﻿VOLCANIC 
  ROCKS 
  OF 
  DARTMOOR. 
  401 
  

  

  the 
  porphyritic 
  crystals 
  singularly 
  perfect 
  in 
  outline 
  and 
  character. 
  

   Though 
  it 
  is 
  not 
  an 
  andesite, 
  anclesitic 
  affinities 
  seem 
  to 
  be 
  suggested. 
  

  

  As 
  it 
  is 
  simply 
  intended 
  to 
  indicate 
  the 
  more 
  typical 
  features 
  of 
  

   the 
  " 
  elvans," 
  and 
  those 
  of 
  Grenofen 
  and 
  Shillamill 
  surpass 
  all 
  

   others 
  in 
  interest, 
  I 
  pass 
  on 
  to 
  consider 
  the 
  evidence 
  afforded 
  by 
  

   Dartmoor 
  of 
  the 
  existence 
  of 
  distinctly 
  volcanic 
  rocks 
  as 
  a 
  part 
  of 
  

   its 
  igneous 
  series. 
  

  

  There 
  are 
  among 
  the 
  fragments 
  of 
  the 
  local 
  Triassic 
  conglomerate 
  

   examples 
  of 
  andesitic 
  and 
  similar 
  rocks 
  that 
  may 
  fairly 
  be 
  classed 
  

   in 
  this 
  category, 
  but 
  I 
  do 
  not 
  desire 
  to 
  lay 
  any 
  special 
  stress 
  upon 
  

   them. 
  

  

  It 
  is 
  more 
  distinctly 
  to 
  the 
  point 
  that 
  in 
  October 
  of 
  last 
  year 
  

   there 
  was 
  found 
  in 
  undisturbed 
  clay 
  on 
  the 
  limestone 
  of 
  Cattedown, 
  

   near 
  Plymouth, 
  a 
  deposit 
  of 
  water-borne 
  and 
  water-worn 
  detritus, 
  

   which 
  indicated 
  a 
  Dartmoor 
  origin 
  for 
  a 
  large 
  proportion 
  of 
  its 
  identi- 
  

   fiable 
  constituents, 
  but 
  contained 
  associated 
  therewith 
  rolled 
  flints 
  and 
  

   pebbles 
  of 
  Carboniferous, 
  Liassic, 
  and 
  Cretaceous 
  limestones, 
  which 
  

   could 
  not 
  have 
  been 
  brought 
  to 
  the 
  place 
  where 
  they 
  were 
  found 
  

   from 
  any 
  existing 
  locality 
  by 
  any 
  existing 
  river. 
  The 
  inference 
  

   seemed 
  clear 
  that 
  these 
  Carboniferous, 
  Liassic, 
  and 
  Cretaceous 
  rem- 
  

   nants 
  represented 
  a 
  very 
  ancient 
  denudation, 
  when 
  the 
  western 
  

   flanks 
  of 
  Dartmoor 
  were 
  partially 
  covered 
  by 
  Carboniferous 
  rocks 
  

   which 
  have 
  now 
  disappeared 
  ; 
  by 
  the 
  Chalk, 
  of 
  which 
  the 
  nearest 
  

   trace 
  is 
  now 
  on 
  Haldon 
  ; 
  and 
  by 
  the 
  Lias, 
  which 
  does 
  not 
  now 
  extend 
  

   further 
  west 
  than 
  Lyme 
  Regis. 
  

  

  And 
  associated 
  with 
  these 
  clearly 
  identifiable 
  and 
  undoubtedly 
  

   local 
  rocks 
  were 
  others 
  hitherto 
  unknown 
  in 
  the 
  West 
  — 
  examples 
  of 
  

   andesites 
  which 
  Professor 
  Bonney 
  has 
  kindly 
  examined 
  for 
  me, 
  and 
  

   has 
  pronounced 
  typical 
  examples, 
  closely 
  resembling 
  specimens 
  from 
  

   the 
  Andes 
  ; 
  and 
  clastic 
  igneous 
  rocks, 
  one 
  of 
  which, 
  a 
  volcanic 
  grit, 
  

   Professor 
  Bonney 
  regards 
  as 
  of 
  very 
  unusual 
  interest, 
  and 
  as 
  the 
  

   result 
  of 
  the 
  denudation 
  of 
  volcanic 
  cones. 
  Some 
  of 
  the 
  fragments 
  

   are 
  felspar, 
  but 
  quartz 
  and 
  viridite 
  also 
  occur. 
  Prof. 
  Bonney 
  remarks 
  

   that 
  the 
  rock-fragments, 
  so 
  far 
  as 
  ascertained, 
  are 
  all 
  of 
  igneous 
  

   origin. 
  " 
  Some 
  are 
  fairly 
  clear, 
  some 
  a 
  rich 
  brown 
  colour, 
  some 
  

   almost 
  black 
  with 
  opacite 
  ; 
  some 
  are 
  homogeneous, 
  except 
  for 
  a 
  

   little 
  opacite 
  and 
  some 
  belonites 
  or 
  trichites 
  of 
  a 
  dark 
  grey 
  colour, 
  

   which 
  often 
  are 
  grouped 
  in 
  more 
  or 
  less 
  dendritic 
  forms 
  or 
  bundles 
  

   like 
  rootlets. 
  A 
  few 
  of 
  these 
  grains 
  are 
  still 
  isotropic, 
  but 
  most 
  of 
  

   those 
  which 
  are 
  transparent 
  exhibit 
  devitrification-structure. 
  Small 
  

   spherulites 
  are 
  rather 
  common 
  ; 
  one 
  fragment 
  seems 
  part 
  of 
  a 
  large 
  

   spherulite. 
  Other 
  fragments 
  show 
  flow-structure 
  ; 
  one 
  is 
  perlitic. 
  

   Clearly 
  several 
  varieties 
  of 
  rock 
  are 
  present, 
  but 
  I 
  think 
  the 
  majority 
  

   may 
  be 
  referred 
  to 
  andesites, 
  some 
  of 
  which 
  may 
  not 
  be 
  far 
  removed 
  

   from 
  basalt 
  ; 
  others 
  may 
  have 
  a 
  tolerably 
  high 
  percentage 
  of 
  silica." 
  

  

  I 
  believe 
  I 
  am 
  perfectly 
  safe 
  in 
  saying 
  that 
  there 
  are 
  no 
  rocks 
  in 
  

   the 
  county 
  or 
  in 
  the 
  West 
  of 
  England 
  yet 
  known 
  that 
  would 
  yield 
  

   either 
  of 
  these 
  andesites 
  or 
  volcanic 
  fragmental 
  examples. 
  If 
  they 
  

   belonged 
  to 
  the 
  superstructure 
  of 
  Dartmoor, 
  the 
  denudation 
  of 
  which 
  

   began 
  in 
  Triassic 
  times, 
  the 
  absence 
  of 
  the 
  parent-rocks 
  is 
  explained. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  179. 
  2 
  g 
  

  

  