﻿352 
  DE. 
  F. 
  H. 
  HATCH 
  ON 
  THE 
  CHARACTERS 
  OF 
  ROCKS 
  

  

  sections 
  present 
  the 
  cross- 
  jointing 
  parallel 
  to 
  the 
  basal 
  plane 
  and 
  the 
  

   central 
  accumulation 
  of 
  inclusions 
  in 
  the 
  direction 
  of 
  the 
  vertical 
  axis, 
  

   characteristic 
  for 
  apatite. 
  The 
  mineralogical 
  composition 
  of 
  the 
  

   rock 
  is 
  that 
  of 
  an 
  augite-andesite 
  ; 
  but 
  its 
  structure 
  is 
  basaltic, 
  

   the 
  felspar 
  being 
  confined 
  to 
  the 
  ground-mass, 
  while 
  the 
  augite 
  is 
  

   porphyritic. 
  With 
  the 
  exception 
  of 
  the 
  absence 
  of 
  olivine 
  it 
  differs 
  

   in 
  no 
  essential 
  respect 
  from 
  the 
  basalt 
  described 
  in 
  section 
  b. 
  

  

  d. 
  Basalt 
  with 
  Hornblende 
  as 
  a 
  Constituent 
  of 
  the 
  Ground-mass. 
  

  

  The 
  rock 
  placed 
  under 
  this 
  head 
  is 
  a 
  basalt 
  of 
  rather 
  unusual 
  

   type. 
  It 
  occurs 
  in 
  one 
  of 
  a 
  series 
  of 
  dykes 
  on 
  the 
  sea-coast 
  at 
  Am- 
  

   bodimadiro, 
  on 
  the 
  north-west 
  coast. 
  Originally 
  vesicular, 
  its 
  cavities 
  

   have 
  become 
  infilled 
  with 
  fibrous 
  zeolites 
  and 
  calcite. 
  The 
  powdered 
  

   rock 
  effervesces 
  strongly 
  with 
  hydrochloric 
  acid 
  ; 
  and 
  on 
  standing 
  in 
  

   the 
  cold, 
  gelatinous 
  silica 
  separates. 
  No 
  cubes 
  of 
  salt 
  being 
  formed 
  

   in 
  the 
  solution 
  when 
  evaporated, 
  the 
  zeolites 
  must 
  be 
  lime-zeolites 
  

   (scolecite, 
  phillipsite). 
  

  

  The 
  chief 
  porphyritic 
  constituent 
  of 
  the 
  rock 
  is 
  augite 
  in 
  nu- 
  

   merous 
  large 
  and 
  well-shaped 
  crystals, 
  which 
  are 
  partially 
  invaded 
  

   by 
  serpentinous 
  and 
  calcareous 
  alteration-products. 
  Plagioclase 
  also 
  

   occurs 
  porphyritically 
  ; 
  but 
  its 
  crystals 
  are 
  few 
  and 
  small. 
  

  

  The 
  ground-mass 
  is 
  a 
  plexus 
  of 
  felspar-laths, 
  between 
  which 
  are 
  

   scattered 
  numerous 
  small 
  crystals 
  and 
  needles 
  of 
  brown 
  hornblende 
  

   and 
  granules 
  of 
  magnetic 
  iron-ore. 
  Interstitial 
  glassy 
  matter 
  

   could 
  not 
  be 
  detected. 
  

  

  In 
  its 
  general 
  character 
  the 
  rock 
  much 
  resembles 
  a 
  tephrite 
  : 
  and 
  an 
  

   attempt 
  was 
  made 
  to 
  prove 
  the 
  presence 
  of 
  nepheline. 
  But 
  the 
  fact 
  

   that 
  the 
  hydrochloric-acid 
  solution 
  of 
  the 
  powdered 
  rock 
  gave 
  no 
  

   cubes 
  of 
  salt, 
  tends 
  to 
  negative 
  this 
  supposition. 
  After 
  etching 
  and 
  

   treatment 
  with 
  fuchsine, 
  however, 
  a 
  number 
  of 
  small 
  specks 
  retained 
  

   the 
  colour 
  ; 
  and 
  it 
  is 
  quite 
  possible 
  that 
  these 
  may 
  be 
  granules 
  of 
  

   nepheline. 
  

  

  It 
  is 
  an 
  interesting 
  fact 
  with 
  regard 
  to 
  this 
  rock 
  that, 
  while 
  augite 
  

   appears 
  as 
  a 
  porphyritic 
  constituent, 
  hornblende 
  is 
  present 
  only 
  

   in 
  the 
  ground-mass, 
  thus 
  reversing 
  the 
  usual 
  order 
  of 
  things. 
  

   The 
  occurrence 
  of 
  hornblende 
  as 
  a 
  constituent 
  of 
  the 
  ground-mass 
  

   of 
  basalts 
  has 
  been 
  mentioned 
  by 
  C. 
  Chelius 
  *. 
  

  

  e. 
  Felspar-free 
  Basalt 
  (Magma-basalt, 
  Boiicky). 
  

  

  Belonging 
  to 
  this 
  class 
  is 
  a 
  black, 
  compact, 
  semi-vitreous 
  rock, 
  

   from 
  a 
  lava-stream 
  near 
  the 
  village 
  of 
  Ambohipolo, 
  in 
  the 
  volcanic 
  

   region 
  to 
  the 
  west 
  of 
  Lake 
  Itasy. 
  

  

  In 
  thin 
  section 
  this 
  rock 
  appears 
  as 
  a 
  clear 
  brown 
  glass, 
  containing 
  

   numerous 
  microlites 
  and 
  crystals 
  of 
  augite 
  and 
  magnetite, 
  the 
  

   latter 
  in 
  square 
  and 
  lozenge-shaped 
  sections, 
  varying 
  in 
  diameter 
  

   from 
  -006 
  mm. 
  to 
  -086 
  mm. 
  Olivine 
  is 
  rare. 
  The 
  augite 
  is 
  pale 
  

  

  * 
  ' 
  Erlautervmgen 
  zu 
  Blatt 
  Messel 
  und 
  Blatt 
  Eossclorf 
  der 
  geolog. 
  Karte 
  

   des 
  Grossk. 
  Hessen.' 
  Darmstadt, 
  1886. 
  Eosenbusch, 
  Die 
  massigen 
  Gest. 
  

   vol. 
  ii. 
  p. 
  711. 
  

  

  