﻿GEOLOGY 
  OF 
  MADAGASCAR. 
  319 
  

  

  Lake 
  Itasy 
  needs 
  no 
  lengthy 
  description 
  here. 
  It 
  covers 
  ground, 
  

   roughly 
  speaking, 
  to 
  the 
  extent 
  of 
  about 
  twenty-five 
  square 
  miles. 
  

   It 
  may 
  not 
  improbably 
  occupy 
  an 
  area 
  of 
  depression 
  due 
  to 
  volcanic 
  

   action 
  *; 
  but 
  be 
  this 
  as 
  it 
  may, 
  there 
  is 
  a 
  cause 
  at 
  its 
  outlet 
  suffi- 
  

   cient 
  to 
  account 
  for 
  its 
  formation. 
  Here, 
  lying 
  in 
  the 
  river-bed, 
  

   are 
  numerous 
  blocks 
  of 
  gneiss, 
  many 
  of 
  them 
  blackened 
  with 
  a 
  

   covering 
  of 
  oxide 
  of 
  iron; 
  and 
  beneath 
  this 
  gneiss 
  lava 
  may 
  be 
  

   seen. 
  Several 
  volcanoes 
  cluster 
  round 
  the 
  outlet 
  ; 
  but 
  there 
  is 
  one 
  

   — 
  an 
  inconsiderable 
  hill 
  — 
  situated 
  on 
  the 
  southern 
  margin 
  of 
  the 
  

   outflowing 
  river, 
  just 
  above 
  the 
  rapids. 
  There, 
  distinctly 
  enough, 
  

   may 
  be 
  seen 
  a 
  low 
  and 
  much-worn 
  crater, 
  with 
  its 
  breached 
  side 
  

   facing 
  the 
  outlet 
  ; 
  and 
  gneiss 
  blocks 
  may 
  be 
  traced 
  from 
  the 
  bed 
  of 
  

   the 
  river 
  all 
  up 
  the 
  hill-side 
  to 
  the 
  crater. 
  There 
  has 
  evidently 
  been 
  

   first 
  an 
  ejection 
  of 
  volcanic 
  matter, 
  followed 
  probably 
  by 
  an 
  explosion 
  

   tearing 
  up 
  and 
  flinging 
  out 
  fragments 
  of 
  the 
  gneiss 
  through 
  which 
  the 
  

   vent 
  was 
  bored 
  ; 
  hence 
  the 
  gneiss 
  blocks 
  are 
  superimposed 
  upon 
  the 
  

   lava. 
  Thus 
  the 
  water 
  has 
  been 
  ponded 
  back. 
  The 
  river 
  has 
  now 
  

   cut 
  its 
  way 
  several 
  feet 
  through 
  the 
  barrier 
  thus 
  thrown 
  across 
  its 
  

   course 
  ; 
  and 
  by 
  this 
  continual 
  erosion 
  at 
  its 
  outlet, 
  and 
  the 
  accumu- 
  

   lation 
  of 
  sediment 
  and 
  the 
  growth 
  of 
  vegetation 
  at 
  its 
  head, 
  the 
  

   lake 
  is 
  slowly, 
  though 
  surely, 
  decreasing 
  in 
  extent 
  year 
  by 
  year. 
  

  

  It 
  may 
  be 
  mentioned 
  that 
  the 
  river 
  Lilia, 
  which 
  flows 
  from 
  the 
  

   lake, 
  passes 
  for 
  several 
  miles 
  over 
  sheets 
  of 
  black 
  lava, 
  through 
  

   which 
  it 
  has 
  cut 
  its 
  way. 
  At 
  Ambohipolo 
  there 
  are 
  three 
  fine 
  

   waterfalls 
  fifty 
  or 
  sixty 
  feet 
  high, 
  and 
  on 
  the 
  north 
  side 
  of 
  the 
  

   river, 
  immediately 
  below 
  the 
  second 
  fall, 
  there 
  is 
  an 
  exposed 
  section 
  

   of 
  columnar 
  basalt. 
  By 
  the 
  river-side 
  near 
  here, 
  too, 
  may 
  be 
  seen 
  

   at 
  the 
  foot 
  of 
  the 
  basaltic 
  cliff 
  carbonate 
  of 
  lime, 
  which 
  has 
  been 
  

   dissolved 
  out 
  from 
  the 
  basalt 
  and 
  been 
  deposited 
  on 
  the 
  shingle, 
  

   forming 
  conglomerate. 
  

  

  A 
  few 
  miles 
  south 
  of 
  Lake 
  Itasy 
  the 
  volcanoes 
  begin 
  to 
  dis- 
  

   appear, 
  though 
  they 
  do 
  not 
  entirely 
  die 
  out 
  for 
  a 
  good 
  distance 
  

   southwards 
  ; 
  indeed 
  the 
  volcanoes 
  of 
  Mandridrano 
  and 
  those 
  of 
  the 
  

   Betafo 
  valley 
  seem 
  to 
  be 
  more 
  or 
  less 
  connected 
  by 
  intermediate 
  ones. 
  

   Between 
  the 
  two 
  districts, 
  it 
  is 
  true, 
  there 
  are 
  but 
  few 
  volcanic 
  

   cones 
  of 
  any 
  size, 
  but 
  craters 
  may 
  be 
  seen 
  here 
  and 
  there. 
  In 
  

   Dr. 
  Mullens's 
  map 
  of 
  the 
  Central 
  Provinces 
  of 
  Madagascar 
  several 
  

   " 
  volcanic 
  hills 
  " 
  are 
  shown 
  somewhat 
  to 
  the 
  west 
  of 
  a 
  straight 
  line 
  

   drawn 
  between 
  the 
  two 
  districts 
  ; 
  but 
  these 
  are 
  not 
  in 
  actual 
  ex- 
  

   istence. 
  

  

  * 
  Mr. 
  Johnson 
  says, 
  " 
  I 
  am 
  told 
  here 
  that 
  Itasy 
  was 
  once 
  a 
  huge 
  swamp, 
  and 
  

   that 
  its 
  becoming 
  a 
  clear 
  lake 
  is 
  within 
  the 
  knowledge, 
  or 
  perhaps 
  the 
  tradi- 
  

   tions, 
  of 
  the 
  people" 
  (Antananarivo 
  Annual, 
  No. 
  I., 
  1875, 
  p. 
  60). 
  If 
  this 
  be 
  

   really 
  true, 
  it 
  can 
  only 
  be 
  explained 
  on 
  the 
  supposition 
  that 
  there 
  has 
  been 
  a 
  

   recent 
  subsidence 
  of 
  what 
  is 
  now 
  the 
  bed 
  of 
  the 
  lake, 
  as 
  in 
  the 
  case 
  of 
  Man- 
  

   dentika 
  mentioned 
  above. 
  

  

  Mr. 
  Sibree 
  says, 
  " 
  The 
  natives 
  say 
  that 
  the 
  lake 
  Itasy 
  was 
  formed 
  by 
  a 
  

  

  Vazimba 
  chieftain, 
  named 
  Rapeto, 
  damming 
  up 
  a 
  river 
  in 
  the 
  vicinity, 
  and 
  so 
  

   the 
  rice-fields 
  of 
  a 
  neighbouring 
  chief 
  with 
  whom 
  he 
  was 
  at 
  variance 
  were 
  

   flooded, 
  and 
  have 
  ever 
  since 
  remained 
  under 
  water." 
  ('The 
  Great 
  African 
  

   Island,' 
  p. 
  136.) 
  

  

  Q. 
  J. 
  G 
  S. 
  No. 
  178. 
  2 
  a 
  

  

  