﻿176 
  PKOF. 
  J. 
  W. 
  JTTDD 
  ON 
  THE 
  GEOWTH 
  OE 
  CETSTALS 
  EST 
  

  

  tained 
  concerning 
  the 
  secondary 
  origin 
  of 
  the 
  characteristic 
  struc- 
  

   tures 
  of 
  the 
  " 
  granophyric 
  " 
  rocks 
  should 
  be 
  placed 
  on 
  record. 
  

  

  The 
  examination 
  of 
  the 
  field-relations 
  of 
  the 
  remarkably 
  varied 
  

   types 
  of 
  acid 
  and 
  intermediate 
  rocks 
  which 
  occur 
  in 
  the 
  Western 
  

   Isles 
  of 
  Scotland 
  and 
  the 
  North 
  of 
  Ireland 
  shows 
  that 
  rocks 
  ex- 
  

   hibiting 
  the 
  structures 
  known 
  as 
  " 
  granophyric 
  " 
  occur 
  under 
  certain 
  

   well-marked 
  conditions. 
  The 
  larger, 
  truly 
  granitic 
  eruptive 
  masses 
  

   usually 
  pass 
  towards 
  their 
  peripheral 
  portions 
  into 
  " 
  granophyres 
  " 
  * 
  ; 
  

   the 
  smaller 
  eruptive 
  bosses 
  and 
  lenticular 
  sheets 
  (" 
  laccolites 
  ") 
  of 
  

   the 
  same 
  rocks 
  exhibit 
  the 
  granophyric 
  character 
  throughout 
  ; 
  while 
  

   the 
  apophyses, 
  and 
  even 
  dykes, 
  proceeding 
  from 
  intrusive 
  masses 
  very 
  

   often 
  display 
  these 
  same 
  structures, 
  which 
  are, 
  in 
  such 
  cases, 
  not 
  un- 
  

   frequ 
  entry 
  developed 
  on 
  a 
  very 
  minute 
  scale. 
  The 
  structures 
  which 
  

   especially 
  distinguish 
  these 
  granophyric 
  rocks 
  are 
  the 
  micropegma- 
  

   titic, 
  the 
  centric 
  or 
  ocellar 
  structure, 
  the 
  pseud 
  ospherulitic, 
  the 
  

   micro-granitic, 
  and 
  the 
  drusy 
  or 
  miarolitic 
  structures. 
  

  

  I 
  hope 
  from 
  the 
  study 
  of 
  the 
  remarkably 
  fresh 
  examples 
  of 
  

   Tertiary 
  rocks, 
  which 
  are 
  in 
  texture 
  intermediate 
  between 
  the 
  

   granitic 
  and 
  the 
  volcanic 
  types, 
  to 
  illustrate 
  the 
  mode 
  of 
  origin 
  of 
  

   these 
  various 
  structures, 
  and 
  to 
  show 
  their 
  relations 
  one 
  to 
  an 
  other 
  f. 
  

  

  2. 
  ENLARGEMENTS 
  OE 
  DETRITAE 
  FRAGMENTS 
  OE 
  CrYSTAES 
  : 
  — 
  QlJARTZ, 
  

  

  Felspar, 
  Hornblende, 
  and 
  Mica. 
  

  

  At 
  the 
  outset 
  of 
  this 
  inquiry 
  I 
  must 
  recall 
  the 
  important 
  and 
  

   suggestive 
  discovery 
  made 
  by 
  Mr. 
  Sorby, 
  and 
  announced 
  to 
  the 
  

   Society 
  in 
  his 
  Presidential 
  Address 
  in 
  1880 
  J. 
  Mr. 
  Sorby 
  showed 
  

   that 
  the 
  so-called 
  " 
  crystalline 
  sands 
  " 
  were 
  produced 
  by 
  secondary 
  

   growths 
  of 
  quartz 
  upon 
  the 
  fragments 
  of 
  quartz- 
  crystals 
  that 
  con- 
  

   stitute 
  sand-grains. 
  

  

  Like 
  so 
  many 
  other 
  of 
  this 
  author's 
  discoveries, 
  the 
  new 
  facts 
  

   then 
  announced 
  have 
  proved 
  wonderfully 
  suggestive 
  and 
  fruitful 
  in 
  

   new 
  departures 
  of 
  thought. 
  

  

  Mr. 
  A. 
  A 
  Young 
  §, 
  the 
  late 
  Dr. 
  Eoland 
  D. 
  Irving 
  ||, 
  Mr. 
  Yan 
  

   Hise 
  ^[, 
  and 
  other 
  authors 
  in 
  the 
  United 
  States 
  have 
  shown 
  how 
  the 
  

   structures 
  of 
  sandstones 
  and 
  quartzites 
  may 
  be 
  explained 
  by 
  the 
  

   application 
  of 
  principles 
  discovered 
  by 
  Mr. 
  Sorby 
  ; 
  while 
  Professor 
  

  

  * 
  Throughout 
  this 
  paper 
  I 
  use 
  this 
  term 
  not 
  in 
  the 
  original 
  sense 
  in 
  

   which 
  it 
  was 
  employed 
  by 
  its 
  author, 
  Yogelsang, 
  but 
  with 
  the 
  meaning 
  which 
  

   Rosenbusch 
  has 
  proposed 
  to 
  attach 
  to 
  it. 
  

  

  t 
  I 
  may 
  mention, 
  in 
  passing, 
  that 
  the 
  study 
  of 
  the 
  processes 
  of 
  crystallization 
  

   by 
  which 
  the 
  amorphous 
  silica, 
  replacing 
  the 
  calcic 
  carbonate 
  of 
  chalk 
  -mud, 
  

   passes 
  into 
  the 
  difi'erent 
  varieties 
  of 
  hypocrystalline 
  flint, 
  throws 
  much 
  light 
  

   upon 
  the 
  changes 
  which 
  have 
  taken 
  place 
  in 
  the 
  ground-mass 
  of 
  igneous 
  rocks. 
  

   The 
  problem 
  is 
  in 
  this 
  case 
  simplified 
  by 
  the 
  fact 
  that 
  there 
  is 
  only 
  one 
  mineral 
  

   species 
  involved 
  in 
  the 
  change, 
  namely, 
  quartz 
  ; 
  while 
  in 
  the 
  devitrification 
  

   of 
  the 
  glasses 
  of 
  igneous 
  rocks 
  there 
  is 
  much 
  greater 
  complexity. 
  

  

  I 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxvi. 
  (1880). 
  

  

  § 
  Am. 
  Journ. 
  Sci. 
  3rd 
  ser. 
  vol. 
  xxiii. 
  (1881), 
  p. 
  257, 
  and 
  vol. 
  xxiv. 
  (1881), 
  

   p. 
  47. 
  

  

  || 
  Ibid. 
  3rd 
  ser. 
  vol. 
  xxv. 
  (1883), 
  p. 
  401. 
  

  

  % 
  Bull. 
  U. 
  S. 
  Geol. 
  Survey, 
  no. 
  8. 
  

  

  