﻿22 
  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  Chapter 
  VIII, 
  he 
  devotes 
  to 
  a 
  discussion 
  of 
  the 
  Five 
  Islands, 
  

   under 
  this 
  caption 
  : 
  "Intervention 
  of 
  Hydro-Thermal 
  and 
  Vol- 
  

   canic 
  Forces 
  in 
  the 
  Formation 
  of 
  Lower 
  Louisiana." 
  (See 
  

   special 
  discussion 
  of 
  this 
  subject, 
  special 
  report 
  No. 
  3.) 
  

  

  Humphreys, 
  and 
  Abbott.'^ 
  — 
  By 
  far 
  the 
  most 
  serious 
  stud}- 
  of 
  

   the 
  lower 
  Mississippi 
  in 
  all 
  its 
  bearings, 
  is 
  that 
  by 
  Humphreys 
  

   and 
  Abbott, 
  first 
  published 
  in 
  1861 
  and 
  afterwards 
  with 
  additions 
  

   in 
  1876. 
  The 
  hydrography 
  and 
  geology 
  of 
  the 
  whole 
  Mississippi 
  

   basin 
  are 
  taken 
  into 
  account 
  in 
  order 
  to 
  form 
  just 
  conclusions 
  

   regarding 
  the 
  special 
  subjects 
  under 
  consideration. 
  

  

  They 
  hold 
  that 
  the 
  river 
  alluvium 
  is 
  a 
  comparatively 
  thin 
  

   stratum 
  underlaid 
  by 
  blue 
  clay 
  of 
  wide 
  geographical 
  distribution 
  

   and 
  of 
  Tertiary 
  or 
  even 
  Cretaceous 
  age. 
  It 
  is 
  said 
  to 
  underlie 
  

   the 
  Vicksburg 
  bluff, 
  the 
  whole 
  Yazoo 
  bottom, 
  and 
  to 
  underlie 
  

   New 
  Orleans 
  at 
  a 
  depth 
  of 
  not 
  more 
  than 
  40 
  feet. 
  It 
  forms 
  

   the 
  bar 
  of 
  tough 
  clay 
  across 
  the 
  efflux 
  of 
  the 
  Atchafalaya 
  35 
  feet 
  

   below 
  the 
  bank 
  and 
  15 
  feet 
  below 
  Gulf 
  level. 
  An 
  artesian 
  well 
  

   boring 
  in 
  the 
  Atchafalaya 
  upon 
  Gen. 
  Welles' 
  plantation, 
  10 
  or 
  

  

  15 
  miles 
  south 
  of 
  Alexandria 
  shows 
  that 
  the 
  alluvial 
  soil 
  there 
  

   is 
  30 
  feet 
  thick, 
  the 
  surface 
  of 
  the 
  older 
  formation 
  being 
  

   about 
  50 
  feet 
  above 
  tide. 
  

  

  For 
  an 
  extensive 
  review 
  of 
  this 
  work 
  see 
  Amer. 
  Jour. 
  Sci., 
  

   vol. 
  33, 
  1862, 
  p. 
  181; 
  vol. 
  35, 
  1863, 
  p. 
  223 
  ; 
  vol. 
  36, 
  1863, 
  pp. 
  

  

  16 
  and 
  147. 
  

  

  In 
  the 
  edition 
  of 
  1876, 
  pp. 
  465-466, 
  it 
  is 
  stated 
  that 
  the 
  

   ' 
  ' 
  original 
  mouth 
  ' 
  ' 
  of 
  the 
  Mississippi 
  was 
  near 
  the 
  efflux 
  of 
  the 
  

   Plaquemine, 
  220 
  miles 
  from 
  the 
  Gulf. 
  

  

  By 
  comparing 
  Talcott's 
  maps, 
  1838, 
  and 
  the 
  U. 
  S. 
  Coast 
  

   Survey 
  maps 
  of 
  1851 
  it 
  is 
  found 
  that 
  the 
  yearly 
  advance 
  of 
  all 
  

   the 
  passes 
  is 
  262 
  feet 
  per 
  annum. 
  The 
  total 
  advance 
  from 
  the 
  

   Plaquemine 
  efflux 
  has 
  taken 
  4,400 
  years. 
  

  

  * 
  Report 
  on 
  the 
  Physics 
  and 
  Hydraulics 
  of 
  the 
  Mississippi 
  River 
  ; 
  upon 
  the 
  

   protection 
  ot 
  the 
  alhivial 
  region 
  against 
  overflow 
  and 
  upon 
  the 
  deepening 
  

   of 
  the 
  mouths. 
  Based 
  upon 
  surveys 
  and 
  investigations 
  (etc.) 
  U. 
  S. 
  Army, 
  

   Corps 
  of 
  Topographical 
  Engineers, 
  Professional 
  papers. 
  No. 
  4, 
  xiii, 
  456, 
  

   cxlvi 
  pages, 
  20 
  plates, 
  4°, 
  Philadelphia, 
  1861 
  ; 
  also 
  Washington, 
  1867. 
  

   Again 
  214 
  pages, 
  i 
  plate, 
  Washington, 
  1867. 
  With 
  additions, 
  691 
  pages, 
  25 
  

   plates 
  (Professional 
  papers, 
  No. 
  13), 
  Washington, 
  1876. 
  

  

  