﻿320 
  EEY. 
  E. 
  BAEOX 
  OX 
  THE 
  

  

  A 
  good 
  deal 
  of 
  what 
  has 
  been 
  said 
  respecting 
  the 
  volcanic 
  district 
  

   of 
  Mandridrano 
  also 
  holds 
  good 
  in 
  regard 
  to 
  that 
  of 
  the 
  Betafo 
  valley 
  

   and 
  neighbourhood, 
  where, 
  however, 
  the 
  volcanic 
  cones 
  are 
  fewer, 
  and 
  

   where 
  trachytic 
  and 
  andesitic 
  domes 
  do 
  not 
  appear 
  to 
  exist. 
  The 
  

   largest 
  volcano 
  in 
  the 
  Betafo 
  valley 
  is 
  probably 
  Iavoko 
  ; 
  it 
  has 
  a 
  

   larger 
  crater 
  than 
  any 
  to 
  be 
  found 
  about 
  Lake 
  Itasy. 
  Prom 
  this 
  

   volcano 
  a 
  large 
  sheet 
  of 
  black 
  lava 
  * 
  has 
  issued, 
  upon 
  which 
  are 
  found 
  

   in 
  abundance 
  various 
  species 
  of 
  plants, 
  notably 
  a 
  Euphorbia 
  and 
  a 
  

   stonecrop 
  (Kitchingia). 
  Almost 
  all 
  the 
  plants 
  growing 
  on 
  this 
  lava 
  

   bed, 
  however, 
  are 
  of 
  a 
  succulent 
  character, 
  and 
  can 
  dispense 
  with 
  

   soil, 
  requiring 
  merely 
  a 
  foothold. 
  On 
  the 
  sides 
  of 
  Iavoko 
  may 
  be 
  

   picked 
  up 
  fragments 
  of 
  what 
  appear 
  to 
  be 
  calcined 
  gneiss, 
  which 
  

   have 
  been 
  torn 
  from 
  the 
  sides 
  of 
  the 
  vent 
  in 
  the 
  passage 
  upwards 
  

   of 
  the 
  volcanic 
  matter. 
  On 
  some 
  of 
  the 
  cones 
  numerous 
  crystals 
  of 
  

   augite 
  as 
  large 
  as 
  marbles 
  may 
  be 
  found 
  among 
  the 
  volcanic 
  debris. 
  

   There 
  is 
  one 
  volcano, 
  Tritriva, 
  near 
  Betafo, 
  which, 
  inasmuch 
  as 
  it 
  

   is 
  different 
  in 
  character 
  from 
  any 
  others 
  mentioned 
  above, 
  deserves 
  

   a 
  few 
  words. 
  It 
  is 
  one 
  of 
  those 
  volcanoes 
  off 
  which 
  the 
  summit 
  

   has 
  been 
  blown 
  by 
  explosive 
  action, 
  leaving 
  what 
  is 
  known 
  as 
  a 
  

   crater-ring, 
  which 
  is 
  now 
  the 
  site 
  of 
  a 
  small 
  lake. 
  The 
  lake 
  is 
  not 
  

   more 
  than 
  100 
  or 
  200 
  feet 
  in 
  diameter, 
  perhaps 
  not 
  so 
  much 
  as 
  

   that 
  ; 
  but 
  there 
  is 
  reason 
  to 
  suppose 
  that 
  it 
  is 
  of 
  great 
  depth. 
  The 
  

   inner 
  sides 
  are 
  steep 
  for 
  the 
  greater 
  part 
  of 
  the 
  circumference, 
  but 
  

   on 
  one 
  side 
  the 
  lake 
  is 
  easily 
  accessible. 
  

  

  The 
  largest 
  crater-lake 
  in 
  these 
  volcanic 
  districts 
  (for 
  there 
  is 
  

   said 
  to 
  be 
  one 
  also 
  to 
  the 
  east 
  of 
  Ingolofotsy) 
  is 
  probably 
  Andrai- 
  

   kiba, 
  a 
  mile 
  or 
  two 
  to 
  the 
  west 
  of 
  Antsirabe, 
  though 
  even 
  this 
  is 
  of 
  

   comparatively 
  small 
  size. 
  It 
  is, 
  perhaps, 
  a 
  quarter 
  of 
  a 
  mile 
  in 
  

   diameter, 
  and 
  is 
  level 
  with 
  the 
  surrounding 
  country. 
  

  

  The 
  volcanoes 
  of 
  Mandridrano 
  and 
  Betafo 
  are 
  situated 
  twenty 
  or 
  

   thirty 
  miles 
  to 
  the 
  west 
  and 
  south-west 
  of 
  Ankaratra, 
  and, 
  judging 
  

   from 
  the 
  small 
  amount 
  of 
  denudation 
  they 
  have 
  undergone, 
  came 
  

   into 
  existence 
  after 
  the 
  volcanic 
  energy 
  of 
  Ankaratra 
  had 
  spent 
  

   itself. 
  

  

  To 
  the 
  east 
  of 
  Imerina, 
  near 
  Ambohidratrimo, 
  on 
  the 
  outskirts 
  of 
  

   the 
  large 
  forest, 
  I 
  found 
  a 
  few 
  years 
  ago 
  several 
  small 
  volcanic 
  craters. 
  

   These 
  also 
  seem 
  to 
  belong 
  to 
  the 
  class 
  of 
  crater-rings 
  or 
  explosion- 
  

   craters. 
  Although 
  fragments 
  of 
  volcanic 
  matter 
  have 
  been 
  ejected 
  

   from 
  them, 
  they 
  are 
  not 
  sufficient 
  to 
  form 
  a 
  cone 
  ; 
  and 
  the 
  craters, 
  

   none 
  of 
  which 
  exceed 
  100 
  yards 
  in 
  diameter 
  and 
  thirty 
  feet 
  in 
  

   depth, 
  have 
  been 
  probably 
  produced 
  by 
  a 
  single 
  explosion 
  of 
  the 
  

   pent-up 
  forces 
  below. 
  With 
  the 
  exception 
  of 
  scoriae 
  and 
  lapilli, 
  

   which 
  are 
  sparingly 
  scattered 
  about, 
  there 
  is 
  no 
  visible 
  sign 
  of 
  

   volcanoes, 
  and 
  one 
  comes 
  to 
  the 
  very 
  verge 
  of 
  the 
  craters 
  before 
  

   being 
  aware 
  of 
  their 
  existenee. 
  Two 
  of 
  the 
  largest 
  craters 
  consist 
  

   of 
  saucer-shaped 
  depressions, 
  but 
  are 
  rather 
  elliptical 
  than 
  circular 
  

   in 
  form 
  ; 
  the 
  others 
  consist 
  mostly 
  of 
  small 
  cavities, 
  deep 
  in 
  pro- 
  

   portion 
  to 
  their 
  width. 
  Several 
  of 
  the 
  craters 
  are 
  occupied 
  by 
  

  

  * 
  For 
  description 
  of 
  this 
  lava 
  see 
  Dr. 
  Hatch's 
  paper, 
  p. 
  346. 
  

  

  