﻿210 
  PROF. 
  J. 
  W. 
  JEDD 
  ON 
  THE 
  TERTIARY 
  VOLCANOES 
  

  

  land, 
  and 
  in 
  the 
  Xorth 
  and 
  West 
  of 
  England 
  are 
  really 
  of 
  Xewer 
  

   Palaeozoic 
  age, 
  and 
  are 
  connected 
  with 
  the 
  ejection 
  of 
  the 
  very 
  

   similar 
  lavas 
  that 
  were 
  erupted 
  during 
  that 
  period. 
  

  

  That 
  a 
  few 
  dykes 
  in 
  the 
  Xorth 
  of 
  England 
  which 
  intersect 
  the 
  

   Mesozoic 
  strata 
  are 
  probably 
  of 
  Tertiary 
  age, 
  has 
  long 
  been 
  very 
  

   generally 
  admitted 
  ; 
  but, 
  as 
  Hr. 
  Teall 
  has 
  so 
  well 
  shown, 
  the 
  

   Tertiary 
  dykes 
  consist 
  of 
  very 
  characteristic 
  materials 
  (augite- 
  

   andesites 
  or 
  " 
  tholeites 
  " 
  of 
  Rosenbusch), 
  while 
  other 
  truly 
  basaltic 
  

   dykes 
  in 
  the 
  same 
  district 
  appear 
  certainly 
  to 
  be 
  of 
  far 
  older 
  date. 
  

  

  That 
  certain 
  dykes, 
  like 
  that 
  of 
  Eskdalemuir, 
  which 
  cannot 
  be 
  

   proved 
  to 
  intersect 
  strata 
  younger 
  than 
  Palaeozoic, 
  are 
  probably 
  also 
  

   of 
  Tertiary 
  age, 
  is 
  highly 
  probable, 
  from 
  the 
  similarity 
  of 
  their 
  

   materials 
  to 
  those 
  of 
  the 
  Cleveland 
  and 
  other 
  late 
  dykes 
  ; 
  but 
  in 
  the 
  

   large 
  and 
  valuable 
  series 
  of 
  facts 
  brought 
  forward 
  by 
  the 
  author 
  

   of 
  the 
  recent 
  memoir 
  — 
  in 
  which 
  he 
  summarizes 
  the 
  results 
  of 
  the 
  

   labours 
  of 
  the 
  Geological 
  Survey 
  in 
  the 
  North 
  of 
  England, 
  in 
  Western 
  

   Scotland, 
  and 
  in 
  Ireland 
  — 
  I 
  fail 
  to 
  find 
  any 
  evidence 
  based 
  either 
  on 
  

   the 
  petrographical 
  character 
  of 
  the 
  dykes, 
  the 
  directions 
  in 
  which 
  

   they 
  run, 
  or 
  any 
  other 
  peculiarities, 
  that 
  we 
  can 
  safely 
  discriminate 
  

   between 
  the 
  Newer 
  Palaeozoic 
  dykes 
  and 
  those 
  which 
  are 
  of 
  Tertiary 
  

   age. 
  Still 
  less 
  can 
  I 
  find 
  any 
  evidence 
  for 
  the 
  confident 
  assertion 
  

   that 
  the 
  great 
  majority 
  of 
  the 
  basic 
  dykes 
  over 
  such 
  a 
  wide 
  area 
  

   must 
  all 
  be 
  of 
  Tertiary 
  age. 
  

  

  It 
  is 
  an 
  interesting 
  fact, 
  to 
  which 
  I 
  called 
  attention 
  in 
  my 
  original 
  

   memoir, 
  and 
  the 
  full 
  evidence 
  for 
  which 
  I 
  hope 
  shortly 
  to 
  lay 
  before 
  

   this 
  Society, 
  that 
  the 
  materials 
  ejected 
  during 
  the 
  latest 
  eruptions 
  

   in 
  the 
  West 
  of 
  Scotland 
  are 
  augite-andesites 
  (" 
  tholeites 
  " 
  of 
  

   Rosenbusch), 
  presenting 
  the 
  most 
  striking 
  similarity 
  to 
  the 
  rocks 
  

   of 
  the 
  undoubtedly 
  later 
  dykes 
  in 
  the 
  Xorth 
  of 
  England, 
  described 
  

   by 
  Mr. 
  Teall 
  *. 
  It 
  is 
  on 
  this 
  ground 
  that 
  I 
  continue 
  to 
  maintain 
  

   that 
  these 
  dykes 
  represent 
  the 
  long 
  radiating 
  fissures 
  which 
  were 
  

   produced 
  after 
  the 
  great 
  central 
  vent 
  became 
  extinct, 
  and 
  that 
  in 
  

   many 
  cases 
  they 
  give 
  rise 
  to 
  lines 
  of 
  cinder-cones 
  with 
  issuing 
  lava- 
  

   streams, 
  similar 
  to 
  the 
  well-known 
  " 
  puys 
  " 
  of 
  the 
  Auvergne 
  

   (Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  260-267). 
  

  

  C. 
  Insufficiency 
  of 
  these 
  Dyhe-Jissures 
  as 
  Sources 
  of 
  the 
  

   Plateau-basalts. 
  

  

  Even 
  if 
  we 
  were 
  to 
  grant 
  the 
  contention 
  that 
  a 
  large 
  proportion 
  

   of 
  the 
  Xorth 
  British 
  basic 
  dykes 
  are 
  really 
  of 
  Tertiary 
  age, 
  I 
  believe 
  

   that 
  the 
  fissures 
  occupied 
  by 
  those 
  dykes 
  are 
  quite 
  inadequate 
  to 
  

   have 
  served 
  as 
  the 
  vents 
  from 
  which 
  such 
  enormous 
  masses 
  as 
  the 
  

   basaltic 
  lavas 
  of 
  the 
  plateaux 
  could 
  have 
  been 
  outpoured. 
  

  

  I 
  am 
  happy 
  to 
  find 
  that 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  is 
  quite 
  

   at 
  one 
  with 
  myself 
  in 
  regarding 
  the 
  existing 
  patches 
  of 
  these 
  basaltic 
  

   lavas 
  as 
  mere 
  vestiges, 
  which 
  have 
  escaped 
  denudation, 
  of 
  masses 
  

   originally 
  having 
  a 
  far 
  greater 
  thickness 
  and 
  covering 
  a 
  much 
  wider 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xl. 
  (1884), 
  p. 
  209. 
  

  

  