﻿496 
  DK. 
  C. 
  CALLAWAY 
  ON 
  SECONDAKY 
  MINEBALS 
  IN 
  THE 
  

  

  mineral 
  rearrangement 
  in 
  the 
  granite, 
  as 
  well 
  as 
  in 
  the 
  diorite, 
  

   arising 
  out 
  of 
  the 
  mutual 
  interaction 
  of 
  the 
  ingredients 
  of 
  the 
  two 
  

   kinds 
  of 
  rock. 
  

  

  The 
  effects 
  here 
  described 
  are 
  very 
  similar 
  to 
  those 
  seen 
  at 
  

   Malvern. 
  They 
  are 
  briefly 
  the 
  production 
  in 
  the 
  diorite 
  of 
  (1) 
  

   allotriomorphic 
  hornblende, 
  (2) 
  chlorite 
  and 
  epidote, 
  (3) 
  black 
  mica, 
  

   especially 
  when 
  the 
  rock 
  has 
  been 
  compressed, 
  (4) 
  linear 
  aggregation 
  

   in 
  the 
  hornblende 
  and 
  mica. 
  

  

  5. 
  Mineral 
  Aggregation 
  and 
  Enlargement. 
  

  

  Aggregation 
  as 
  the 
  result 
  of 
  contact-action 
  was 
  noticed 
  by 
  Mr. 
  S. 
  

   Allport* 
  in 
  1876. 
  The 
  slate 
  near 
  the 
  Land's-End 
  granite 
  is 
  altered 
  

   to 
  mica-schist, 
  and 
  in 
  places 
  the 
  quartz 
  and 
  the 
  mica 
  are 
  aggregated 
  

   into 
  spheroidal 
  and 
  elliptical 
  nodules. 
  Prof. 
  Bonney 
  f 
  states 
  that 
  

   in 
  Brittany 
  the 
  granitic 
  intrusions 
  produce 
  an 
  enlargement 
  of 
  

   minerals 
  in 
  detrital 
  rocks. 
  Dr. 
  Sorby 
  and 
  others 
  have 
  made 
  similar 
  

   observations. 
  Both 
  aggregation 
  and 
  enlargement 
  are 
  not 
  unfrequent 
  

   at 
  the 
  junction 
  of 
  granite-veins 
  in 
  the 
  Malvern 
  district. 
  In 
  a 
  speci- 
  

   men 
  (322, 
  PI. 
  XVI. 
  fig. 
  4) 
  from 
  Swinyard's 
  Hill, 
  in 
  which 
  a 
  narrow 
  

   vein 
  of 
  granite 
  is 
  enclosed 
  in 
  foliated 
  black 
  diorite, 
  the 
  hornblende 
  

   is 
  massed 
  together 
  at 
  the 
  junction 
  in 
  crystals, 
  which 
  are 
  enormous 
  

   compared 
  with 
  the 
  average 
  of 
  size 
  in 
  this 
  variety 
  of 
  diorite. 
  One 
  of 
  

   them 
  shows 
  a 
  prismatic 
  section 
  6 
  millim. 
  in 
  length 
  (the 
  average 
  

   length 
  not 
  exceeding 
  1 
  millim), 
  and 
  within 
  it 
  is 
  enclosed 
  a 
  crystal 
  

   of 
  plagioclase. 
  Similar 
  effects 
  may 
  be 
  seen 
  in 
  a 
  biotite-gneiss 
  north 
  

   of 
  the 
  Wych. 
  Even 
  in 
  hand-specimens, 
  small 
  granite-veins, 
  4 
  or 
  5 
  

   millim. 
  thick, 
  are 
  surrounded 
  by 
  an 
  aureole 
  of 
  black 
  mica 
  in 
  large 
  

   plates. 
  Under 
  the 
  microscope 
  (396) 
  the 
  large 
  crystallization 
  of 
  

   the 
  mica 
  (PI. 
  XYI. 
  fig. 
  5) 
  is 
  very 
  well 
  marked. 
  

  

  We 
  have 
  seen 
  that 
  aggregation 
  is 
  a 
  common 
  effect 
  in 
  gneiss- 
  

   formation, 
  the 
  mica 
  and 
  the 
  felspar 
  of 
  the 
  kersantite 
  respectively 
  

   separating 
  from 
  each 
  other 
  and 
  forming 
  alternating 
  bands, 
  and 
  the 
  

   newly 
  formed 
  quartz 
  coalescing 
  into 
  clusters 
  of 
  granules. 
  How 
  far 
  

   the 
  gneissic 
  structure 
  results 
  from 
  contact-action 
  and 
  how 
  far 
  from 
  

   pressure 
  will 
  not 
  be 
  easy 
  to 
  determine. 
  

  

  6. 
  Introduction 
  of 
  new 
  Minerals 
  by 
  Infiltration. 
  

  

  The 
  infiltration 
  of 
  the 
  mineral 
  which 
  is 
  now 
  biotite 
  is 
  suggested 
  

   by 
  some 
  of 
  the 
  preceding 
  specimens 
  (example, 
  No. 
  277), 
  but 
  it 
  is 
  

   clearly 
  shown 
  in 
  the 
  following 
  slides 
  from 
  a 
  shear-zone 
  situated 
  a 
  

   hundred 
  yards 
  or 
  so 
  to 
  the 
  south 
  of 
  the 
  Wych. 
  The 
  first 
  four 
  are 
  

   taken 
  in 
  the 
  order 
  of 
  their 
  occurrence, 
  the 
  extremes 
  of 
  the 
  series 
  

   being 
  within 
  one 
  foot 
  of 
  each 
  other. 
  

  

  No. 
  349. 
  Medium 
  black 
  diorite 
  with 
  rough 
  parallelism. 
  Horn- 
  

   blende 
  drawn 
  out 
  at 
  shear-planes 
  and 
  somewhat 
  corroded. 
  De- 
  

   composition 
  has 
  commenced, 
  some 
  chlorite, 
  epidote, 
  and 
  opacite 
  

  

  * 
  Quart. 
  Jo 
  urn. 
  Geol. 
  Soc. 
  vol. 
  xxxii. 
  p. 
  409. 
  

   t 
  Ibid. 
  vol. 
  xliv. 
  p. 
  15. 
  

  

  