﻿IGNEOUS 
  ROCKS 
  AFTER 
  THEIR 
  CONSOLIDATION". 
  

  

  183 
  

  

  7. 
  KeLATION 
  OE 
  THESE 
  FACTS 
  TO 
  Dr. 
  LeHIIANN's 
  THEORY 
  OF 
  THE 
  

  

  Origin 
  of 
  Perthite-structtjre, 
  and 
  other 
  recent 
  researches. 
  

  

  There 
  are 
  some 
  recent 
  researches 
  that 
  give 
  very 
  considerable 
  

   support 
  to 
  the 
  views 
  I 
  have 
  been 
  advocating. 
  Prof. 
  Lehmann 
  has 
  

   shown 
  that, 
  on 
  heating 
  and 
  cooling, 
  crystals 
  undergo 
  contraction 
  along 
  

   planes 
  which 
  he 
  calls 
  "Contractionsrisse." 
  These 
  " 
  Contractions- 
  

   risse 
  " 
  he 
  shows 
  are 
  sometimes 
  planes 
  of 
  cleavage 
  and 
  at 
  other 
  times 
  

   gliding-planes. 
  He 
  refers 
  the 
  beautiful 
  structure 
  known 
  as 
  perthite 
  

   and 
  microperthite 
  — 
  so 
  commonly 
  seen 
  in 
  micorocline 
  — 
  to 
  the 
  sepa- 
  

   ration 
  of 
  the 
  orthoclase-crystals 
  along 
  their 
  " 
  Contractionsrisse," 
  and 
  

   the 
  deposition 
  of 
  secondary 
  albite 
  in 
  the 
  fissures 
  thus 
  formed. 
  

  

  In 
  this 
  case 
  we 
  have 
  an 
  example 
  of 
  an 
  action 
  strikingly 
  similar 
  

   to 
  that 
  which 
  I 
  have 
  been 
  describing 
  *. 
  

  

  Dr. 
  Erasmus 
  Haworth, 
  in 
  the 
  very 
  interesting 
  communication 
  to 
  

   which 
  I 
  have 
  already 
  referred, 
  gives 
  descriptions 
  and 
  drawings 
  of 
  a 
  

   striking 
  series 
  of 
  appearances 
  which 
  he 
  detected 
  in 
  certain 
  Missouri 
  

   granites. 
  He 
  shows 
  that 
  idiomorphic 
  crystals 
  of 
  felspar 
  sometimes 
  

   exhibit 
  enlargements 
  similar 
  to 
  those 
  I 
  have 
  been 
  describing 
  (figs. 
  

   b 
  and 
  c) 
  and 
  at 
  other 
  times 
  a 
  pseudo-spherulitic 
  fringe 
  (fig. 
  a). 
  

  

  Orihoclase-Crystals 
  from 
  Granite 
  of 
  Missouri. 
  

   (After 
  Dr. 
  E. 
  Haworth.) 
  

  

  In 
  a 
  the 
  crystal 
  is 
  surrounded 
  by 
  a 
  pseudo-spherulitic 
  border 
  ; 
  in 
  b 
  and 
  c 
  

   there 
  are 
  enlargements 
  with 
  continuous 
  cleavage 
  ; 
  and 
  in 
  d 
  " 
  the 
  idiomor- 
  

   phic 
  orthoclase 
  is 
  partly 
  surrounded 
  by 
  a 
  secondary 
  growth, 
  and 
  partly 
  by 
  

   a 
  micropegmatite 
  in 
  which 
  the 
  felspar 
  is 
  attached 
  to 
  the 
  original 
  crystal 
  

   and 
  oriented 
  with 
  it." 
  

  

  Still 
  more 
  remarkable 
  are 
  the 
  cases 
  which 
  he 
  cites 
  of 
  idiomorphic 
  

   orthoclase, 
  surrounded 
  by 
  secondary 
  outgrowths, 
  which 
  in 
  parts 
  

   exhibit 
  all 
  the 
  characters 
  of 
  a 
  true 
  micropegmatite, 
  the 
  central 
  

  

  * 
  " 
  Ueber 
  die 
  Mikroklin- 
  und 
  Perthit-Structur 
  der 
  Kalifeldspathe 
  und 
  deren 
  

   Abhangigkeit 
  von 
  ausseren 
  z. 
  Th. 
  mecbanischen 
  Einfliissen." 
  Jahres-Bericht 
  

   der 
  schles. 
  Gresellschaft 
  fur 
  vaterlandische 
  Cultur, 
  fur 
  1885, 
  Sitzung 
  vom 
  11 
  

   Feb., 
  also 
  in 
  same 
  Journal, 
  p. 
  92, 
  and 
  1886, 
  pp. 
  119-120. 
  See 
  also 
  Zeitschr. 
  f. 
  

   Kryst. 
  Bd. 
  ii. 
  1886, 
  pp. 
  608-612. 
  

  

  Dr. 
  Lehmann 
  informs 
  me 
  that 
  he 
  hopes 
  soon 
  to 
  publish 
  full 
  details, 
  with 
  

   illustrations, 
  of 
  these 
  very 
  important 
  researches. 
  

  

  