﻿346 
  DE. 
  F. 
  H. 
  HATCH 
  ON 
  THE 
  CHARACTERS 
  OF 
  ROCKS 
  

  

  activity. 
  Again, 
  near 
  Betafo, 
  some 
  50 
  miles 
  to 
  the 
  south 
  of 
  this 
  

   district, 
  there 
  is 
  a 
  group 
  of 
  a 
  dozen 
  or 
  more 
  volcanoes. 
  Volcanic 
  

   rocks 
  also 
  occur 
  sporadically 
  as 
  dykes, 
  lava-flows, 
  and 
  dome-shaped 
  

   hummocks 
  in 
  other 
  parts 
  of 
  the 
  island 
  *. 
  The 
  greater 
  proportion 
  

   of 
  the 
  lavas 
  are 
  basaltic 
  ; 
  but 
  trachytes 
  and 
  andesites 
  also 
  occur. 
  

   The 
  basalts 
  are 
  here 
  taken 
  first. 
  

  

  1. 
  Basalts. 
  

  

  They 
  may 
  be 
  divided, 
  according 
  to 
  their 
  mineralogical 
  composi- 
  

   tion, 
  into 
  the 
  following 
  groups 
  : 
  — 
  

  

  (a) 
  Olivine-basalt. 
  

  

  (6) 
  Olivine-basalt 
  with 
  hornblende 
  and 
  biotite 
  in 
  porphyritic 
  

   cry 
  stals 
  (hornblende-oli 
  vine-basalt). 
  

  

  (c) 
  Olivine-free 
  basalt 
  with 
  porphyritic 
  hornblende 
  (hornblende- 
  

  

  basalt). 
  

  

  (d) 
  Basalt 
  with 
  hornblende 
  as 
  a 
  constituent 
  of 
  the 
  ground-mass. 
  

  

  (e) 
  Felspar-free 
  basalt 
  (magma-basalt, 
  limburgite). 
  

  

  a. 
  OUvine-basalts. 
  

  

  Basaltic 
  lavas, 
  rich 
  in 
  olivine, 
  are 
  of 
  frequent 
  occurrence 
  among 
  

   the 
  volcanic 
  rocks 
  of 
  Madagascar. 
  The 
  specimens, 
  here 
  described 
  in 
  

   detail, 
  were 
  collected 
  at 
  a 
  lava-stream 
  at 
  the 
  south 
  foot 
  of 
  the 
  

   volcano 
  of 
  Iavoko, 
  the 
  largest 
  of 
  the 
  group 
  of 
  volcanoes 
  in 
  the 
  

   neighbourhood 
  of 
  the 
  village 
  of 
  Betafo, 
  80 
  or 
  90 
  miles 
  S.S.W. 
  of 
  

   the 
  capital. 
  

  

  This 
  rock, 
  which 
  is 
  slightly 
  vesicular 
  in 
  the 
  hand-specimen, 
  is 
  

   found, 
  when 
  examined 
  with 
  the 
  microscope, 
  to 
  be 
  composed 
  mainly 
  

   of 
  a 
  dark 
  brown 
  ground-mass, 
  speckled 
  over 
  with 
  innumerable 
  

   minute 
  granules 
  of 
  magnetic 
  iron-ore, 
  but 
  containing 
  granules 
  of 
  

   augite 
  and 
  olivine 
  and, 
  in 
  lesser 
  quantity, 
  small 
  needles 
  of 
  striated 
  

   felspar. 
  Unaltered 
  glass 
  does 
  not 
  appear 
  to 
  be 
  present 
  ; 
  for, 
  under 
  

   high 
  powers, 
  the 
  brown 
  interstitial 
  matter 
  is 
  resolved 
  into 
  a 
  plexus 
  of 
  

   colourless 
  or 
  faintly 
  tinged 
  microlites 
  which, 
  despite 
  their 
  minute 
  

   size, 
  show 
  indications 
  of 
  double 
  refraction. 
  

  

  Olivine 
  and 
  augite 
  also 
  occur 
  in 
  porphyritic 
  crystals. 
  Those 
  of 
  

   the 
  first-named 
  mineral 
  are 
  for 
  the 
  most 
  part 
  sharply 
  contoured, 
  

   sometimes 
  giving 
  six-sided 
  sections 
  (those 
  in 
  the 
  zone 
  (010), 
  (001)). 
  

   Finely 
  marked 
  cleavage- 
  cracks 
  parallel 
  to 
  (010) 
  and 
  (001) 
  were 
  

   observed. 
  In 
  some 
  cases 
  the 
  interior 
  of 
  the 
  crystals 
  is 
  filled 
  with 
  

   dark-coloured 
  glassy 
  material 
  similar 
  to 
  the 
  external 
  base, 
  with 
  

   which 
  it 
  is 
  usually 
  connected 
  by 
  a 
  narrow 
  canal 
  ; 
  detached 
  inclusions 
  

   of 
  the 
  same 
  substance 
  are 
  also 
  not 
  unfrequent 
  (figs. 
  1, 
  2). 
  Some 
  

   fine 
  examples 
  of 
  interpenetration-twinning 
  were 
  observed 
  (see 
  figs. 
  3 
  

   and 
  4). 
  The 
  twinning-plane 
  is 
  (011) 
  f. 
  Fluidal 
  movements 
  in 
  

  

  * 
  For 
  full 
  particulars 
  as 
  to 
  the 
  occurrence 
  of 
  the 
  yolcanic 
  rocks, 
  see 
  Mr. 
  

   Baron's 
  paper. 
  

  

  t 
  Kalkowsky, 
  "Ueber 
  Olivinzwillinge 
  in 
  Gresteinen," 
  Zeitschr. 
  fiir 
  Krystall. 
  

   und 
  Min. 
  1888, 
  x. 
  p. 
  17. 
  

  

  