﻿GEOLOGY 
  OF 
  MADAGASCAE. 
  307 
  

  

  the 
  district 
  of 
  Androna 
  ; 
  how 
  much 
  further 
  north 
  T 
  cannot 
  say. 
  

   And 
  as 
  its 
  southern 
  extremity 
  extended 
  to 
  about 
  latitude 
  19°, 
  its 
  

   total 
  length 
  has 
  once 
  exceeded 
  200 
  miles. 
  Its 
  breadth, 
  however, 
  

   has 
  been 
  small 
  compared 
  with 
  its 
  length, 
  averaging 
  probably 
  from 
  

   20 
  to 
  30 
  miles. 
  Lake 
  Alaotra 
  is 
  situated 
  in 
  the 
  deepest 
  part 
  of 
  this 
  

   great 
  longitudinal 
  trough, 
  and 
  is 
  2600 
  feet 
  above 
  the 
  sea. 
  It 
  is 
  a 
  

   shallow 
  sheet 
  of 
  water 
  almost 
  adjoining 
  the 
  eastern 
  range, 
  with 
  

   weeds 
  reaching 
  to 
  the 
  surface 
  almost 
  throughout 
  its 
  whole 
  extent, 
  

   and, 
  I 
  may 
  add, 
  literally 
  alive 
  with 
  water-fowl. 
  To 
  the 
  north-east 
  

   of 
  Mandritsara 
  there 
  is 
  another 
  small 
  sheet 
  of 
  water 
  ; 
  this 
  is 
  pro- 
  

   bably 
  also 
  a 
  remnant 
  of 
  the 
  same 
  ancient 
  lake. 
  

  

  At 
  a 
  slight 
  elevation 
  above 
  Lake 
  Alaotra, 
  on 
  the 
  southern 
  and 
  

   western 
  sides, 
  there 
  is 
  an 
  extensive 
  marsh, 
  succeeded 
  a 
  little 
  higher 
  

   by 
  a 
  marginal 
  plain. 
  Above 
  this, 
  again, 
  level 
  surfaces 
  at 
  various 
  

   heights 
  may 
  be 
  observed, 
  showing 
  how 
  far 
  the 
  water 
  has 
  once 
  ex- 
  

   tended. 
  Against 
  the 
  western 
  hill 
  s, 
  however, 
  old 
  lake-terraces, 
  several 
  

   hundred 
  feet 
  high, 
  may 
  be 
  distinctly 
  traced 
  ; 
  and 
  away 
  again 
  to 
  the 
  

   north 
  of 
  Antsihanaka 
  there 
  may 
  be 
  seen 
  numerous 
  old 
  lake-bottoms, 
  

   containing 
  rounded, 
  water-worn, 
  quartz 
  pebbles 
  and 
  small 
  iron 
  

   nodules. 
  Indeed 
  I 
  traced 
  these 
  old 
  lake-bottoms 
  to 
  over 
  100 
  miles 
  

   north 
  of 
  Lake 
  Alaotra, 
  and 
  found 
  that 
  one 
  of 
  them 
  actually 
  reached 
  

   to 
  the 
  astonishing 
  height 
  of 
  1140 
  feet 
  above 
  the 
  surface 
  of 
  the 
  

   existing 
  lake. 
  They 
  were 
  to 
  be 
  seen 
  also 
  at 
  various 
  intermediate 
  

   elevations. 
  The 
  highest 
  and 
  therefore 
  the 
  longest 
  exposed 
  of 
  these 
  

   lake-bottoms 
  were 
  so 
  much 
  worn 
  by 
  denudation 
  as 
  to 
  be 
  almost 
  un- 
  

   recognizable, 
  but 
  the 
  existence 
  of 
  numerous 
  water-worn 
  pebbles 
  

   and, 
  in 
  some 
  places, 
  of 
  deposits 
  of 
  iron 
  ore, 
  bore 
  witness 
  to 
  their 
  

   character. 
  

  

  About 
  15 
  or 
  16 
  miles 
  to 
  the 
  south 
  of 
  the 
  town 
  of 
  Mandritsara 
  

   there 
  is 
  an 
  abrupt 
  break 
  in 
  the 
  western 
  hill-range, 
  the 
  northern 
  end 
  

   of 
  it 
  forming 
  the 
  mountain 
  of 
  Ambiniviny, 
  which 
  rises 
  as 
  a 
  magni- 
  

   ficent 
  precipice 
  to 
  the 
  height 
  of 
  about 
  2000 
  feet 
  above 
  what 
  may 
  be 
  

   called 
  the 
  Mandritsara 
  valley. 
  This 
  precipice 
  is 
  continued 
  for 
  some 
  

   miles 
  in 
  a 
  semicircular 
  form 
  in 
  a 
  north-west 
  direction. 
  

  

  The 
  old 
  lake-bed 
  runs 
  past 
  Ambiniviny 
  somewhat 
  to 
  the 
  east 
  of 
  it, 
  

   but 
  in 
  its 
  northern 
  prolongation 
  occupies 
  high 
  ground, 
  and 
  is 
  not 
  so 
  

   distinctly 
  hemmed 
  in 
  by 
  hill-ranges 
  ; 
  indeed 
  on 
  its 
  western 
  edge, 
  

   which 
  is 
  about 
  3000 
  feet 
  above 
  the 
  sea, 
  there 
  is 
  a 
  steep 
  descent 
  of 
  

   nearly 
  2000 
  feet 
  into 
  the 
  Mandritsara 
  valley. 
  Moreover 
  the 
  old 
  

   lake-bottom 
  is 
  here 
  about 
  300 
  feet 
  lower 
  on 
  its 
  eastern 
  side 
  than 
  on 
  

   its 
  western. 
  In 
  some 
  parts 
  of 
  the 
  beds 
  east 
  of 
  Mandritsara 
  there 
  

   are 
  very 
  numerous 
  tubular 
  burrows, 
  from 
  \ 
  to 
  f 
  inch 
  in 
  diameter 
  

   and 
  a 
  foot 
  or 
  more 
  long. 
  These 
  are 
  filled 
  with 
  ruler-like 
  hardened 
  

   earth, 
  which 
  may 
  be 
  taken 
  out 
  in 
  cylindrical 
  pieces. 
  Have 
  these 
  

   been 
  the 
  homes 
  of 
  some 
  burrowing 
  animal? 
  

  

  Pour 
  or 
  five 
  years 
  ago 
  I 
  found 
  imbedded 
  in 
  some 
  ochreous 
  iron- 
  

   stone 
  in 
  the 
  Ankay 
  part 
  of 
  this 
  old 
  lake-bottom 
  (for 
  bog-iron 
  ore 
  

   abounds 
  in 
  it) 
  a 
  goodly 
  number 
  of 
  fossils, 
  being 
  chiefly 
  the 
  stems 
  of 
  

   what 
  were 
  probably 
  species 
  of 
  Cyperus, 
  leaves, 
  and 
  a 
  depresso-globose 
  

   fruit 
  about 
  the 
  size 
  of 
  a 
  marble, 
  five-celled 
  and 
  five-seeded. 
  In 
  some 
  

  

  