﻿212 
  PROF. 
  J. 
  W. 
  JIJDD 
  ON 
  THE 
  TERTIARY 
  VOLCANOES 
  

  

  I 
  have 
  shown 
  that 
  it 
  is 
  not 
  necessary 
  to 
  resort 
  to 
  such 
  a 
  theory 
  of 
  

   the 
  origin 
  of 
  " 
  trap-rocks.'' 
  

  

  D. 
  Arguments 
  drawn 
  from 
  the 
  Lava-fields 
  of 
  the 
  Western 
  

   Territories 
  of 
  the 
  United 
  States. 
  

  

  In 
  the 
  year 
  1876 
  the 
  author 
  of 
  the 
  memoir 
  which 
  we 
  are 
  now 
  

   considering 
  published 
  a 
  Geological 
  Hap 
  of 
  Scotland, 
  in 
  which 
  he. 
  

   for 
  the 
  first 
  time, 
  publicly 
  renounced 
  the 
  view 
  that 
  the 
  gabbros 
  of 
  the 
  

   Western 
  Isles 
  are 
  of 
  metamorphic 
  origin 
  and 
  of 
  Laurentian 
  age. 
  

   Tjie 
  relations 
  of 
  the 
  igneous 
  rocks 
  to 
  one 
  another, 
  as 
  indicated 
  upon 
  

   this 
  map, 
  were 
  precisely 
  those 
  which 
  I 
  had 
  announced 
  two 
  years 
  

   before. 
  

  

  But 
  in 
  1879 
  the 
  same 
  author, 
  as 
  he 
  himself 
  informs 
  us, 
  undertook 
  

   an 
  excursion 
  to 
  the 
  Yellowstone 
  Park, 
  and, 
  while 
  on 
  his 
  journey 
  to 
  

   that 
  famous 
  locality, 
  observed 
  facts 
  which 
  profoundly 
  modified 
  his 
  

   views 
  concerning 
  the 
  mode 
  of 
  origin 
  of 
  the 
  igneous 
  rocks 
  of 
  his 
  own 
  

   country. 
  Riding 
  across 
  the 
  plains 
  of 
  the 
  Snake 
  River, 
  he 
  was 
  

   struck 
  by 
  the 
  absence 
  of 
  volcanic 
  cones 
  in 
  a 
  wide 
  area 
  covered 
  by 
  

   basaltic 
  lavas 
  ; 
  and 
  he 
  came 
  to 
  the 
  conclusion 
  that 
  the 
  lavas 
  must 
  

   have 
  reached 
  the 
  surface 
  by 
  means 
  of 
  fissures 
  opened 
  in 
  the 
  sub- 
  

   jacent 
  rocks. 
  

  

  Now 
  I 
  cannot 
  help 
  remarking, 
  at 
  the 
  outset, 
  that 
  this 
  generalization, 
  

   in 
  the 
  case 
  of 
  the 
  lavas 
  of 
  the 
  Snake-River 
  district, 
  appears 
  to 
  have 
  

   been 
  a 
  somewhat 
  hasty 
  one. 
  The 
  opportunities 
  for 
  careful 
  observa- 
  

   tion 
  during 
  a 
  rapid 
  ride 
  of 
  this 
  kind 
  must 
  have 
  been 
  necessarily 
  

   few 
  ; 
  and, 
  as 
  Darwin 
  pointed 
  out 
  long 
  ago, 
  the 
  most 
  careful 
  observer 
  

   is 
  very 
  prone 
  1o 
  ascribe 
  to 
  a 
  single 
  outflow 
  of 
  lava 
  the 
  materials 
  

   which 
  were 
  actually 
  accumulated 
  by 
  a 
  number 
  of 
  distinct 
  ejections. 
  

   As 
  Darwin 
  well 
  showed, 
  in 
  his 
  ' 
  Yolcanic 
  Islands,' 
  it 
  is 
  often 
  

   impossible 
  to 
  detect 
  the 
  limits 
  of 
  the 
  different, 
  comparatively 
  fresh 
  

   flows, 
  except 
  by 
  stud}ung 
  the 
  age 
  of 
  the 
  vegetation 
  which 
  has 
  sprung 
  

   up 
  on 
  each 
  of 
  them. 
  

  

  The 
  statement 
  that 
  the 
  basalts 
  of 
  the 
  Snake 
  River 
  welled 
  out 
  in 
  

   floods 
  from 
  fissures 
  in 
  the 
  subjacent 
  rocks, 
  is 
  admitted 
  to 
  be 
  purely 
  

   conjectural 
  ; 
  and 
  several 
  of 
  the 
  observations 
  which 
  the 
  author 
  of 
  

   the 
  statement 
  has 
  made 
  with 
  regard 
  to 
  the 
  district 
  appear 
  to 
  me 
  to 
  be 
  

   quite 
  irreconcilable 
  with 
  his 
  theory. 
  He 
  admits 
  that 
  cinder-cones 
  

   were 
  seen 
  here 
  and 
  there 
  during 
  his 
  rapid 
  ride 
  ; 
  and 
  when 
  we 
  

   remember 
  how 
  liable 
  such 
  cinder-cones 
  are 
  to 
  be 
  swept 
  away, 
  firstly 
  

   by 
  the 
  outwelling 
  of 
  lava 
  from 
  their 
  craters, 
  and 
  secondly 
  by 
  the 
  

   ordinary 
  agents 
  of 
  subaerial 
  denudation, 
  this 
  fact 
  is 
  a 
  sufficiently 
  

   significant 
  one. 
  That 
  a 
  considerable 
  amount 
  of 
  denudation 
  has 
  

   taken 
  place 
  in 
  the 
  Snake-River 
  district 
  is 
  shown 
  by 
  the 
  circumstance 
  

   that 
  it 
  presents, 
  according 
  to 
  the 
  author's 
  own 
  description, 
  " 
  low 
  

   hummocks 
  or 
  ridges 
  of 
  bare 
  black 
  basalt, 
  the 
  surfaces 
  of 
  which 
  

   exhibited 
  a 
  reticulated 
  pavement 
  of 
  the 
  ends 
  of 
  columns 
  " 
  *. 
  These 
  

   appearances, 
  which 
  are 
  illustrated 
  \>y 
  a 
  drawing, 
  are 
  scarcely 
  con- 
  

   sistent 
  with 
  quite 
  fresh 
  lava- 
  streams, 
  but 
  are 
  exactly 
  what 
  is 
  met 
  

  

  * 
  See 
  Text-Book 
  of 
  Geology 
  (1882), 
  p. 
  257. 
  

  

  