﻿280 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  direction 
  of 
  the 
  flow, 
  while 
  the 
  actual 
  surfaces 
  of 
  the 
  stream 
  exhibit 
  

   the 
  most 
  strikingly 
  scoriaceous 
  aspect. 
  The 
  same 
  weathering 
  pro- 
  

   cess 
  sometimes 
  develops 
  in 
  these 
  old 
  lavas 
  other 
  original 
  structures, 
  

   which 
  had 
  become 
  wholly 
  obliterated 
  by 
  infiltration, 
  crystallization, 
  

   and 
  other 
  processes 
  taking 
  place 
  in 
  the 
  mass 
  of 
  the 
  rock 
  ; 
  and 
  these 
  

   structures 
  would 
  have 
  remained 
  altogether 
  unsuspected 
  but 
  for 
  the 
  

   action 
  of 
  this 
  cause. 
  Thus 
  some 
  highly 
  crystalline 
  and 
  porphyritic 
  

   rocks, 
  when 
  weathered, 
  resume 
  their 
  earthy 
  or 
  compact 
  texture 
  ; 
  and 
  

   in 
  certain 
  cases 
  structures 
  like 
  the 
  ephaerulites 
  of 
  pearlstone, 
  which 
  

   had 
  become 
  wholly 
  obscured 
  in 
  the 
  mass, 
  are 
  again 
  revealed. 
  Simi- 
  

   larly 
  many 
  rocks 
  of 
  very 
  solid 
  appearance 
  are 
  seen, 
  when 
  the 
  infil- 
  

   trated 
  materials 
  are 
  removed 
  by 
  weathering, 
  to 
  have 
  been 
  originally 
  

   aggregates 
  of 
  ashy, 
  pumiceous, 
  and 
  scoriaceous 
  fragments, 
  among 
  

   which 
  " 
  volcanic 
  bombs 
  " 
  or 
  their 
  fragments 
  may 
  not 
  unfrequently 
  

   be 
  detected*. 
  

  

  3. 
  Relations 
  of 
  the 
  Volcanic 
  Roclcs 
  of 
  Lorn. 
  — 
  In 
  considering 
  this 
  

   question 
  it 
  is 
  important 
  at 
  the 
  outset 
  to 
  notice 
  two 
  striking 
  facts 
  with 
  

   regard 
  to 
  the 
  positions 
  of 
  the 
  lavas 
  and 
  associated 
  beds 
  of 
  Lorn. 
  In 
  

   the 
  first 
  place 
  they 
  always 
  rest 
  directly, 
  but 
  unconformably, 
  upon 
  

   the 
  Lower 
  Silurian 
  gneissose 
  and 
  schistose 
  rocks, 
  and 
  never 
  exhibit 
  

   any 
  of 
  the 
  fossiliferous 
  Secondary 
  strata 
  at 
  their 
  base 
  ; 
  in 
  this 
  respect 
  

   they 
  present 
  a 
  striking 
  difference 
  in 
  their 
  relations 
  from 
  the 
  Terti- 
  

   ary 
  lavas 
  of 
  the 
  adjoining 
  districts. 
  And, 
  secondly, 
  they 
  are, 
  like 
  

   the 
  older 
  rocks 
  upon 
  which 
  they 
  repose, 
  penetrated 
  in 
  every 
  direc- 
  

   tion 
  by 
  numerous 
  dykes 
  of 
  dolerite 
  and 
  basalt 
  ; 
  these 
  are 
  precisely 
  

   similar 
  to 
  the 
  intrusive 
  masses 
  associated 
  with 
  the 
  Tertiary 
  volcanic 
  

   rocks 
  — 
  of 
  which 
  series, 
  indeed, 
  we 
  can 
  scarcely 
  hesitate 
  to 
  regard 
  

   them 
  as 
  forming 
  a 
  part. 
  

  

  These 
  later 
  dykes 
  of 
  dolerite 
  and 
  basalt, 
  which 
  are 
  sometimes 
  of 
  

   great 
  width, 
  and 
  often 
  present 
  a 
  prismatic 
  or 
  columnar 
  structure, 
  con- 
  

   stitute 
  a 
  most 
  interesting 
  and 
  striking 
  feature 
  in 
  the 
  district. 
  In 
  

   consequence 
  of 
  their 
  usually 
  greater 
  relative 
  hardness, 
  they 
  frequently 
  

   stand 
  up 
  like 
  gigantic 
  walls 
  amidst 
  the 
  rocks 
  of 
  slate 
  and 
  felstone 
  

   which 
  once 
  enclosed 
  them 
  but 
  have 
  now 
  been 
  weathered 
  away 
  from 
  

   their 
  sides. 
  

  

  The 
  remarkable 
  series 
  of 
  volcanic 
  rocks 
  of 
  Lorn 
  is 
  probably 
  several 
  

   thousand 
  feet 
  in 
  thickness 
  — 
  though, 
  its 
  upper 
  portions 
  having 
  been 
  

   everywhere 
  removed 
  by 
  denudation, 
  its 
  original 
  limits 
  in 
  this 
  respect 
  

   are 
  quite 
  unknown 
  to 
  us. 
  Its 
  relations 
  to 
  the 
  slate 
  rocks 
  and 
  certain 
  

   conglomerates, 
  sandstones, 
  and 
  breccias 
  appear, 
  at 
  first 
  sight, 
  to 
  be 
  

  

  * 
  That 
  the 
  geologist 
  never 
  meets 
  with 
  glassy 
  lavas 
  (obsidians, 
  pitchstones 
  &c.) 
  

   among 
  the 
  older 
  rocks 
  will 
  not 
  occasion 
  any 
  surprise 
  when 
  we 
  reflect 
  upon 
  the 
  

   facility 
  with 
  which 
  artificial 
  glasses 
  undergo 
  devitrification. 
  That 
  glassy 
  lavas 
  

   were 
  formed 
  even 
  in 
  palaeozoic 
  times 
  appears 
  to 
  be 
  indicated 
  by 
  the 
  fact 
  recorded 
  

   in 
  the 
  text 
  that 
  weathering 
  sometimes 
  reveals 
  the 
  characteristic 
  sphcerulitic 
  struc- 
  

   ture 
  in 
  some 
  of 
  the 
  lavas 
  of 
  Lorn. 
  Many 
  of 
  the 
  Newer 
  Palaeozoic 
  " 
  porphyrites 
  " 
  

   of 
  Scotland 
  are 
  quite 
  undistinguishable 
  in 
  appearance 
  from 
  some 
  trachytes, 
  such 
  

   as 
  those 
  of 
  Hungary. 
  The 
  white 
  granular 
  siliceous 
  rocks 
  into 
  which 
  the 
  former 
  

   are 
  sometimes 
  found 
  altered, 
  appear 
  to 
  be 
  equally 
  undistinguishable 
  from 
  the 
  

   products 
  of 
  the 
  decomposition 
  of 
  the 
  latter 
  (occasioned 
  by 
  the 
  passage 
  of 
  acid 
  

   vapours 
  through 
  them), 
  and 
  constituting 
  the 
  so-called 
  "Alaunstein."' 
  

  

  