﻿272 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OV 
  SCOTLAND. 
  

  

  ting 
  in 
  the 
  intervals 
  between 
  the 
  several 
  outflows 
  of 
  lava 
  (as 
  in 
  the 
  

   chalk- 
  and 
  flint-detritus 
  of 
  Ardtun 
  and 
  Carsaig). 
  

  

  That, 
  on 
  the 
  other 
  hand, 
  this 
  remarkable 
  series 
  of 
  events, 
  even 
  

   the 
  latest 
  of 
  them, 
  took 
  place 
  at 
  a 
  period 
  very 
  remote 
  from 
  the 
  

   present, 
  is 
  proved 
  by 
  the 
  enormous 
  amount 
  of 
  denudation 
  which 
  the 
  

   volcanic 
  products 
  have 
  undergone. 
  

  

  I 
  know 
  of 
  no 
  more 
  striking 
  evidence 
  of 
  the 
  vast 
  duration 
  of 
  geo- 
  

   logical 
  periods, 
  than 
  that 
  which 
  is 
  afforded 
  to 
  us 
  in 
  the 
  Hebrides, 
  

   when 
  we 
  contrast 
  the 
  wonderful 
  freshness 
  of 
  the 
  contours, 
  polished 
  

   surfaces, 
  and 
  striae 
  produced 
  during 
  the 
  Glacial 
  period, 
  with 
  the 
  

   everywhere 
  abundant 
  proofs 
  of 
  enormous 
  denudation 
  suffered 
  by 
  

   rocks 
  of 
  undoubted 
  Tertiary 
  age, 
  many 
  of 
  these, 
  moreover, 
  being 
  

   very 
  conspicuous 
  for 
  their 
  intense 
  hardness 
  and 
  great 
  capability 
  of 
  

   resisting 
  weathering 
  influences. 
  How 
  almost 
  infinitesimal 
  on 
  such 
  

   a 
  comparison 
  appears 
  the 
  time 
  which 
  has 
  elapsed 
  since 
  the 
  Glacial 
  

   epoch, 
  to 
  the 
  duration 
  of 
  the 
  great 
  Tertiary 
  periods 
  ! 
  

  

  The 
  facts 
  adduced 
  in 
  the 
  present 
  paper 
  show 
  that 
  there 
  were 
  three 
  

   well-marked 
  periods 
  of 
  igneous 
  activity 
  in 
  the 
  district, 
  which 
  were 
  

   characterized 
  as 
  follows 
  : 
  — 
  

  

  First 
  Period. 
  The 
  outburst 
  in 
  the 
  midst 
  of 
  a 
  terrestrial 
  surface, 
  com- 
  

   posed 
  of 
  various 
  Palaeozoic 
  and 
  Secondary 
  rocks, 
  of 
  ashes, 
  scoriae, 
  and 
  

   fragments 
  of 
  the 
  rocks 
  in 
  which 
  the 
  vents 
  were 
  opened, 
  alternating 
  

   with 
  the 
  outflow 
  of 
  streams 
  of 
  highly 
  felspathic 
  lava. 
  With 
  these 
  

   subaerial 
  phenomena 
  was 
  connected 
  the 
  ascent 
  of 
  molten 
  masses, 
  in 
  

   a 
  manner 
  to 
  a 
  great 
  extent 
  peculiar 
  to 
  rocks 
  of 
  acid 
  composition, 
  

   which, 
  injecting 
  the 
  fissures 
  of 
  the 
  surrounding 
  and 
  superposed 
  rocks, 
  

   consolidated 
  into 
  felsitic 
  rocks 
  in 
  their 
  outer 
  portions, 
  but 
  in 
  their 
  

   deeper 
  and 
  more 
  central 
  portions, 
  under 
  different 
  conditions 
  of 
  slow 
  

   cooling 
  and 
  great 
  pressure, 
  assumed 
  well-marked 
  granitic 
  characters. 
  

   Second 
  Period. 
  The 
  extrusion 
  from 
  the 
  same 
  volcanic 
  foci 
  of 
  

   masses 
  of 
  basic 
  igneous 
  materials, 
  which 
  on 
  reaching 
  the 
  surface 
  

   spread, 
  after 
  the 
  usual 
  manner 
  of 
  lavas 
  of 
  this 
  class, 
  into 
  streams 
  

   which, 
  following 
  one 
  another 
  at 
  intervals, 
  sometimes 
  of 
  long 
  duration, 
  

   gradually 
  built 
  up 
  those 
  enormous 
  plateaux 
  of 
  basalt 
  rock 
  of 
  which 
  

   only 
  mere 
  fragments 
  have 
  escaped 
  denudation. 
  The 
  same 
  eruptions 
  

   of 
  basic 
  rocks 
  evidently 
  gave 
  rise 
  to 
  the 
  formation 
  of 
  the 
  moun- 
  

   tain-masses 
  of 
  gabbro 
  and 
  the 
  intrusion 
  between 
  and 
  among 
  the 
  

   surrounding 
  strata 
  of 
  innumerable 
  and 
  widely-spread 
  sheets 
  and 
  

   dykes 
  of 
  dolerite 
  and 
  basalt. 
  

  

  Third 
  Period. 
  The 
  appearance 
  in 
  a 
  sporadic 
  manner, 
  in 
  the 
  neigh- 
  

   bourhood 
  of 
  the 
  grand 
  old 
  extinct 
  volcanoes, 
  of 
  numerous 
  minor 
  out- 
  

   bursts 
  of 
  lava 
  (felspathic, 
  basaltic, 
  or 
  intermediate 
  in 
  composition), 
  

   which, 
  with 
  the 
  accompanying 
  fragmentary 
  ejections, 
  gave 
  rise 
  to 
  

   the 
  formation 
  of 
  a 
  series 
  of 
  volcanic 
  cones, 
  small, 
  indeed, 
  as 
  com- 
  

   pared 
  with 
  the 
  vast 
  mountain 
  masses 
  formed 
  during 
  the 
  two 
  pre- 
  

   ceding 
  periods 
  of 
  eruption, 
  but 
  of 
  which 
  some 
  at 
  least 
  were 
  of 
  no 
  

   mean 
  dimensions. 
  The 
  greater 
  number 
  of 
  these 
  would, 
  to 
  some 
  ex- 
  

   tent, 
  compensate 
  for 
  their 
  inferior 
  dimensions. 
  

  

  But 
  from 
  the 
  facts 
  which 
  I 
  have 
  adduced 
  in 
  this 
  paper 
  it 
  is 
  also 
  

   evident 
  that 
  periods 
  of 
  enormous 
  duration 
  must 
  have 
  separated 
  these 
  

  

  