﻿348 
  DK. 
  F. 
  H. 
  HATCH 
  ON 
  THE 
  CHAEACTEES 
  OF 
  EOCKS 
  

  

  eruption. 
  J. 
  S. 
  Diller 
  * 
  and 
  J. 
  P. 
  Iddiugs 
  f, 
  however, 
  have 
  recently 
  

   argued 
  against 
  this 
  notion. 
  They 
  consider 
  them 
  to 
  he 
  indigenous 
  

   to 
  the 
  "basalt, 
  having 
  separated 
  at 
  an 
  early 
  period 
  in 
  the 
  rock's 
  

   history, 
  prohahly 
  hefore 
  its 
  eruption 
  and 
  under 
  conditions 
  of 
  pressure 
  

   and 
  " 
  chemical 
  equilibrium 
  " 
  at 
  variance 
  with 
  those 
  that 
  obtain 
  at 
  

   the 
  surface 
  J. 
  If 
  this 
  be 
  so, 
  the 
  law 
  of 
  increasing 
  acidity 
  during 
  

   consolidation 
  cannot 
  be 
  universally 
  true. 
  With 
  our 
  present 
  know- 
  

   ledge 
  it 
  is 
  difficult 
  to 
  conceive 
  olivine 
  crystallizing 
  out 
  from 
  a 
  magma 
  

   from 
  which 
  quartz 
  has 
  already 
  separated 
  §. 
  

  

  A 
  very 
  similar 
  olivine-basalt, 
  but 
  without 
  the 
  quartz, 
  forms 
  the 
  

   highest 
  peak 
  (Tsiafajavona) 
  of 
  the 
  mountain 
  of 
  Ankaratra. 
  

  

  An 
  olivine-basalt 
  from 
  Antongodrahoja, 
  120 
  miles 
  N.N."W. 
  of 
  the 
  

   capital, 
  approximates 
  to 
  an 
  andesitic 
  type. 
  The 
  felspar 
  is 
  abundant 
  

   and 
  porphyritic, 
  the 
  crystals 
  are 
  of 
  a 
  prismatic 
  habit, 
  giving 
  rect- 
  

   angular 
  sections 
  parallel 
  to 
  the 
  long 
  axis, 
  and 
  square 
  cross-sections. 
  

   As 
  in 
  the 
  andesites, 
  the 
  felspars 
  contain 
  much 
  included 
  matter, 
  glass 
  

   and 
  iron-ore. 
  A 
  pale-brown 
  augite 
  occurs 
  in 
  allotriomorphic 
  grains, 
  

   sometimes 
  penetrated 
  by 
  a 
  felspar 
  lath 
  (see 
  fig. 
  5). 
  Olivine 
  is 
  sub- 
  

   Fig. 
  5. 
  — 
  Grain 
  of 
  Augite 
  penetrated 
  by 
  a 
  felspar-lath. 
  

  

  ordinate. 
  The 
  ground-mass 
  is 
  a 
  mesh 
  of 
  felspar-microlites, 
  together 
  

   with 
  granules 
  of 
  augite, 
  particles 
  of 
  iron-ore, 
  and 
  patches 
  of 
  inter- 
  

   stitial 
  glass 
  (the 
  " 
  hyalopilitic 
  " 
  structure 
  of 
  Eosenbusch). 
  

  

  * 
  " 
  The 
  Latest 
  Volcanic 
  Eruption 
  in 
  Northern 
  California 
  and 
  its 
  peculiar 
  

   Lava," 
  Ainer. 
  Journ. 
  Sci. 
  (3) 
  xxxiii. 
  1887, 
  p. 
  45. 
  

  

  t 
  " 
  On 
  the 
  Origin 
  of 
  Primary 
  Quartz 
  in 
  Basalt," 
  ibid, 
  xxxvi. 
  Sept. 
  1888, 
  p. 
  208. 
  

  

  \ 
  Mr. 
  Iddings 
  suggests 
  the 
  influence 
  of 
  water-vapour 
  at 
  a 
  great 
  pressure. 
  

  

  § 
  In 
  his 
  latest 
  work 
  (' 
  Theoretische 
  Geologie,' 
  p. 
  214) 
  Dr. 
  E. 
  Beyer 
  offers 
  

   an 
  ingenious 
  explanation 
  of 
  this 
  unnatural 
  association. 
  On 
  account 
  of 
  its 
  

   suogestiveness 
  I 
  give 
  the 
  passage 
  in 
  his 
  own 
  words: 
  — 
  "Ich 
  vermute, 
  dass 
  es 
  

   sieh 
  hier 
  uberhaupt 
  nur 
  urn 
  Grenzgebiete 
  zwischen 
  sauren 
  und 
  basischen 
  

   Sohlieren 
  handelt, 
  wo 
  heterogene 
  Associations-Kreise 
  fortwabrend 
  einander 
  

   bege^nen. 
  Tritt 
  die 
  Mischung 
  erst 
  kurze 
  Zeit 
  vor 
  dem 
  Erstarren 
  ein, 
  so 
  kann 
  

   natiirlich 
  jene 
  absurde 
  Association 
  erhalten 
  bleiben." 
  

  

  