﻿ACADEMY 
  OF 
  SCIENCES. 
  

  

  215 
  

  

  He 
  describes 
  the 
  lava-flood 
  as 
  probably 
  the 
  greatest 
  ia 
  the 
  world. 
  Issuing 
  

   from 
  fissures 
  in 
  the 
  Cascade 
  and 
  Blue 
  Mountain 
  ranges, 
  it 
  spread 
  over 
  nearly 
  

   the 
  whole 
  of 
  Oregon, 
  Washington, 
  and 
  Idaho, 
  and 
  far 
  into 
  California 
  on 
  the 
  

   south, 
  Montana 
  on 
  the 
  east, 
  and 
  British 
  Columbia 
  on 
  the 
  north. 
  Its 
  area 
  is 
  

   certainly 
  200,000 
  to 
  300,000 
  square 
  miles. 
  Its 
  thickness 
  in 
  the 
  axis 
  of 
  the 
  Cas- 
  

   cade 
  Mountains, 
  where 
  it 
  is 
  cut 
  through 
  by 
  the 
  Columbia 
  River, 
  is 
  more 
  than 
  

   3,500 
  feet. 
  The 
  section 
  shown 
  by 
  the 
  Des 
  Chutes 
  River, 
  fifty 
  miles 
  from 
  the 
  

   axis, 
  is 
  2,000 
  to 
  3.000 
  feet. 
  The 
  average 
  thickness 
  over 
  the 
  whole 
  Cascade 
  

   region 
  (100,000 
  square 
  miles) 
  is 
  probably 
  not 
  less 
  than 
  2,000 
  feet. 
  

  

  II. 
  — 
  Structure 
  of 
  the 
  Cascade 
  Mountains. 
  

   The 
  Columbia 
  River, 
  on 
  its 
  way 
  to 
  the 
  Pacific, 
  cuts 
  through 
  the 
  Cascade 
  

   range 
  almost 
  to 
  the 
  sea 
  level. 
  Its 
  magnificent 
  canon 
  reveals 
  the 
  structure 
  of 
  

   this 
  range 
  for 
  nearly 
  100 
  miles. 
  Except 
  120 
  feet 
  at 
  the 
  base 
  and 
  in 
  the 
  axis, 
  

   the 
  whole 
  range 
  consists 
  of 
  layers 
  of 
  lava, 
  piled 
  one 
  on 
  top 
  of 
  another 
  .to 
  the 
  

   thickness 
  of 
  about 
  3,700 
  feet. 
  The 
  lava 
  is 
  cut 
  entirely 
  through 
  only 
  in 
  the 
  axis 
  

   of 
  the 
  range, 
  and 
  there 
  only 
  for 
  two 
  or 
  three 
  miles 
  ; 
  at 
  this 
  place, 
  therefore, 
  is 
  

   1-evealed 
  the 
  old 
  ground 
  surface 
  upon 
  which 
  the 
  lava 
  was 
  first 
  outpoured. 
  

  

  The 
  river 
  at 
  this 
  place 
  (Cas- 
  

   cades 
  of 
  the 
  Columbia) 
  washes 
  

   against 
  the 
  southern 
  cliff, 
  and 
  

   makes 
  a 
  fine 
  section. 
  The 
  best 
  

   place 
  to 
  examine 
  is 
  nearly 
  op- 
  

   posite 
  the 
  lower 
  steamboat-land- 
  

   ing. 
  The 
  diagrams, 
  figs. 
  1 
  and 
  

   2, 
  represent 
  the 
  front 
  view 
  and 
  

   a 
  section 
  of 
  the 
  cliS' 
  at 
  this 
  

   place. 
  

  

  From 
  the 
  water 
  level 
  to 
  fif- 
  

   teen 
  feet 
  above, 
  there 
  is 
  a 
  

   coarse 
  conglomerate 
  ('() 
  of 
  por- 
  

   FiG. 
  1. 
  phyritic 
  pebbles 
  and 
  bouldi 
  rs 
  

  

  in 
  a 
  cemented 
  earthy 
  pa.ste. 
  This 
  is 
  limited 
  above 
  by 
  an 
  irregular 
  dark 
  line 
  

   (b), 
  a 
  veritable 
  ground-surface. 
  On 
  this 
  stood 
  silicified 
  stumps, 
  with 
  roots 
  ram- 
  

   ified 
  in 
  the 
  boulder 
  soil 
  beneath. 
  Above 
  this 
  ground-surface 
  lay 
  a 
  stratified 
  

   sandstone 
  (c), 
  two 
  feet 
  thick, 
  filled 
  with 
  leaf 
  impressions. 
  Above 
  this 
  lay 
  a 
  

   conglomerate 
  (d), 
  irregularly 
  stratified 
  in 
  spots, 
  like 
  modified 
  drift, 
  containing 
  

   scattered 
  fragments 
  of 
  silicified 
  drift-wood. 
  Upon 
  the 
  uneven 
  surface 
  (e) 
  of 
  

   this 
  rested 
  lava 
  layers, 
  one 
  above 
  another, 
  to 
  the 
  height 
  at 
  this 
  point 
  of 
  about 
  

   3,000 
  feet. 
  

  

  That 
  there 
  might 
  be 
  no 
  uncertainty 
  about 
  the 
  actual 
  relation 
  of 
  the 
  lava 
  to 
  

   the 
  conglomerate, 
  several 
  of 
  the 
  stream-beds 
  running 
  into 
  the 
  Columbia 
  at 
  this 
  

   point— 
  and 
  making 
  actual 
  sections 
  similar 
  to 
  the 
  ideal 
  section, 
  fig. 
  2 
  — 
  were 
  

   ascended. 
  In 
  several 
  instances, 
  in 
  the 
  stream-beds 
  as 
  well 
  as 
  on 
  the 
  Columbia 
  

   River 
  at 
  Tooth 
  Bridge, 
  the 
  actual 
  contact 
  of 
  the 
  lava 
  with 
  the 
  underlying 
  

  

  -^r=z 
  

  

  