﻿196 
  PROF. 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  TERTIARY 
  VOLCANOES 
  

  

  that 
  this 
  particular 
  class 
  of 
  rock 
  is 
  characteristic 
  of 
  the 
  Lauren 
  tian 
  

   period. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  accuracy 
  of 
  the 
  conclusions 
  announced 
  by 
  

   me 
  in 
  1874 
  has 
  heen 
  again 
  and 
  again 
  disputed, 
  on 
  the 
  ground 
  that 
  

   the 
  "basalt 
  and 
  gahbros 
  had 
  been 
  severally 
  referred 
  to 
  the 
  Jurassic 
  

   and 
  Laurentian 
  systems. 
  

  

  In 
  the 
  year 
  1874, 
  Mr. 
  Allport, 
  in 
  his 
  classical 
  paper 
  on 
  the 
  

   British 
  Carboniferous 
  Dolerites 
  *, 
  showed 
  conclusively 
  that 
  many 
  of 
  

   the 
  basaltic 
  rocks 
  of 
  Palaeozoic 
  age 
  were 
  as 
  fresh 
  and 
  unaltered 
  as 
  

   many 
  Tertiary 
  lavas. 
  In 
  the 
  paper 
  which 
  I 
  published 
  in 
  the 
  same 
  

   year, 
  it 
  was 
  pointed 
  out 
  that 
  many 
  of 
  the 
  rocks 
  occurring 
  among 
  the 
  

   Tertiaries 
  present 
  all 
  the 
  characters 
  which 
  would, 
  if 
  found 
  among 
  

   older 
  rocks, 
  cause 
  them 
  to 
  be 
  classed 
  as 
  " 
  porphyrites," 
  " 
  mela- 
  

   phyres," 
  " 
  diabases," 
  &c. 
  It 
  is 
  therefore 
  satisfactory 
  to 
  find 
  that 
  

   the 
  author 
  of 
  the 
  recently 
  published 
  memoir 
  is 
  able 
  to 
  testify 
  that 
  he 
  

   has 
  " 
  been 
  unable 
  to 
  recognize 
  any 
  essential 
  difference 
  of 
  structure 
  

   or 
  composition 
  " 
  between 
  the 
  Tertiary 
  igneous 
  rocks 
  and 
  those 
  of 
  

   far 
  earlier 
  geological 
  age 
  (T. 
  R. 
  S. 
  E. 
  1888, 
  pp. 
  74, 
  145). 
  

  

  IY. 
  Conclusions 
  announced 
  in 
  1874, 
  which 
  are 
  Disputed 
  in 
  

   the 
  recent 
  Memoir. 
  

  

  There 
  are 
  several 
  points 
  of 
  detail 
  in 
  which 
  it 
  is 
  asserted 
  that 
  the 
  

   results 
  obtained 
  by 
  recent 
  studies 
  are 
  at 
  variance 
  with 
  those 
  stated 
  in 
  

   1874. 
  In 
  some 
  of 
  these 
  cases 
  I 
  am 
  perfectly 
  willing 
  to 
  confess 
  that 
  

   I 
  fell 
  into 
  error 
  ; 
  but 
  in 
  others 
  I 
  shall 
  certainly 
  be 
  able 
  to 
  show 
  that 
  

   the 
  mistake 
  has 
  not 
  been 
  on 
  my 
  side. 
  As 
  none 
  of 
  these 
  minor 
  

   details 
  in 
  any 
  way 
  affects 
  the 
  main 
  questions 
  at 
  issue, 
  I 
  may, 
  for 
  the 
  

   present, 
  pass 
  them 
  by. 
  

  

  There 
  are 
  two 
  contentions 
  in 
  the 
  recent 
  memoir 
  which 
  do, 
  how- 
  

   ever, 
  more 
  or 
  less 
  seriously 
  affect 
  the 
  conclusions 
  which 
  my 
  memoir 
  

   of 
  1874 
  aimed 
  at 
  establishing. 
  One 
  of 
  these 
  is 
  a 
  matter 
  of 
  fact 
  and 
  

   of 
  observation 
  — 
  namely, 
  the 
  relations 
  between 
  the 
  different 
  kinds 
  of 
  

   igneous 
  rock-masses 
  in 
  the 
  district 
  ; 
  the 
  other 
  is 
  a 
  subject 
  of 
  in- 
  

   ference 
  and 
  theory 
  — 
  namely, 
  the 
  explanation 
  that 
  is 
  to 
  be 
  given 
  of 
  

   what 
  I 
  have 
  shown 
  in 
  the 
  preceding 
  pages 
  to 
  be 
  now 
  mutually 
  

   admitted 
  facts. 
  I 
  will 
  take 
  up 
  these 
  two 
  questions 
  in 
  the 
  order 
  

   named. 
  

  

  A. 
  The 
  General 
  Order 
  in 
  which 
  the 
  several 
  Varieties 
  of 
  Igneous 
  

   JRocTcs 
  were 
  erupted. 
  

  

  The 
  memoir 
  of 
  1874 
  stated 
  that, 
  as 
  a 
  whole, 
  the 
  more 
  acid 
  rocks 
  

   of 
  the 
  district 
  were 
  erupted 
  before 
  the 
  more 
  basic 
  ones 
  ; 
  but 
  that 
  

   after 
  the 
  extrusion 
  of 
  the 
  great 
  mass 
  of 
  the 
  basaltic 
  lavas 
  there 
  

   were 
  numerous 
  sporadic 
  outbursts, 
  and, 
  as 
  the 
  result 
  of 
  these, 
  various 
  

   acid 
  and 
  intermediate 
  varieties 
  of 
  rock 
  made 
  their 
  way 
  to 
  the 
  sur- 
  

   face 
  (see 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  272 
  &c). 
  

  

  It 
  may 
  be 
  pointed 
  out 
  that 
  this 
  conclusion 
  was 
  not 
  one 
  for 
  which 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxx. 
  (1874), 
  pp. 
  529-566. 
  

  

  