﻿GEOLOGY 
  OF 
  MADAGASCAE. 
  317 
  

  

  tered 
  " 
  on 
  the 
  slope, 
  only 
  a 
  small 
  portion 
  of 
  it 
  having 
  reached 
  the 
  

   valley 
  helow. 
  This 
  ridge 
  of 
  hills, 
  through 
  which 
  the 
  volcanic 
  orifice 
  

   has 
  been 
  drilled, 
  is 
  composed 
  entirely 
  of 
  gneiss 
  ; 
  and, 
  indeed, 
  it 
  may 
  

   be 
  here 
  stated 
  that 
  the 
  whole 
  of 
  these 
  volcanoes, 
  as 
  is 
  the 
  case 
  also 
  

   with 
  those 
  abont 
  Betafo, 
  presently 
  to 
  be 
  mentioned, 
  rest 
  upon 
  a 
  

   platform 
  of 
  gneiss. 
  

  

  As 
  to 
  the 
  nature 
  of 
  the 
  volcanic 
  rocks 
  of 
  the 
  district, 
  it 
  may 
  be 
  

   said 
  that 
  these 
  comprise 
  basalt, 
  andesite, 
  trachyte, 
  trachytic 
  tuff, 
  

   palagonite-tuff, 
  and 
  basaltic 
  and 
  trachytic 
  conglomerate. 
  Some 
  

   of 
  the 
  trachytic 
  rocks 
  contain 
  large 
  crystals 
  of 
  glassy 
  felspar. 
  

   Pumice, 
  obsidian, 
  and 
  pitchstone 
  do 
  not 
  seem 
  to 
  be 
  found 
  anywhere. 
  

  

  In 
  addition 
  to 
  the 
  numerous 
  scoria-cones, 
  there 
  may 
  be 
  seen 
  here 
  

   and 
  there 
  in 
  the 
  district 
  some 
  dozen 
  or 
  more 
  other 
  volcanoes, 
  

   differing 
  entirely 
  in 
  character 
  from 
  those 
  which 
  have 
  been 
  spoken 
  

   of 
  above. 
  These 
  are 
  large 
  bell-shaped 
  hummocks 
  of 
  trachyte 
  or 
  

   andesite. 
  They 
  are 
  without 
  definite 
  craters, 
  though 
  one 
  or 
  two 
  of 
  

   them 
  ha-ye 
  more 
  or 
  less 
  conspicuous 
  depressions 
  on 
  their 
  summits, 
  

   showing 
  that 
  eruptive 
  action 
  has 
  not 
  been 
  altogether 
  wanting. 
  

   These 
  hammocks 
  are 
  chiefly 
  composed 
  of 
  a 
  light-coloured 
  compact 
  

   rock. 
  This 
  rock, 
  having 
  originally 
  had 
  a 
  highly 
  viscid 
  or 
  pasty 
  

   consistency, 
  has 
  accumnlated 
  and 
  set 
  immediately 
  over 
  the 
  orifice 
  

   through 
  which 
  it 
  was 
  extruded. 
  Such 
  hummocks 
  are 
  Ingolofotsy, 
  

   Beteheza, 
  Andranonatoha, 
  Angavo, 
  Ambasy, 
  Isahadimy, 
  Ambohibe 
  9 
  

   Lazaina, 
  Antsahondra, 
  Antsangarahara 
  (?), 
  &c. 
  Ingolofotsy, 
  situ- 
  

   ated 
  to 
  the 
  north-west 
  of 
  Lake 
  Itasy, 
  is 
  perhaps 
  the 
  most 
  striking 
  in 
  

   appearance 
  of 
  these 
  hummocks. 
  It 
  bears 
  some 
  resemblance 
  to 
  a 
  

   bell 
  or 
  a 
  Turkish 
  fez, 
  except 
  that 
  its 
  sides 
  are 
  furrowed 
  with 
  water- 
  

   channels, 
  and 
  its 
  truncated 
  summit 
  is 
  notched 
  in 
  a 
  remarkable 
  

   manner. 
  Its 
  height 
  above 
  the 
  plain 
  is 
  665 
  feet 
  ; 
  the 
  inclination 
  of 
  

   its 
  sides 
  averages 
  probably 
  50°. 
  Adjoining 
  Ingolofotsy 
  on 
  the 
  

   south-west 
  is 
  Beteheza, 
  and 
  further 
  still 
  Andranonatoha, 
  the 
  latter 
  

   of 
  which 
  consists 
  of 
  andesite 
  *, 
  which 
  has 
  probably 
  welled 
  out 
  from 
  

   orifices 
  on 
  the 
  same 
  line 
  of 
  fissure 
  from 
  which 
  Ingolofotsy 
  was 
  ex- 
  

   truded. 
  From 
  Andranonatoha, 
  which 
  seems 
  to 
  be 
  the 
  highest 
  of 
  

   all 
  the 
  domes, 
  the 
  lava 
  has 
  flowed 
  in 
  thick 
  masses, 
  but 
  has 
  stopped 
  

   apparently 
  at 
  the 
  foot 
  of 
  the 
  mountain. 
  Angavo 
  is 
  another 
  of 
  these 
  

   fez-shaped 
  domes. 
  One 
  singular 
  feature 
  in 
  this 
  mountain 
  is 
  its 
  

   numerous 
  shallow 
  water-channels, 
  which, 
  on 
  the 
  north 
  side, 
  make 
  

   their 
  way 
  down 
  from 
  the 
  summit 
  in 
  a 
  surprisingly 
  regular 
  manner, 
  

   giving 
  the 
  appearance 
  of 
  an 
  open 
  umbrella 
  with 
  numerous 
  ribs. 
  

   From 
  one 
  point 
  of 
  view 
  I 
  counted 
  as 
  many 
  as 
  thirty-four 
  of 
  these 
  

   channels. 
  It 
  may 
  be 
  mentioned 
  in 
  passing 
  that, 
  in 
  a 
  valley 
  at 
  the 
  

   west 
  foot 
  of 
  Angavo, 
  there 
  is 
  a 
  small 
  crater 
  the 
  lips 
  of 
  which 
  are 
  level 
  

   with 
  the 
  surface 
  of 
  the 
  ground. 
  This 
  may, 
  perhaps, 
  be 
  accounted 
  

   for 
  by 
  supposing 
  that 
  the 
  ejected 
  materials 
  from 
  this 
  and 
  other 
  

   craters 
  near 
  have 
  so 
  accumulated 
  as 
  to 
  raise 
  the 
  level 
  of 
  the 
  valley 
  

   between 
  up 
  to 
  the 
  rim 
  of 
  the 
  crater, 
  and 
  so 
  to 
  obliterate 
  the 
  cone, 
  

   probably 
  never 
  of 
  any 
  great 
  height. 
  

  

  * 
  See 
  Dr 
  Hatch's 
  description 
  of 
  this 
  andesite 
  on 
  p. 
  355. 
  

  

  