﻿FROM 
  ALGIERS 
  TO 
  THE 
  SAHARA. 
  119 
  

  

  of 
  very 
  bright 
  red 
  and 
  green 
  marls 
  is 
  exposed 
  as 
  a 
  low 
  escarpment 
  

   in 
  the 
  north 
  Sahara 
  plain 
  between 
  the 
  Tell 
  and 
  the 
  Hauts 
  Plateaux, 
  

   near 
  the 
  Rocher 
  de 
  Sel 
  at 
  the 
  foot 
  of 
  the 
  Hauts 
  Plateaux, 
  at 
  Ai'n- 
  

   el-Ibel 
  at 
  the 
  foot 
  of 
  the 
  southern 
  escarpment, 
  and 
  again 
  on 
  the 
  

   south 
  side 
  of 
  the 
  Sidi 
  Makhelouf 
  sandstone 
  anticlinal 
  from 
  which 
  the 
  

   marls 
  have 
  been 
  denuded. 
  It 
  is 
  probable 
  the 
  red 
  marls 
  are 
  brought 
  

   up 
  by 
  rolls 
  of 
  the 
  surface 
  in 
  several 
  parts 
  of 
  the 
  north 
  Sahara 
  plain 
  

   both 
  north 
  and 
  south 
  of 
  the 
  Guelt-el-Stel 
  anticlinal, 
  but 
  are 
  obscured 
  

   by 
  the 
  calcareous 
  surface-crust, 
  sand, 
  and 
  other 
  superficial 
  deposits. 
  

  

  Grey 
  Marls 
  and 
  rocky 
  bands 
  of 
  the 
  Hauts 
  Plateaux. 
  — 
  Immediately 
  

   in 
  upward 
  succession 
  from 
  the 
  band 
  of 
  red 
  and 
  green 
  marls 
  is 
  an 
  

   immense 
  series 
  of 
  dark 
  grey 
  marls, 
  interstratified 
  with 
  argillaceo- 
  

   calcareous 
  bands, 
  resembling 
  the 
  Lias, 
  the 
  collective 
  thickness 
  of 
  

   which 
  cannot 
  be 
  less 
  than 
  1500 
  feet. 
  They 
  compose 
  the 
  great 
  syn- 
  

   clinal 
  of 
  the 
  Hauts 
  Plateaux 
  and 
  the 
  contorted 
  mass 
  of 
  the 
  Tell 
  

   Plateau, 
  separated 
  by 
  the 
  denuded 
  plain 
  of 
  the 
  northern 
  Sahara. 
  

   From 
  these 
  beds 
  I 
  obtained 
  no 
  fossils 
  ; 
  but 
  as 
  they 
  immediately 
  overlie 
  

   the 
  saliferous 
  marls 
  of 
  the 
  Rochers 
  de 
  Sel 
  containing 
  Exogyra, 
  they 
  

   are 
  probably 
  Cretaceous. 
  These 
  beds 
  also 
  extend 
  south 
  of 
  the 
  Hauts 
  

   Plateaux 
  to 
  the 
  borders 
  of 
  the 
  Sahara 
  at 
  L'Aghouat, 
  where 
  they 
  are 
  

   overlain 
  by 
  

  

  The 
  Fossiliferous 
  Beds 
  of 
  L'Aghouat. 
  — 
  These 
  occur 
  in 
  conformable 
  

   succession 
  from 
  the 
  marls 
  of 
  the 
  Hauts 
  Plateaux, 
  and 
  consist 
  of 
  

   gypseous 
  marls 
  interstratified 
  with 
  bands 
  of 
  fine-grained 
  stone, 
  and 
  

   capped 
  with 
  limestone 
  containing 
  curious 
  black 
  concretionary 
  nodules 
  

   full 
  of 
  fossils, 
  which 
  Mr. 
  Etheridge 
  considers 
  are 
  of 
  Miocene 
  age. 
  

   It 
  is 
  probable 
  that 
  these 
  beds 
  may 
  also 
  occur 
  on 
  the 
  summit 
  of 
  the 
  

   Hauts 
  Plateaux, 
  but 
  were 
  not 
  observed 
  along 
  my 
  line 
  of 
  route, 
  

   which 
  was 
  below 
  the 
  highest 
  platforms 
  of 
  rock 
  seen 
  to 
  the 
  right 
  

   and 
  left. 
  

  

  Tertiary 
  Beds 
  of 
  the 
  Tell 
  and 
  Algiers. 
  — 
  The 
  only 
  other 
  Tertiary 
  

   beds 
  included 
  in 
  the 
  section 
  are, 
  first, 
  the 
  soft 
  yellow 
  calcareous 
  

   freestone 
  flanking 
  the 
  anticlinal 
  of 
  gneiss 
  and 
  mica-schist 
  of 
  the 
  

   Algerian 
  promontory, 
  at 
  a 
  height 
  of 
  from 
  100 
  to 
  900 
  feet 
  above 
  the 
  

   sea 
  ; 
  and, 
  secondly, 
  the 
  red 
  and 
  grey 
  marls 
  and 
  ferruginous 
  freestone 
  

   of 
  Medeah, 
  capping 
  the 
  Tell 
  Plateau 
  as 
  an 
  irregular 
  synclinal, 
  and 
  

   occurring 
  at 
  a 
  height 
  of 
  from 
  2500 
  to 
  4000 
  feet, 
  probably 
  of 
  Miocene 
  

   age. 
  The 
  great 
  difference 
  in 
  height 
  at 
  which 
  these 
  two 
  masses 
  of 
  

   Tertiary 
  beds 
  occur 
  within 
  a 
  short 
  distance, 
  is 
  an 
  important 
  point 
  in 
  

   the 
  physical 
  geology 
  of 
  the 
  district 
  presently 
  to 
  be 
  referred 
  to. 
  

  

  Posttertiary 
  Deposits. 
  — 
  To 
  the 
  north 
  of 
  the 
  Lesser 
  Atlas, 
  the 
  plain 
  

   of 
  the 
  Mitidja 
  consists 
  of 
  grey 
  loam 
  interstratified 
  with 
  shingle-beds, 
  

   commencing 
  at 
  the 
  sea-level 
  south 
  of 
  Algiers, 
  and 
  ascending 
  inland 
  by 
  

   a 
  gentle 
  slope 
  to 
  a 
  height 
  of 
  from 
  200 
  to 
  300 
  feet. 
  Again, 
  on 
  the 
  

   south 
  side 
  of 
  the 
  Atlas 
  and 
  Tell, 
  the 
  great 
  plain 
  of 
  the 
  northern 
  

   Sahara 
  is 
  covered 
  with 
  grey 
  loam 
  and 
  occasional 
  sandhills 
  from 
  a 
  

   height 
  of 
  2000 
  feet 
  on 
  its 
  northern 
  side 
  to 
  a 
  height 
  of 
  2700 
  feet 
  on 
  

   its 
  southern 
  boundary 
  ; 
  and 
  after 
  crossing 
  the 
  Hauts 
  Plateaux, 
  the 
  

   same 
  deposit 
  sets 
  in 
  on 
  its 
  southern 
  side, 
  commencing 
  in 
  the 
  inter- 
  

   vening 
  valleys 
  at 
  a 
  height 
  of 
  2700 
  feet, 
  and 
  gradually 
  descending 
  

  

  