
  Bands 
  of 
  clay 
  or 
  earth, 
  usually 
  only 
  a 
  few 
  inches 
  in 
  thick- 
  

   ness, 
  and 
  of 
  a 
  bright 
  red 
  colour, 
  very 
  frequently 
  occur 
  between 
  

   the 
  sheets 
  of 
  Tertiary 
  basalt 
  both 
  in 
  Scotland 
  and 
  Ireland. 
  These 
  

   appear 
  to 
  be 
  identical 
  in 
  character 
  with 
  the 
  beds 
  of 
  soil 
  formed 
  by 
  

   the 
  weathering 
  of 
  the 
  surface 
  of 
  one 
  lava 
  stream 
  and 
  destroyed 
  by 
  

   burning 
  when 
  overwhelmed 
  by 
  a 
  new 
  sheet 
  of 
  lava. 
  This 
  pheno- 
  

   menon, 
  as 
  pointed 
  out 
  by 
  Sir 
  Charles 
  Lyell, 
  is 
  frequently 
  exhibited 
  in 
  

   existing 
  volcanoes 
  J. 
  

  

  * 
  By 
  this 
  infiltration 
  various 
  minerals 
  have 
  been 
  formed 
  in 
  the 
  cavities 
  

   of 
  the 
  vesicular 
  lava. 
  By 
  very 
  simple 
  chemical 
  reactions 
  the 
  felspars 
  yield 
  

   zeolitic 
  minerals, 
  while 
  the 
  pyroxenic 
  constituents 
  are 
  converted 
  into 
  various 
  

   hvdrous, 
  magnesian, 
  and 
  ferruginous 
  silicates. 
  More 
  complete 
  decompositions 
  

   result 
  in 
  the 
  production 
  of 
  chalcedony 
  and 
  calcspar. 
  It 
  is 
  a 
  noteworthy 
  cir- 
  

   cumstance, 
  capable 
  of 
  frequent 
  verification 
  in 
  the 
  district 
  under 
  consideration, 
  

   that 
  in 
  rocks 
  containing 
  numerous 
  amygdaloidal 
  cavities 
  the 
  mass 
  of 
  the 
  rock 
  

   is 
  always 
  much 
  decomposed, 
  and 
  often 
  has 
  passed 
  into 
  a 
  " 
  wackose 
  " 
  condition, 
  

   through 
  the 
  partial 
  removal 
  of 
  its 
  materials 
  by 
  solution. 
  

  

  f 
  These 
  beds 
  precisely 
  resemble 
  in 
  character 
  those 
  which 
  are 
  found 
  in 
  con* 
  

   nexion 
  with 
  volcanoes 
  but 
  recently 
  extinct. 
  Thus, 
  for 
  example, 
  in 
  the 
  island 
  of 
  

   Lipari 
  we 
  find 
  a 
  fine-grained 
  ash 
  of 
  chocolate-brown 
  colour, 
  full 
  of 
  small 
  decom- 
  

   posed 
  fragments 
  of 
  white 
  colour 
  (probably 
  fragments 
  of 
  crystals 
  of 
  felspar), 
  

   which 
  is 
  quite 
  undistinguishable 
  from 
  the 
  rock 
  referred 
  to 
  in 
  the 
  text. 
  

  

  X 
  "On 
  the 
  Lavas 
  of 
  Etna," 
  &o. 
  Phil. 
  Trans. 
  1858, 
  pt. 
  ii. 
  p. 
  711. 
  During 
  a 
  

   recent 
  examination 
  of 
  many 
  of 
  the 
  most 
  interesting 
  volcanic 
  districts 
  of 
  the 
  

  

  