﻿506 
  Transactions. 
  — 
  Geology. 
  

  

  has 
  proposed 
  another 
  theory, 
  namely, 
  that 
  the 
  principal 
  cause 
  of 
  vnlcanicity 
  

   is 
  to 
  be 
  sought 
  in 
  the 
  compressing 
  and 
  crushing 
  action 
  taking 
  place 
  beneath 
  

   the 
  crust 
  of 
  the 
  earth, 
  and 
  by 
  which 
  such 
  a 
  great 
  amount 
  of 
  heat 
  is 
  

   generated 
  that 
  a 
  fusion 
  of 
  rocks, 
  often 
  on 
  a 
  large 
  scale, 
  is 
  easily 
  produced. 
  

   This 
  theory 
  would 
  so 
  far 
  explain 
  Very 
  well 
  the 
  difference 
  in 
  the 
  composition 
  

   of 
  the 
  rocks 
  varying 
  according 
  to 
  the 
  depth 
  where 
  the 
  crushing 
  action 
  was 
  

   actually 
  taking 
  place 
  ; 
  thus, 
  if 
  the 
  same 
  action 
  were 
  to 
  act 
  upon 
  granites, 
  

   trachytes, 
  and 
  other 
  acidic 
  rocks, 
  the 
  result 
  would 
  be 
  the 
  production 
  of 
  

   trachytes, 
  whilst 
  if 
  basic 
  rocks 
  were 
  fused, 
  basalts 
  would 
  ascend 
  towards 
  or 
  

   to 
  the 
  surface. 
  Here, 
  however, 
  another 
  great 
  difficulty 
  presents 
  itself 
  in 
  the 
  

   fact 
  that, 
  although 
  the 
  number 
  of 
  volcanic 
  eruptions 
  during 
  which 
  the 
  

   caldera 
  walls 
  were 
  built 
  up 
  must 
  have 
  been 
  very 
  great, 
  no 
  trachytic 
  lava 
  

   streams, 
  with 
  one 
  single 
  exception, 
  have 
  made 
  their 
  appearance, 
  the 
  whole 
  

   series 
  being 
  of 
  a 
  basic, 
  whilst 
  most 
  of 
  the 
  principal 
  dykes 
  are 
  of 
  an 
  acidic 
  

   nature. 
  In 
  such 
  a 
  case, 
  the 
  crushing 
  of 
  acidic 
  rocks 
  would 
  have 
  exclusively 
  

   taken 
  place 
  when 
  the 
  dykes 
  were 
  being 
  formed, 
  and 
  never 
  when 
  lava-streams 
  

   issued 
  from 
  the 
  crater's 
  mouth, 
  which 
  is 
  altogether 
  improbable. 
  

  

  Although 
  I 
  have 
  carefully 
  read 
  every 
  work 
  accessible 
  to 
  me 
  in 
  English, 
  

   German, 
  French, 
  and 
  Italian, 
  treating 
  on 
  vulcanicity, 
  I 
  have 
  not 
  been 
  able 
  

   to 
  find 
  either 
  any 
  account 
  of 
  the 
  existence 
  of 
  dykes 
  in 
  other 
  volcanic 
  regions 
  

   converging 
  so 
  regularly 
  to 
  a 
  few 
  centres 
  close 
  to 
  each 
  other, 
  or 
  continuing 
  

   over 
  such 
  a 
  large 
  area 
  (always 
  keeping 
  the 
  general 
  direction 
  with 
  which 
  

   they 
  set 
  out), 
  as 
  do 
  those 
  of 
  the 
  Lyttelton 
  caldera; 
  or 
  again, 
  offering 
  an 
  

   explanation 
  for 
  the 
  difference 
  in 
  the 
  composition 
  of 
  the 
  dyke 
  rocks 
  when 
  

   compared 
  with 
  the 
  lava-streams 
  or 
  agglomeratic 
  beds 
  through 
  which 
  they 
  

   pass. 
  Mr. 
  E. 
  Mallet's 
  excellent 
  paper 
  on 
  the 
  " 
  Mechanism 
  of 
  Production 
  

   of 
  Volcanic 
  Dykes,"* 
  and 
  of 
  those 
  of 
  Mount 
  Somma, 
  in 
  which 
  an 
  

   exhaustive 
  account 
  of 
  the 
  physical 
  features 
  of 
  the 
  dykes 
  in 
  the 
  old 
  caldera 
  

   wall 
  of 
  Mount 
  Vesuvius 
  is 
  given, 
  unfortunately 
  does 
  not 
  contain 
  any 
  

   physical 
  theory 
  to 
  account 
  for 
  the 
  mode 
  by 
  which 
  fissures 
  are 
  produced, 
  

   forming, 
  when 
  filled, 
  volcanic 
  dykes. 
  If 
  we 
  take 
  the 
  heterogeneous 
  nature 
  

   of 
  the 
  material 
  of 
  which 
  the 
  caldera 
  wall 
  has 
  been 
  built 
  up 
  into 
  account, 
  it 
  

   is 
  astonishing 
  that 
  the 
  dykes 
  show 
  such 
  a 
  remarkable 
  regularity, 
  always 
  

   starting 
  from 
  a 
  few 
  points 
  not 
  far 
  from 
  each 
  other, 
  from 
  which 
  they 
  radiate 
  

   in 
  aU 
  directions. 
  It 
  is 
  still 
  more 
  remarkable 
  to 
  observe 
  that 
  all 
  dykea 
  

   which 
  are 
  cu.t 
  by 
  the 
  Christchurch 
  and 
  Lyttelton 
  railway 
  tunnel 
  have 
  such 
  

   a 
  constant 
  direction 
  that 
  they 
  all, 
  with 
  one 
  or 
  two 
  exceptions, 
  appear 
  to 
  

   converge 
  to 
  one 
  single 
  axis 
  behind 
  Quail 
  Island, 
  a 
  fact 
  worthy 
  of 
  note 
  if 
  we 
  

   consider 
  the 
  distance, 
  which 
  is 
  more 
  than 
  four 
  miles, 
  measured 
  to 
  the 
  most 
  

   distant 
  dyke 
  in 
  that 
  tunnel. 
  The 
  only 
  dyke 
  with 
  which 
  I 
  am 
  acquainted, 
  

  

  * 
  (Quarterly 
  Journal 
  of 
  the 
  Geological 
  Society 
  of 
  London, 
  No. 
  128, 
  Nov., 
  1876, 
  

  

  