﻿39 
  S 
  ME. 
  R. 
  N. 
  WORTH 
  OS 
  THE 
  ELVAKS 
  AND 
  

  

  26. 
  The 
  Elvaxs 
  and 
  Volcanic 
  Bocks 
  0/ 
  Dartmoor. 
  

   By 
  E. 
  N. 
  \Vorih, 
  Esq., 
  F.G.S. 
  (Bead 
  April 
  3, 
  1889. 
  | 
  

  

  In 
  the 
  course 
  of 
  au 
  inquiry 
  into 
  the 
  physical 
  history 
  of 
  Dartmoor, 
  

   which 
  led 
  to 
  the 
  conclusion 
  that 
  this 
  great 
  granitic 
  upland 
  represents 
  

   the 
  basal 
  portion 
  of 
  a 
  volcano 
  some 
  16,000 
  feet 
  high, 
  special 
  attention 
  

   was 
  necessarily 
  paid 
  to 
  the 
  felsites 
  or 
  elvans 
  of 
  the 
  district, 
  as 
  

   filli 
  ng 
  the 
  gap 
  between 
  the 
  plutonic 
  members 
  of 
  the 
  series, 
  the 
  

   granites, 
  and 
  the 
  true 
  volcanic 
  rocks 
  no 
  longer 
  found 
  in 
  situ. 
  Search 
  

   for 
  traces 
  of 
  the 
  latter 
  was 
  also 
  made. 
  Hence 
  were 
  gathered 
  the 
  facts 
  

   now 
  briefly 
  set 
  forth. 
  The 
  less 
  definite 
  term 
  " 
  elvan 
  '" 
  is 
  used 
  here 
  in 
  

   preference 
  to 
  the 
  more 
  distinct 
  •• 
  felstone 
  '" 
  or 
  " 
  quartz-"'' 
  or 
  M 
  felspar- 
  

   porphvry." 
  because 
  the 
  intention 
  is 
  to 
  include 
  under 
  that 
  head 
  all 
  

   the 
  dyke-rocks 
  of 
  granitoid 
  material 
  connected 
  with 
  the 
  Dartmoor 
  

   granite 
  : 
  and 
  because 
  it 
  is 
  one 
  of 
  the 
  chief 
  objects 
  of 
  this 
  paper 
  to 
  

   show 
  that 
  there 
  is 
  no 
  fundamental 
  distinction 
  between 
  the 
  rocks 
  so 
  

   named, 
  as 
  they 
  occur 
  in 
  the 
  Dartmoor 
  environment, 
  but 
  that 
  different 
  

   portions 
  of 
  the 
  same 
  dyke 
  may 
  exhibit 
  all 
  these 
  varieties 
  of 
  structure, 
  

   and 
  present 
  a 
  graduated 
  series 
  between 
  granite 
  and 
  a 
  compact 
  

   felsite, 
  in 
  which 
  differentiation 
  has 
  yet 
  to 
  be 
  set 
  up. 
  An 
  " 
  elvan 
  " 
  

   may 
  be 
  any 
  species 
  of 
  rock 
  that 
  has 
  developed 
  from 
  a 
  granitic 
  magma 
  

   under 
  conditions 
  intermediate 
  between 
  plutonic 
  and 
  volcanic. 
  

  

  One 
  of 
  the 
  first 
  points 
  to 
  attract 
  attention 
  in 
  the 
  inquiry 
  was 
  the 
  

   great 
  disproportion 
  existing 
  between 
  the 
  characteristic 
  varieties 
  of 
  

   elvans 
  to 
  be 
  found 
  in 
  situ 
  (I 
  am 
  speaking 
  generally 
  of 
  the 
  south- 
  

   western 
  borders 
  of 
  Dartmoor) 
  and 
  those 
  occurring 
  in 
  the 
  detritus 
  

   which 
  covers 
  large 
  areas 
  of 
  bottom 
  land 
  in 
  the 
  border 
  valleys 
  of 
  the 
  

   Moor. 
  Elvans. 
  too. 
  are 
  far 
  more 
  largely 
  represented 
  in 
  the 
  latter 
  

   than 
  in 
  the 
  adjacent 
  modern 
  river-beds. 
  

  

  Tracing 
  back 
  the 
  history 
  of 
  denudation, 
  next 
  came 
  the 
  allied 
  fact 
  

   that 
  while 
  granitoid 
  pebbles 
  are 
  to 
  be 
  found 
  in 
  quantity 
  on 
  many 
  

   of 
  the 
  beaches 
  near 
  the 
  mouths 
  of 
  the 
  rivers 
  of 
  Dartmoor 
  origin, 
  on 
  

   the 
  shores 
  of 
  the 
  English 
  Channel, 
  there, 
  again, 
  elvans 
  largely 
  pre- 
  

   dominate. 
  That 
  these 
  accumulations 
  represent 
  in 
  the 
  main 
  an 
  ancient 
  

   as 
  distinct 
  from 
  a 
  modern 
  denudation 
  seems 
  clear. 
  For 
  example, 
  

   while 
  in 
  the 
  beaches 
  immediately 
  within 
  the 
  mouth 
  of 
  the 
  Tealm 
  no 
  

   such 
  pebbles 
  occur, 
  on 
  the 
  beaches 
  immediately 
  outside 
  they 
  are 
  

   plentiful. 
  Mr. 
  Pengelly. 
  F.E.S., 
  has 
  described 
  similar 
  phenomena 
  

   at 
  the 
  mouth 
  of 
  the 
  Ernie*. 
  Granitoid 
  pebbles 
  abound 
  in 
  a 
  cove 
  

   close 
  to 
  the 
  mouth 
  of 
  that 
  river 
  ; 
  but 
  for 
  two 
  and 
  a 
  half 
  miles 
  upward 
  

   thence 
  Mr. 
  Pengelly 
  failed 
  to 
  detect 
  any 
  granitoid 
  rocks 
  on 
  the 
  

   course 
  of 
  the 
  Ernie. 
  The 
  conclusion 
  that 
  these 
  pebbles 
  were 
  not 
  

   brought 
  down 
  by 
  the 
  rivers. 
  ; 
  * 
  but 
  cast 
  up 
  by 
  the 
  sea," 
  seemed 
  there- 
  

   fore 
  a 
  natural 
  one. 
  So 
  far 
  as 
  my 
  experience 
  goes, 
  however, 
  these 
  

   pebbles 
  are 
  quite 
  as 
  clearly 
  of 
  Dartmoor 
  origin 
  as 
  those 
  of 
  the 
  old 
  

   valley-detritus 
  : 
  and 
  whether 
  they 
  were 
  brought 
  to 
  the 
  shore 
  in 
  

  

  * 
  Trans. 
  Dev. 
  Association, 
  xi. 
  pp. 
  329-30. 
  

  

  