﻿NAE.CONDAM. 
  33 
  

  

  the 
  rock 
  induced 
  thereby, 
  the 
  lower 
  specific 
  gravity, 
  and 
  the 
  substitu- 
  

   tion 
  of 
  hornblende 
  for 
  augite. 
  

  

  Volcanic 
  agglomerates, 
  composed 
  of 
  rounded 
  and 
  angular 
  fragments 
  

   of 
  lava 
  of 
  every 
  size, 
  up 
  to 
  3 
  or 
  4 
  feet 
  in 
  diameter, 
  

   embedded 
  in 
  a 
  finer 
  matrix, 
  are 
  largely 
  exposed 
  

   in 
  section 
  along 
  several 
  parts 
  of 
  the 
  coast. 
  Rounded 
  nodules 
  of 
  rock, 
  

   often 
  darker-colored 
  and 
  finer-grained 
  than 
  the 
  surrounding 
  matrix, 
  

   and 
  standing 
  out 
  prominently 
  on 
  weathered 
  surfaces, 
  owing 
  to 
  their 
  

   yielding 
  less 
  than 
  the 
  matrix 
  to 
  atmospheric 
  attack, 
  also 
  occur 
  fre- 
  

   quently 
  in 
  rocks 
  which 
  have 
  every 
  appearance 
  of 
  being 
  true 
  lava. 
  It 
  

   is 
  a 
  matter 
  of 
  great 
  difficulty, 
  in 
  some 
  cases, 
  to 
  distinguish 
  between 
  the 
  

   lavas 
  and 
  the 
  agglomerates, 
  owing 
  to 
  the 
  quasi-agglomeratic 
  character 
  of 
  

   rocks 
  which 
  apparently 
  should 
  be 
  classed 
  with 
  the 
  former, 
  and 
  the 
  con- 
  

   solidation 
  of 
  the 
  latter. 
  

  

  Taking 
  into 
  account 
  the 
  comparatively 
  acid 
  character 
  of 
  the 
  Narcon- 
  

   dam 
  lavas, 
  and 
  the 
  absence 
  of 
  any 
  recognisable 
  

   crater, 
  the 
  question 
  arises 
  whether 
  the 
  volcano 
  may 
  

   not 
  be 
  of 
  the 
  * 
  endogenous 
  ' 
  type, 
  and 
  produced 
  by 
  the 
  extrusion 
  of 
  viscid 
  

   lava 
  without 
  the 
  accompaniment 
  of 
  crater-forming 
  materials. 
  This 
  view 
  

   certainly 
  derives 
  some 
  support 
  from 
  the 
  general 
  uniformity 
  in 
  character 
  

   of 
  the 
  lavas, 
  and 
  the 
  absence 
  of 
  clearly 
  marked 
  alternations 
  of 
  lava 
  and 
  ash 
  

   like 
  those 
  seen 
  at 
  Barren 
  Island. 
  Viewing 
  the 
  question 
  in 
  this 
  light, 
  the 
  

   agglomerates 
  must 
  be 
  regarded 
  as 
  of 
  aqueous 
  origin, 
  having 
  been 
  formed 
  

   by 
  the 
  pluvial 
  wash 
  of 
  material 
  down 
  the 
  flanks 
  of 
  the 
  mountain. 
  That 
  

   they 
  are 
  not 
  marine 
  is 
  indicated 
  by 
  the 
  absence 
  of 
  coral, 
  &c, 
  like 
  that 
  

   found 
  so 
  abundantly 
  in 
  rolled 
  lumps 
  on 
  the 
  present 
  beach. 
  

  

  There 
  are 
  cases, 
  indeed, 
  one 
  of 
  which 
  is 
  to 
  be 
  found 
  in 
  the 
  small 
  island 
  

   near 
  the 
  anchorage, 
  where 
  lava 
  seems 
  to 
  overlie 
  agglomerate, 
  as 
  if 
  it 
  had 
  

   been 
  poured 
  forth 
  in 
  a 
  separate 
  flow. 
  But 
  the 
  difficulty, 
  already 
  alluded 
  

   to, 
  of 
  distinguishing, 
  beyond 
  the 
  possibility 
  of 
  doubt, 
  between 
  some 
  of 
  

   the 
  lavas 
  and 
  tuffs, 
  render 
  these 
  cases 
  not 
  so 
  clear 
  as 
  would 
  be 
  desirable. 
  1 
  

  

  1 
  In 
  the 
  case 
  just 
  mentioned, 
  microscopic 
  sections 
  of 
  the 
  overlying 
  and 
  underlying 
  

   rocks 
  do 
  not 
  show 
  any 
  essential 
  difference. 
  

  

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