﻿Geology 
  and 
  Mineralogy 
  247 
  

  

  The 
  Columbia 
  lava 
  in 
  Washington 
  to 
  the 
  west 
  of 
  the 
  Columbia 
  

   and 
  south 
  of 
  the 
  Big 
  Bend, 
  as 
  ascertained 
  during 
  a 
  previous 
  recon- 
  

   noissance, 
  is 
  broken 
  by 
  extensive 
  faults 
  and 
  the 
  blocks 
  thus 
  

   formed 
  variously 
  tilted. 
  In 
  the 
  region 
  here 
  treated, 
  however, 
  the 
  

   basalt 
  is 
  horizontal 
  over 
  extensive 
  areas, 
  and 
  deeply 
  dissected 
  by 
  

   Snake 
  river 
  and 
  its 
  tributaries. 
  

  

  Many 
  lava 
  sheets, 
  one 
  resting 
  on 
  another, 
  were 
  seen. 
  Between 
  

   some 
  of 
  the 
  flows 
  there 
  are 
  widely 
  extended 
  sheets 
  of 
  lacustral 
  

   clays, 
  sand, 
  gravel, 
  volcanic 
  dust 
  and 
  lapilli. 
  In 
  some 
  instances 
  

   leaves 
  and 
  the 
  silicified 
  stumps 
  and 
  trunks 
  of 
  trees 
  occur 
  in 
  these 
  

   layers. 
  

  

  The 
  Columbia 
  lava 
  flowed 
  about 
  the 
  bases 
  of 
  the 
  mountains 
  of 
  

   eastern 
  Washington 
  and 
  the 
  adjacent 
  portion 
  of 
  Idaho, 
  in 
  a 
  series 
  

   of 
  inundations 
  which 
  covered 
  the 
  low 
  country 
  to 
  the 
  south. 
  The 
  

   level 
  basaltic 
  plateau. 
  meets 
  the 
  mountains 
  of 
  metamorphic 
  rock 
  

   in 
  much 
  the 
  same 
  manner 
  that 
  the 
  sea 
  joins 
  a 
  rugged 
  and 
  deeply 
  

   indented 
  coast. 
  The 
  lava 
  entered 
  the 
  valleys 
  and 
  gave 
  them 
  level 
  

   floors 
  of 
  basalt 
  ; 
  the 
  deeply 
  sculptured 
  ridges 
  between 
  the 
  valleys 
  

   were 
  transformed 
  into 
  capes 
  and 
  headlands 
  ; 
  outstanding 
  moun- 
  

   tain 
  peaks 
  became 
  islands 
  in 
  the 
  sea 
  of 
  molten 
  rock. 
  

  

  After 
  the 
  last 
  of 
  the 
  lava 
  sheets 
  was 
  spread 
  out, 
  the 
  rivers 
  

   flowing 
  from 
  the 
  mountains 
  began 
  the 
  excavation 
  of 
  channels 
  

   across 
  the 
  basaltic 
  plateau 
  and 
  have 
  deeply 
  dissected 
  it. 
  The 
  

   most 
  important 
  of 
  these 
  channels 
  is 
  the 
  one 
  cut 
  by 
  Snake 
  river. 
  

   From 
  the 
  mouth 
  of 
  Snake 
  river 
  to 
  Lewiston, 
  the 
  stream 
  is 
  in 
  a 
  com- 
  

   paratively 
  narrow 
  steep-sided 
  canyon 
  about 
  2000 
  feet 
  deep. 
  Where 
  

   Snake 
  river 
  forms 
  the 
  boundary 
  between 
  Washington 
  and 
  Idaho, 
  

   its 
  gorge 
  is 
  about 
  4000 
  feet 
  deep 
  and 
  15 
  miles 
  broad. 
  Within 
  

   this 
  vast 
  gorge 
  there 
  are 
  many 
  lateral 
  ridges, 
  and 
  a 
  great 
  variety 
  

   of 
  topographic 
  forms 
  due 
  to 
  erosion. 
  This 
  portion 
  of 
  Snake 
  

   river 
  canyon 
  compares 
  favorably 
  with 
  even 
  the 
  most 
  magnificent 
  

   parts 
  of 
  the 
  Grand 
  Canyon 
  of 
  the 
  Colorado, 
  except 
  that 
  it 
  lacks 
  

   the 
  gorgeous, 
  coloring 
  to 
  which 
  so 
  much 
  of 
  the 
  charm 
  of 
  its 
  

   southern 
  rival 
  is 
  due. 
  

  

  The 
  thickness 
  of 
  horizontally-bedded 
  basalt 
  exposed 
  in 
  the 
  

   walls 
  of 
  Snake 
  river 
  canyon 
  and 
  in 
  the 
  adjacent 
  Blue 
  mountains, 
  

   is 
  in 
  the 
  neighborhood 
  of 
  5000 
  feet, 
  but 
  the 
  maximum 
  thickness 
  

   is 
  not 
  revealed. 
  In 
  the 
  walls 
  of 
  the 
  canyon 
  at 
  three 
  localities 
  the 
  

   summits 
  of 
  steep, 
  angular 
  mountain 
  ranges 
  are 
  exposed 
  ; 
  one 
  of 
  

   these 
  buried 
  peaks 
  rises 
  about 
  2500 
  feet 
  above 
  the 
  river 
  and 
  is 
  

   covered 
  by 
  fully 
  1500 
  feet 
  of 
  horizontally-bedded 
  basalt. 
  

  

  The 
  sheets 
  of 
  clay, 
  sand 
  and 
  gravel 
  interleaved 
  with 
  the 
  basalt, 
  

   especially 
  near 
  its 
  junction 
  with 
  the 
  bordering 
  mountains, 
  furnish 
  

   conditions 
  favorable 
  for 
  obtaining 
  artesian 
  water. 
  A 
  number 
  of 
  

   flowing 
  wells 
  derive 
  their 
  water 
  supply 
  from 
  this 
  source. 
  

  

  The 
  Blue 
  mountains, 
  at 
  least 
  at 
  their 
  northern 
  extremity, 
  con- 
  

   sist 
  of 
  a 
  broad, 
  low, 
  flat-topped 
  dome 
  of 
  basalt 
  which 
  has 
  been 
  

   deeply 
  dissected 
  by 
  consequent 
  streams. 
  The 
  layers 
  of 
  basalt 
  in 
  

   the 
  uplifted 
  region 
  are 
  still 
  horizontal 
  except 
  about 
  its 
  borders, 
  

   where 
  gentle 
  outward 
  dips 
  occur. 
  

  

  