﻿288 
  J. 
  W. 
  JUDD 
  ON 
  IHE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  porphyritic 
  structure. 
  Occasionally, 
  however, 
  streams 
  of 
  a 
  more 
  

   basic 
  character, 
  and 
  approaching 
  to 
  basalts 
  in 
  composition, 
  alternate 
  

   with 
  the 
  felstone 
  lavas. 
  With 
  regard 
  to 
  the 
  nomenclature 
  of 
  these 
  

   rocks 
  great 
  confusion 
  and 
  uncertainty 
  still 
  unfortunately 
  prevails. 
  

   By 
  Jameson 
  and 
  his 
  pupils 
  they 
  were 
  distinguished 
  as 
  porphyries, 
  

   clinkstones, 
  compact 
  felspars, 
  claystones 
  and 
  basalts. 
  In 
  the 
  earlier 
  

   publications 
  of 
  the 
  Geological 
  Survey 
  they 
  are 
  referred 
  to 
  as 
  " 
  fel- 
  

   stones 
  ;" 
  and 
  in 
  the 
  later 
  works 
  published 
  by 
  that 
  department 
  an 
  

   attempt 
  has 
  been 
  made 
  to 
  assimilate 
  their 
  nomenclature 
  to 
  that 
  

   adopted 
  on 
  the 
  Continent 
  by 
  applying 
  to 
  them 
  the 
  terms 
  porphyrite, 
  

   melaphyre 
  &c. 
  

  

  The 
  lavas 
  of 
  Central 
  Scotland, 
  like 
  those 
  of 
  Lorn, 
  have, 
  as 
  a 
  

   rnle, 
  undergone 
  a 
  far 
  greater 
  amount 
  of 
  change 
  from 
  chemical 
  action 
  

   than 
  those 
  of 
  the 
  Tertiary 
  period. 
  Consequently 
  in 
  many 
  cases 
  

   their 
  mode 
  of 
  origin 
  is 
  by 
  no 
  means 
  so 
  obvious 
  as 
  in 
  the 
  case 
  of 
  

   these 
  latter; 
  and 
  in 
  some 
  places 
  not 
  only 
  have 
  they 
  undergone 
  

   a 
  very 
  great 
  amount 
  of 
  general 
  alteration, 
  but 
  mineral 
  veins 
  have 
  

   been 
  formed 
  in 
  their 
  mass. 
  

  

  Strikingly 
  similar 
  as 
  the 
  lavas 
  of 
  Lorn 
  are 
  to 
  those 
  of 
  Central 
  

   Scotland 
  in 
  their 
  petrological 
  characters, 
  they 
  nevertheless 
  present 
  

   in 
  their 
  relations 
  one 
  very 
  marked 
  point 
  of 
  contrast. 
  While 
  the 
  

   lavas 
  of 
  the 
  northern 
  flanks 
  of 
  the 
  Grampian 
  Mountains 
  are, 
  with 
  

   the 
  exception 
  of 
  the 
  conglomerates 
  and 
  sandstone 
  near 
  their 
  base, 
  

   only 
  separated 
  by 
  thin 
  bands 
  of 
  interstratified 
  ash 
  &c, 
  the 
  similar 
  

   lavas 
  of 
  Central 
  Scotland 
  alternate 
  with 
  great 
  masses 
  of 
  sedimentary 
  

   rocks, 
  conglomerates, 
  sandstones, 
  limestones, 
  shales, 
  and 
  beds 
  of 
  

   stratified 
  ash. 
  While 
  the 
  rocks 
  of 
  the 
  former 
  appear 
  to 
  be 
  the 
  

   products 
  of 
  a 
  series 
  of 
  subaerial 
  volcanic 
  eruptions, 
  those 
  of 
  the 
  

   latter 
  are 
  not 
  less 
  clearly 
  seen 
  to 
  have 
  been 
  formed 
  by 
  the 
  outburst 
  

   from 
  time 
  to 
  time 
  of 
  streams 
  of 
  lava, 
  which 
  flowed 
  over 
  the 
  beds 
  of 
  

   seas 
  or 
  lakes 
  in 
  which 
  the 
  accumulation 
  of 
  stratified 
  sediments 
  was 
  

   going 
  on. 
  

  

  With 
  regard 
  to 
  the 
  age 
  of 
  the 
  great 
  lava 
  sheets 
  of 
  the 
  Scottish 
  

   Lowlands 
  we 
  are 
  fortunately 
  supplied 
  with 
  the 
  most 
  satisfactory 
  

   evidence. 
  The 
  associated 
  contemporaneous 
  sedimentary 
  rocks 
  

   occasionally 
  contain 
  fossils 
  ; 
  and 
  thus 
  we 
  are 
  able 
  to 
  define 
  the 
  age 
  

   of 
  different 
  parts 
  of 
  them 
  as 
  Lower, 
  Middle, 
  and 
  Upper 
  Old 
  Red 
  

   Sandstone 
  and 
  the 
  lower 
  part 
  of 
  the 
  Carboniferous 
  (Calciferous 
  

   Sandstone). 
  Some 
  difficulties 
  still 
  remain, 
  it 
  is 
  true, 
  with 
  regard 
  to 
  

   the 
  ages 
  of 
  some 
  of 
  the 
  isolated 
  portions 
  of 
  what 
  once 
  constituted 
  

   great 
  plateaux 
  of 
  volcanic 
  rocks 
  covering 
  a 
  very 
  large 
  portion 
  of 
  

   Scotland. 
  This 
  is 
  due 
  to 
  the 
  fact 
  that 
  many 
  of 
  the 
  fossils, 
  especially 
  

   those 
  of 
  the 
  Old 
  Eed 
  Sandstone, 
  are 
  very 
  local 
  in 
  their 
  mode 
  of 
  

   occurrence 
  ; 
  but 
  that 
  strata 
  belonging 
  to 
  all 
  the 
  periods 
  enumerated 
  

   are 
  found 
  interbedded 
  with 
  these 
  ancient 
  lavas 
  is 
  indisputable. 
  

   Towards 
  the 
  latter 
  part 
  of 
  the 
  period 
  indicated 
  (namely, 
  during 
  the 
  

   deposition 
  of 
  the 
  Calciferous 
  Sandstone) 
  the 
  lavas 
  show 
  a 
  general 
  

   tendency 
  to 
  a 
  more 
  basic 
  character 
  than 
  those 
  of 
  earlier 
  periods, 
  and 
  

   thus 
  form 
  a 
  transition 
  to 
  the 
  basaltic 
  eruptions 
  of 
  a 
  sporadic 
  cha- 
  

   racter 
  by 
  which 
  the 
  great 
  series 
  of 
  volcanic 
  phenomena 
  extending 
  

  

  