﻿HAYDEN.] 
  LETTER 
  TO 
  THE 
  SECRETARY. 
  27 
  

  

  Elevations 
  ", 
  the 
  first 
  edition 
  being 
  by 
  Professor 
  Cyrus 
  Thomas, 
  the 
  

   three 
  succeeding 
  ones 
  by 
  Mr. 
  Henry 
  Gannett. 
  The 
  fourth 
  edition, 
  to 
  be 
  

   published 
  in 
  1877, 
  embodies 
  the 
  results 
  of 
  this 
  collection 
  up 
  to 
  date. 
  It 
  

   contains, 
  among 
  a 
  variety 
  of 
  other 
  matter, 
  profiles 
  of 
  nearly 
  all 
  the 
  rail- 
  

   roads 
  in 
  the 
  part 
  of 
  the 
  United 
  States 
  above 
  mentioned. 
  The 
  results 
  

   given 
  by 
  these 
  profiles 
  have 
  been 
  made 
  to 
  accord, 
  and 
  the 
  heights 
  of 
  

   several 
  thousands 
  of 
  points 
  on 
  them 
  have 
  been 
  determined 
  with 
  an 
  ap- 
  

   proach 
  to 
  accuracy. 
  This 
  edition 
  contains 
  also 
  the 
  heights 
  of 
  many 
  

   thousands 
  of 
  points 
  determined 
  approximately 
  by 
  means 
  of 
  the 
  barom- 
  

   eter. 
  Heights 
  of 
  many 
  thousands 
  of 
  mountain-peaks 
  are 
  given, 
  from 
  

   "which 
  very 
  correct 
  ideas 
  of 
  the 
  ruling 
  heights 
  of 
  the 
  principal 
  ranges 
  

   may 
  be 
  derived. 
  

  

  The 
  mean 
  heights 
  of 
  the 
  States 
  and 
  Territories 
  and 
  of 
  the 
  United 
  

   States 
  have 
  been 
  determined 
  with 
  an 
  approach, 
  at 
  least, 
  to 
  correctness. 
  

  

  It 
  contains 
  also 
  tables 
  of 
  the 
  slopes 
  of 
  the 
  principal 
  streams 
  of 
  the 
  

   West, 
  which 
  are 
  of 
  value 
  in 
  studying 
  the 
  important 
  question 
  of 
  irriga- 
  

   tion. 
  

  

  Besides 
  the 
  tables 
  of 
  heights, 
  it 
  contains 
  a 
  map 
  of 
  the 
  United 
  States, 
  

   in 
  approximate 
  contours 
  of 
  1,000 
  feet 
  of 
  vertical 
  intervals, 
  which, 
  in 
  a 
  

   measure, 
  embodies 
  all 
  the 
  results 
  of 
  this 
  department. 
  Toward 
  the 
  im- 
  

   provement 
  and 
  ultimate 
  perfection 
  of 
  this 
  map, 
  this 
  work 
  is 
  to 
  be 
  mainly 
  

   directed 
  in 
  future. 
  

  

  This 
  map 
  indicates 
  not 
  only, 
  in 
  a 
  rough 
  way, 
  the 
  height 
  of 
  any 
  part 
  

   of 
  the 
  country, 
  and 
  the 
  areas 
  above 
  certain 
  heights, 
  but 
  the 
  magnitude, 
  

   height, 
  and 
  general 
  slopes 
  of 
  the 
  great 
  plateau 
  underlying 
  the 
  mount- 
  

   ain 
  ranges 
  of 
  the 
  West, 
  the 
  true 
  Sierra 
  Madre; 
  the 
  trends, 
  mean 
  heights, 
  

   and 
  geographical 
  character 
  of 
  the 
  ranges; 
  and 
  the 
  areas 
  covered 
  by 
  

   plains, 
  plateaus, 
  and 
  mountains. 
  

  

  To 
  express 
  still 
  more 
  clearly 
  the 
  facts 
  brought 
  out 
  by 
  the 
  map, 
  it 
  is 
  

   the 
  intention 
  of 
  the 
  Survey 
  to 
  make 
  shortly 
  a 
  relief 
  model 
  of 
  the 
  United 
  

   States, 
  on 
  the 
  basis 
  of 
  this 
  map. 
  

  

  The 
  Survey 
  for 
  the 
  past 
  two 
  years 
  has 
  been 
  accumulating 
  the 
  mate- 
  

   rials 
  for 
  a 
  general 
  geological 
  map 
  of 
  all 
  the 
  States 
  and 
  Territories 
  west 
  

   of 
  the 
  94th 
  meridian, 
  on 
  a 
  scale 
  best 
  adopted 
  to 
  the 
  purpose 
  of 
  cor- 
  

   relating, 
  as 
  far 
  as 
  possible, 
  the 
  different 
  formations 
  known 
  over 
  that 
  

   area. 
  Great 
  confusion 
  is 
  liable 
  to 
  ensue 
  by 
  ignoring 
  well 
  established 
  

   names 
  and 
  giving 
  new 
  and 
  very 
  restricted 
  local 
  names. 
  Most 
  of 
  these 
  

   names 
  must 
  eventually 
  yield 
  to 
  the 
  law 
  of 
  priority, 
  and 
  disappear 
  in 
  the 
  

   light 
  of 
  a 
  more 
  general 
  and 
  systematic 
  classification. 
  The 
  Cretaceous 
  

   divisions 
  can 
  undoubtedly 
  be 
  extended 
  all 
  over 
  the 
  West, 
  and 
  the 
  numer- 
  

   ous 
  Tertiary 
  lake-basins 
  may 
  be 
  correlated 
  in 
  time. 
  In 
  this 
  way 
  we 
  may 
  

   be 
  able 
  to 
  understand 
  more 
  clearly 
  the 
  remarkable 
  simplicity 
  and 
  unity 
  of 
  

   the 
  structure 
  of 
  the 
  entire 
  Eocky 
  Mountain 
  region. 
  It 
  is 
  true 
  that 
  the 
  

   geology 
  of 
  very 
  much 
  of 
  our 
  country 
  is 
  as 
  yet 
  practically 
  unknown, 
  but 
  

   some 
  method 
  can 
  be 
  adopted 
  which 
  shall 
  distinguish 
  such 
  portions 
  from 
  

   those 
  on 
  which 
  the 
  work 
  has 
  reached 
  a 
  certain 
  grade 
  of 
  excellence. 
  

  

  