﻿350 
  

  

  I)K. 
  E. 
  H. 
  HATCH 
  ON 
  THE 
  CHAEACTEES 
  OE 
  EOCKS 
  

  

  The 
  crystals 
  almost 
  invariably 
  show 
  zonal 
  structure, 
  being 
  composed 
  

   of 
  successive 
  isomorphous 
  layers, 
  in 
  which 
  there 
  is 
  a 
  gradually 
  in- 
  

   creasing 
  difference 
  in 
  chemical 
  composition, 
  directly 
  influencing 
  the 
  

   position 
  of 
  the 
  axes 
  of 
  elasticity: 
  the 
  extinction 
  between 
  crossed 
  

   nicols 
  is 
  consequently 
  not 
  uniform 
  ; 
  it 
  takes 
  the 
  form 
  of 
  a 
  dark 
  

   shadow, 
  which 
  on 
  rotation 
  of 
  the 
  section 
  moves 
  from 
  the 
  centre 
  

   outwards 
  towards 
  the 
  periphery. 
  In 
  one 
  instance 
  a 
  difference 
  of 
  as 
  

   much 
  as 
  20° 
  was 
  measured 
  between 
  the 
  peripheral 
  layer 
  and 
  the 
  

   central 
  portion. 
  Even 
  in 
  ordinary 
  light 
  a 
  difference 
  between 
  the 
  

   peripheral 
  and 
  central 
  portions 
  can 
  sometimes 
  be 
  distinguished, 
  

   the 
  former 
  being 
  of 
  a 
  deeper 
  tint 
  than 
  the 
  latter. 
  In 
  sections 
  

   parallel 
  to 
  the 
  vertical 
  axis 
  the 
  central 
  portion 
  often 
  simulates 
  the 
  

   shape 
  of 
  an 
  hour-glass 
  (see 
  fig. 
  6). 
  This 
  structure 
  has 
  been 
  

   explained 
  in 
  the 
  following 
  way 
  : 
  — 
  During 
  the 
  consolidation 
  of 
  the 
  

   rock 
  the 
  augite 
  first 
  separates 
  in 
  skeleton-crystals 
  of 
  the 
  shape 
  of 
  

   an 
  hour-glass. 
  Later 
  on 
  the 
  depressions 
  in 
  the 
  sides 
  of 
  these 
  become 
  

   filled 
  in 
  by 
  augitic 
  material 
  of 
  a 
  slightly 
  different 
  composition 
  and 
  

   possessing, 
  in 
  consequence, 
  an 
  optical 
  character 
  deviating 
  from 
  that 
  of 
  

   the 
  first-formed 
  central 
  portion. 
  A 
  curious 
  instance 
  of 
  zoning 
  in 
  

   augite 
  is 
  shown 
  in 
  fig. 
  8. 
  Twinning 
  parallel 
  to 
  the 
  orthopinacoid 
  

  

  Fig. 
  6. 
  — 
  Hour-glass 
  Fig. 
  7.— 
  Augite 
  with 
  

   structure 
  in 
  Augite. 
  glass-inclusion. 
  

  

  Fig. 
  8. 
  — 
  Zoned 
  Augite. 
  

  

  i 
  

  

  r 
  

  

  is 
  of 
  frequent 
  occurrence. 
  In 
  some 
  cases 
  this 
  is 
  polysynthetic, 
  

   as 
  in 
  triclinic 
  felspar 
  (see 
  fig. 
  9) 
  ; 
  in 
  one 
  case 
  two 
  twins 
  are 
  united 
  

   along 
  a 
  common 
  face 
  (fig. 
  10). 
  

  

  Among 
  the 
  earliest 
  minerals 
  to 
  separate 
  were 
  the 
  hornblende 
  and 
  

   mica. 
  Of 
  these 
  minerals, 
  however, 
  the 
  major 
  portion 
  has 
  undergone 
  

   resorption 
  into 
  the 
  molten 
  magma 
  during 
  the 
  later 
  stages 
  of 
  consoli- 
  

   dation. 
  In 
  some 
  cases 
  there 
  remain 
  small 
  fragments, 
  encircled 
  

   by 
  a 
  broad 
  zone 
  of 
  the 
  products 
  of 
  fusion 
  {Schmelzzone). 
  In 
  

   the 
  case 
  of 
  the 
  hornblende 
  this 
  secondary 
  material 
  usually 
  retains 
  

   perfectly 
  the 
  original 
  form 
  of 
  the 
  crystals 
  after 
  which 
  it 
  is 
  pseudo- 
  

   morphous. 
  It 
  consists 
  of: 
  1, 
  opaque 
  particles 
  (magnetite); 
  2, 
  doubly 
  

  

  