﻿VOLCANIC 
  ROCKS 
  OF 
  DARTMOOR. 
  399 
  

  

  connexion 
  with 
  the 
  present 
  river-system 
  or 
  not, 
  I 
  have 
  no 
  hesitation 
  

   in 
  giving 
  the 
  great 
  bulk 
  of 
  them, 
  at 
  least, 
  a 
  Dartmoor 
  birthplace. 
  

  

  Some 
  of 
  these 
  pebbles, 
  found 
  on 
  Slapton 
  Sands, 
  suggested 
  a 
  possible 
  

   secondary 
  origin 
  in 
  the 
  degradation 
  of 
  the 
  Triassic 
  conglomerate 
  

   which 
  extends 
  westward 
  in 
  mass 
  to 
  Torbay, 
  and 
  remnants 
  of 
  a 
  still 
  

   further 
  extension 
  of 
  which 
  are 
  to 
  be 
  found 
  at 
  Thurlestone 
  in 
  Bigbury 
  

   Bav, 
  and 
  at 
  Cawsand 
  in 
  Plymouth 
  Sound. 
  This 
  led 
  to 
  an 
  examination 
  

   of 
  a 
  number 
  of 
  the 
  fragments 
  of 
  igneous 
  rocks 
  included 
  in 
  the 
  

   Triassic 
  couglomerates 
  of 
  Devon. 
  Among 
  these 
  indubitable 
  pieces 
  of 
  

   Dartmoor 
  granite 
  have 
  been 
  from 
  time 
  to 
  time 
  identified, 
  but 
  far 
  less 
  

   numerous 
  in 
  comparison 
  than 
  the 
  examples 
  of 
  so-called 
  " 
  porphy- 
  

   ritic 
  trap," 
  with 
  which 
  they 
  are 
  largely 
  associated. 
  The 
  results 
  of 
  

   that 
  examination 
  speedily 
  convinced 
  me 
  that 
  many 
  of 
  these 
  " 
  porphy- 
  

   ritic 
  traps 
  " 
  were 
  really 
  " 
  elvans," 
  whilst 
  others 
  presented 
  a 
  nearer 
  

   approach 
  to 
  volcanic 
  types. 
  Probably 
  this 
  would 
  have 
  been 
  re- 
  

   cognized 
  long 
  since 
  had 
  there 
  not 
  been 
  a 
  kind 
  of 
  tacit 
  assumption 
  

   that 
  the 
  covering 
  rocks 
  under 
  which 
  the 
  granite 
  of 
  Dartmoor 
  con- 
  

   solidated 
  were 
  wholly 
  sedimentary. 
  Had 
  the 
  volcanic 
  character 
  

   of 
  at 
  least 
  the 
  central 
  pile 
  been 
  suggested, 
  it 
  would 
  have 
  been 
  clear 
  

   that 
  in 
  such 
  an 
  early 
  stage 
  of 
  denudation 
  as 
  that 
  which 
  afforded 
  

   material 
  for 
  the 
  Triassic 
  conglomerates 
  of 
  Devon, 
  rocks 
  of 
  inter- 
  

   mediate 
  and 
  volcanic 
  types 
  must 
  play 
  a 
  far 
  more 
  prominent 
  part 
  

   than 
  granites. 
  

  

  It 
  is 
  evident 
  that 
  the 
  " 
  felsites 
  " 
  of 
  the 
  higher 
  stage 
  of 
  Dartmoor 
  

   must 
  have 
  been 
  much 
  more 
  important 
  and 
  wide-spread 
  than 
  the 
  

   "elvans" 
  or 
  dyke-rocks 
  by 
  which 
  they 
  are 
  now 
  alone 
  represented, 
  

   or 
  they 
  never 
  could 
  have 
  yielded 
  the 
  enormous 
  quantity 
  of 
  material 
  

   which 
  is 
  still 
  traceable. 
  

  

  The 
  " 
  elvans 
  " 
  which 
  remain 
  in 
  situ 
  are 
  practically 
  confined 
  to 
  

   the 
  borders 
  of 
  the 
  moorland, 
  ranging 
  at 
  points 
  up 
  to 
  the 
  main 
  mass 
  

   of 
  granite 
  in 
  the 
  covering 
  rocks, 
  and 
  stretching 
  westward 
  in 
  

   parallel 
  lines 
  towards 
  the 
  next 
  granitic 
  boss, 
  on 
  the 
  western 
  side 
  of 
  

   the 
  Tamar, 
  at 
  Kingston 
  Down. 
  Pive 
  of 
  these 
  dykes 
  are 
  set 
  

   out 
  on 
  the 
  Geological 
  Map 
  of 
  Devon 
  between 
  Shillamill 
  near 
  Tavistock, 
  

   on 
  the 
  north, 
  and 
  Cann 
  Quarry 
  near 
  Plymouth, 
  on 
  the 
  south 
  ; 
  there 
  are 
  

   others, 
  but 
  of 
  minor 
  importance 
  and 
  yielding 
  no 
  additional 
  facts. 
  

  

  Now 
  the 
  northern 
  of 
  these 
  elvans 
  traverse 
  the 
  lowest 
  rocks 
  exposed 
  

   in 
  this 
  area, 
  and 
  the 
  southern 
  the 
  highest 
  ; 
  moreover 
  the 
  northern 
  are 
  

   nearest 
  to 
  the 
  main 
  mass 
  of 
  granite, 
  and 
  the 
  southern 
  most 
  distant 
  

   from 
  it. 
  The 
  northern, 
  therefore, 
  have 
  been 
  formed, 
  to 
  all 
  appearance, 
  

   under 
  conditions 
  of 
  greater 
  depth 
  and 
  pressure 
  than 
  their 
  successors 
  

   southward. 
  As 
  a 
  result 
  of 
  this, 
  we 
  find 
  the 
  most 
  distinctly 
  granitoid 
  

   orplutonic 
  varieties 
  in 
  the 
  northern 
  elvans, 
  Shillamill 
  and 
  Grenofen, 
  

   and 
  the 
  most 
  even-grained 
  and 
  felsitic 
  in 
  the 
  southern 
  at 
  Cann 
  Quarry 
  ; 
  

   while 
  the 
  Boborough 
  Down 
  elvan, 
  which 
  is 
  intermediate, 
  to 
  some 
  

   extent, 
  combines 
  the 
  two 
  characteristics, 
  in 
  its 
  compact 
  felsitic 
  

   ground-mass 
  and 
  its 
  well-developed 
  porphyritic 
  quartz-crystals. 
  

  

  But 
  the 
  chief 
  point 
  to 
  which 
  I 
  wish 
  to 
  direct 
  attention 
  here 
  is 
  

   the 
  wide 
  amount 
  of 
  variation 
  within 
  narrow 
  limits 
  in 
  the 
  same 
  

   dyke. 
  

  

  