﻿ACADEMY 
  OF 
  SCIENCES. 
  219 
  

  

  The 
  wide 
  part 
  inclosed 
  between 
  the 
  cliffs 
  a 
  a 
  (which 
  is 
  about 
  five 
  miles 
  wide) 
  

   he 
  thinks 
  has 
  been 
  made 
  during 
  the 
  post-Tertiary, 
  either 
  by 
  ice 
  or 
  swollen 
  waters, 
  

   or 
  both. 
  During 
  the 
  present 
  epoch 
  the 
  greatly 
  shrunken 
  river 
  has 
  cut 
  its 
  way 
  

   into 
  the 
  underlying 
  conglomerate, 
  moving 
  meanwhile 
  steadily 
  to 
  the 
  south, 
  and 
  

   making 
  there 
  a 
  perpendicuUir 
  cliff. 
  If 
  so, 
  then 
  the 
  recession 
  of 
  the 
  perpen- 
  

   dicular 
  falls 
  of 
  the 
  tributaries 
  back 
  to 
  their 
  present 
  position, 
  about 
  two 
  or 
  three 
  

   miles 
  from 
  the 
  main 
  river, 
  has 
  also 
  been 
  the 
  work 
  of 
  the 
  present 
  epoch. 
  

  

  VI. 
  — 
  Prairie 
  Mounds. 
  

  

  Oregon 
  and 
  Washington 
  are 
  covered 
  with 
  a 
  drift 
  ten 
  to 
  thirty 
  feet 
  thick, 
  

   composed 
  of 
  impalpable, 
  unsorted 
  earth 
  (rock 
  meal), 
  mixed 
  with 
  pebbles 
  and 
  

   rounded 
  boulders. 
  The 
  upper 
  portion 
  of 
  this 
  earth 
  is 
  finer, 
  the 
  pebbles 
  becom- 
  

   ing 
  more 
  numerous 
  and 
  larger 
  as 
  we 
  pass 
  downward. 
  Thus 
  it 
  may 
  be 
  regarded 
  

   as 
  consisting 
  of 
  two 
  parts 
  — 
  a 
  finer 
  and 
  more 
  movable 
  layer 
  above, 
  and 
  a 
  coarser 
  

   and 
  less 
  movable 
  one 
  below, 
  graduating 
  more 
  or 
  less 
  perfectly 
  into 
  each 
  other. 
  

  

  Again 
  : 
  At 
  the 
  southern 
  extremity 
  of 
  Paget 
  Sound, 
  in 
  the 
  midst 
  of 
  tlie 
  

   dense 
  fir 
  forests, 
  and 
  separated 
  from 
  them 
  by 
  the 
  sharpest 
  line 
  of 
  definition, 
  there 
  

   are 
  remarkable 
  narrow, 
  irregularly 
  ramifying 
  glades, 
  or 
  prairies, 
  entirely 
  destitute 
  

   of 
  trees 
  or 
  shrubs. 
  These 
  are 
  doubtless 
  old 
  bottoms 
  of 
  Puget 
  Sound, 
  made 
  dry 
  

   by 
  elevation. 
  They 
  are 
  covered 
  with 
  drift-soil. 
  These 
  grassy 
  prairies 
  are 
  cov- 
  

   ered 
  as 
  thickly 
  as 
  possible 
  with 
  mounds, 
  about 
  three 
  to 
  four 
  feet 
  high, 
  and 
  thirty 
  

   to 
  forty 
  feet 
  diameter. 
  There 
  are 
  probably 
  millions 
  of 
  them. 
  The 
  general 
  

   appearance 
  is 
  that 
  of 
  almost 
  perfect 
  regularity 
  of 
  size 
  and 
  shape. 
  The 
  soil 
  of 
  

   the 
  mounds 
  is 
  a 
  rather 
  fine 
  drift, 
  with 
  pebbles 
  not 
  larger 
  than 
  a 
  pigeon's 
  egg. 
  

   The 
  intervals 
  between 
  the 
  mounds 
  are 
  strewed 
  with 
  larger 
  pebbles. 
  The 
  mounds 
  

   are 
  occupied 
  by 
  ferns, 
  the 
  intervals 
  only 
  by 
  grass. 
  These 
  treeless 
  spaces 
  are 
  

   called 
  " 
  mound-prairies." 
  

  

  There 
  has 
  been 
  much 
  speculation 
  as 
  to 
  the 
  origin 
  of 
  these 
  remarkable 
  mounds. 
  

   Some 
  suppose 
  them 
  burial-mounds, 
  and 
  that 
  we 
  have 
  here 
  veritable 
  cities 
  of 
  the 
  

   dead 
  ; 
  others, 
  that 
  they 
  were 
  raised 
  as 
  foundations 
  for 
  huts, 
  on 
  a 
  wet 
  soil 
  ; 
  others, 
  

   that 
  they 
  were 
  made 
  by 
  a 
  species 
  of 
  fish, 
  when 
  these 
  spots 
  were 
  still 
  the 
  

   shallow 
  bottom 
  of 
  the 
  Sound 
  — 
  that 
  they 
  are 
  huge 
  fish-nests. 
  No 
  careful 
  ob- 
  

   server 
  can 
  for 
  a 
  moment 
  entertain 
  any 
  of 
  these 
  views. 
  The 
  mounds 
  have 
  been 
  

   frequently 
  explored, 
  and 
  although 
  from 
  time 
  to 
  time 
  there 
  have 
  been 
  reports 
  of 
  

   relics 
  having 
  been 
  found 
  in 
  them, 
  the 
  author 
  feels 
  quite 
  confident 
  that 
  nothing 
  

   has 
  ever 
  been 
  found. 
  

  

  The 
  author 
  states 
  that 
  his 
  observations 
  in 
  Eastern 
  Oregon 
  — 
  where 
  they 
  occur 
  

   in 
  every 
  variety 
  of 
  form, 
  size, 
  and 
  regularity 
  — 
  and 
  in 
  California, 
  convince 
  him 
  

   that 
  they 
  are 
  the 
  result 
  of 
  surface 
  erosion 
  under 
  peculiar 
  conditions 
  ; 
  these 
  con- 
  

   ditions 
  being 
  a 
  bare 
  counirij 
  and 
  a 
  drift-soil 
  more 
  movable 
  above 
  and 
  less 
  mov- 
  

   able 
  below. 
  Erosion 
  removes 
  the 
  finer 
  top-soil, 
  leaving 
  it, 
  however, 
  in 
  spots. 
  

   The 
  process 
  once 
  commenced, 
  weeds, 
  shrubs, 
  and 
  ferns 
  take 
  possession 
  of 
  these 
  

   spots 
  as 
  the 
  better 
  soil, 
  or 
  sometimes 
  as 
  the 
  drier 
  soil, 
  and 
  hold 
  them, 
  and 
  by 
  

   their 
  roots 
  retard 
  the 
  erosion 
  there. 
  In 
  some 
  cases 
  a 
  departing 
  vegetation 
  — 
  a 
  

   vegetation 
  gradually 
  destroyed 
  by 
  an 
  increasing 
  dryness 
  of 
  climate 
  — 
  is 
  an 
  im- 
  

   portant 
  condition. 
  

  

  