﻿402 
  ON 
  THE 
  ELVANS 
  AND 
  VOLCANIC 
  EOCKS 
  OE 
  DAETMOOE. 
  

  

  If 
  they 
  did 
  not 
  belong 
  to 
  Dartmoor, 
  their 
  association 
  with 
  rocks 
  all 
  

   of 
  Dartmoor 
  origin, 
  assured 
  or 
  probable, 
  has 
  to 
  be 
  accounted 
  for. 
  

   And 
  that 
  enormous 
  changes 
  have 
  taken 
  place 
  on 
  the 
  moor 
  since 
  the 
  

   existing 
  river-systems 
  have 
  been 
  in 
  operation, 
  the 
  deep 
  gorges 
  cut 
  

   by 
  the 
  principal 
  streams, 
  and 
  the 
  fact 
  that 
  I 
  have 
  recently 
  found 
  

   remnants 
  of 
  an 
  ancient 
  Dartmoor 
  river-gravel 
  300 
  feet 
  above 
  the 
  

   present 
  bed 
  of 
  the 
  Tavy, 
  near 
  Tavistock, 
  will 
  attest. 
  

  

  However, 
  the 
  point 
  need 
  not 
  be 
  left 
  to 
  inference 
  only. 
  Among 
  

   the 
  specimens 
  collected 
  by 
  me 
  from 
  the 
  Dartmoor 
  detritus 
  at 
  Lee 
  

   Moor, 
  within 
  the 
  granitic 
  boundary, 
  was 
  a 
  piece 
  of 
  compact 
  grey 
  

   felsitic 
  rock 
  which 
  indicated 
  fluxion-structure. 
  This, 
  on 
  being 
  sliced, 
  

   proved 
  to 
  be 
  a 
  felsitic 
  lava-breccia, 
  containing 
  numerous 
  fragmental 
  

   grains 
  of 
  quartz 
  and 
  also 
  fragments 
  of 
  volcanic 
  rock, 
  some 
  of 
  which 
  

   closely 
  resemble 
  certain 
  fragments 
  in 
  the 
  volcanic 
  grit 
  of 
  Cattedown. 
  

  

  That 
  the 
  forces 
  which 
  upheaved 
  the 
  great 
  granitic 
  boss 
  of 
  Dartmoor 
  

   found 
  relief 
  in 
  a 
  volcanic 
  outburst 
  seems 
  therefore 
  clear, 
  though 
  not 
  

   a 
  vestige 
  of 
  the 
  volcanic 
  pile 
  remains. 
  

  

  The 
  purpose 
  of 
  this 
  paper 
  is 
  threefold 
  : 
  — 
  

  

  1. 
  To 
  give 
  reasons 
  for 
  the 
  belief 
  that 
  the 
  present 
  granite 
  of 
  Dart- 
  

   moor 
  passed 
  upwards 
  into 
  felsitic 
  and 
  volcanic 
  rocks, 
  remnants 
  of 
  

   which 
  are 
  to 
  be 
  found 
  in 
  the 
  Triassic 
  conglomerate 
  of 
  Devon, 
  in 
  

   the 
  detritus 
  of 
  the 
  bottom 
  lands 
  of 
  the 
  moor 
  itself, 
  on 
  the 
  beaches 
  

   of 
  the 
  Channel, 
  and 
  in 
  ancient 
  river-gravels 
  and 
  pebble-beds. 
  

  

  2. 
  To 
  indicate 
  the 
  wide 
  range 
  of 
  character 
  taken 
  by 
  the 
  Devonian 
  

   felsites 
  of 
  the 
  Dartmoor 
  district. 
  

  

  3. 
  To 
  point 
  out 
  some 
  of 
  the 
  evidence 
  that 
  exists 
  in 
  the 
  elvans 
  

   in 
  situ 
  for 
  the 
  development 
  of 
  the 
  most 
  varied 
  of 
  their 
  forms 
  from 
  

   one 
  common 
  magma, 
  and 
  the 
  peculiar 
  value 
  of 
  their 
  study 
  as 
  bearing 
  

   upon 
  the 
  wider 
  questions 
  of 
  penological 
  research. 
  

  

  Discussion. 
  

  

  Prof. 
  Bonnet 
  stated 
  that 
  the 
  specimen 
  of 
  volcanic 
  grit 
  which 
  

   he 
  had 
  examined 
  for 
  Mr. 
  Worth 
  was 
  one 
  of 
  the 
  most 
  extraordinary 
  

   samples 
  ever 
  sent 
  to 
  him. 
  He 
  agreed 
  with 
  the 
  Author 
  as 
  to 
  the 
  

   importance 
  of 
  studying 
  the 
  Triassic 
  conglomerates 
  of 
  the 
  south- 
  

   west. 
  The 
  term 
  " 
  elvan 
  " 
  should 
  not 
  be 
  used 
  as 
  a 
  scientific 
  term, 
  

   having 
  been 
  so 
  vaguely 
  applied. 
  

  

  Dr. 
  Hinde 
  called 
  attention 
  to 
  the 
  absence 
  of 
  specimens 
  on 
  the 
  

   table 
  as 
  a 
  check 
  upon 
  discussion. 
  

  

  