﻿1 
  78 
  PROF. 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  GROWTH 
  OF 
  CRYSTALS 
  IN 
  

  

  growths 
  around 
  crystals 
  of 
  both 
  hornblende 
  and 
  augite 
  in 
  certain 
  

   massive 
  rocks 
  from 
  Michigan 
  and 
  "Wisconsin 
  *. 
  In 
  the 
  following 
  

   year 
  Mr. 
  G. 
  P. 
  Merrill 
  described 
  " 
  Secondary 
  Enlargements 
  of 
  

   Angites 
  in 
  a 
  Peridotite 
  from 
  Little 
  Deer 
  Isle, 
  Maine," 
  and, 
  like 
  

   Mr. 
  Yan 
  Hise, 
  distinctly 
  asserts 
  that 
  such 
  enlargements 
  must 
  have 
  

   occurred 
  subsequently 
  to 
  the 
  consolidation 
  of 
  the 
  rock 
  t. 
  

  

  The 
  bearing 
  of 
  Dr. 
  J. 
  Lehmann's 
  important 
  researches 
  on 
  " 
  Con- 
  

   tractionsrisse 
  in 
  Krystallen 
  " 
  J 
  upon 
  this 
  question 
  will 
  be 
  discussed 
  

   in 
  a 
  later 
  page 
  of 
  this 
  paper. 
  

  

  4. 
  Proofs 
  that 
  Growth 
  of 
  Porphtrttic 
  Felspars 
  mat 
  take 
  place 
  

   after 
  the 
  consolidation 
  and 
  the 
  alteration 
  by 
  weathering 
  

   of 
  an 
  Igneous 
  Rock. 
  

  

  In 
  my 
  studies 
  of 
  the 
  Tertiary 
  granophyric 
  rocks 
  of 
  the 
  Western 
  

   Isles 
  of 
  Scotland 
  and 
  the 
  North 
  of 
  Ireland, 
  I 
  have 
  found 
  abundant 
  

   evidence 
  that 
  the 
  growth 
  of 
  crystals 
  of 
  felspar 
  and 
  quartz 
  goes 
  on, 
  

   at 
  the 
  expense 
  of 
  a 
  more 
  or 
  less 
  vitreous 
  matrix, 
  long 
  after 
  the 
  

   solidification 
  of 
  the 
  rock, 
  and 
  that 
  in 
  this 
  fact 
  we 
  have 
  a 
  satisfactory 
  

   explanation 
  of 
  the 
  mode 
  of 
  origin 
  of 
  the 
  several 
  " 
  granophyric 
  " 
  

   structures. 
  I 
  think 
  that 
  it 
  will 
  be 
  desirable 
  at 
  the 
  outset 
  to 
  de- 
  

   scribe 
  a 
  case 
  in 
  which 
  the 
  evidence 
  of 
  this 
  action 
  appears 
  to 
  me 
  to 
  

   be 
  so 
  clear 
  as 
  to 
  place 
  it 
  altogether 
  beyond 
  question. 
  

  

  The 
  rock 
  in 
  which 
  the 
  illustration 
  occurs 
  is 
  one 
  belonging 
  to 
  a 
  

   group 
  that, 
  as 
  I 
  shall 
  hereafter 
  show, 
  is 
  very 
  frequently 
  represented 
  

   among 
  the 
  lavas 
  of 
  the 
  district 
  ; 
  in 
  chemical 
  composition 
  it 
  lies 
  on 
  

   the 
  borders 
  of 
  the 
  intermediate 
  and 
  basic 
  groups, 
  and 
  would 
  be 
  desig- 
  

   nated 
  by 
  Prench 
  petrographers 
  a 
  " 
  Labradorite 
  ; 
  " 
  in 
  fact, 
  it 
  cor- 
  

   responds 
  very 
  closely 
  indeed 
  with 
  the 
  Icelandic 
  labradorites 
  so 
  

   well 
  described 
  by 
  M. 
  Rene 
  Breon 
  §. 
  

  

  As 
  it 
  is 
  manifestly 
  inconvenient, 
  however, 
  to 
  employ 
  the 
  same 
  

   term 
  both 
  for 
  a 
  rock 
  and 
  for 
  a 
  mineral, 
  I 
  would 
  suggest 
  that 
  the 
  rocks 
  

   in 
  question 
  should 
  be 
  called 
  " 
  labradorite-andesites." 
  They 
  consist 
  

   of 
  large 
  crystals 
  of 
  a 
  lime-soda 
  felspar, 
  closely 
  corresponding 
  to 
  

   labradorite 
  (or 
  sometimes 
  to 
  a 
  felspar 
  intermediate 
  between 
  that 
  

   species 
  and 
  anorthite), 
  scattered 
  through 
  a 
  glassy 
  base 
  containing 
  

   microlites 
  of 
  felspar, 
  augite, 
  and 
  magnetite 
  ; 
  olivine 
  is 
  usually 
  so 
  

   rare 
  in 
  the 
  rock 
  that 
  we 
  can 
  regard 
  it 
  only 
  as 
  an 
  accessory 
  con- 
  

   stituent. 
  I 
  shall 
  show 
  that, 
  as 
  the 
  result 
  of 
  certain 
  alterations, 
  this 
  

   Tertiary 
  rock 
  assumes 
  the 
  characters 
  which 
  are 
  universally 
  accepted 
  

   as 
  distinguishing 
  the 
  " 
  labradorite-porphyrites," 
  of 
  which 
  the 
  well- 
  

   known 
  verde-antique 
  and 
  Lambay-Island 
  porphyries 
  are 
  such 
  

   excellent 
  examples. 
  In 
  Iceland, 
  however, 
  these 
  rocks 
  are 
  found 
  in 
  

  

  * 
  Am. 
  Joum. 
  Sci. 
  3rd 
  ser. 
  vol. 
  xxxiii. 
  (1887), 
  p. 
  385. 
  

  

  t 
  Ibid. 
  vol. 
  xxxy. 
  (1888), 
  p. 
  488-496. 
  

  

  \ 
  Zeitscbr. 
  f. 
  Kryst. 
  Bd. 
  xi. 
  (1886), 
  pp. 
  608-612. 
  

  

  § 
  Notes 
  pour 
  servir 
  a 
  l'etude 
  de 
  la 
  Geologie 
  de 
  l'lslande 
  et 
  des 
  lies 
  Faeroe 
  

   (1884). 
  I 
  am 
  greatly 
  indebted 
  to 
  M. 
  Breon 
  for 
  supplying 
  me 
  with 
  an 
  interest- 
  

   ing 
  series 
  of 
  these 
  rocks 
  for 
  comparison 
  with 
  those 
  which 
  I 
  am 
  studying 
  in 
  our 
  

   own 
  country. 
  

  

  