﻿480 
  DR. 
  C. 
  CALLAWAY 
  OX 
  SECONDARY 
  MHSTERALB 
  IN 
  THE 
  

  

  the 
  folia 
  are 
  seen 
  to 
  consist 
  mainly 
  of 
  alternations 
  of 
  quartz 
  and 
  

   felspar 
  in 
  small 
  grannies. 
  

  

  In 
  the 
  centre 
  of 
  the 
  exposed 
  part 
  of 
  the 
  zone 
  is 
  another 
  band 
  of 
  

   sheared 
  granite 
  (407). 
  This 
  is 
  as 
  much 
  like 
  a 
  true 
  gneiss 
  as 
  it 
  

   can 
  very 
  well 
  be. 
  Seams 
  of 
  quartz, 
  of 
  a 
  coarser 
  grain 
  than 
  

   any 
  of 
  the 
  preceding, 
  alternate 
  with 
  seams 
  of 
  mixed 
  felspar, 
  white 
  

   mica, 
  and 
  finely 
  granular 
  quartz. 
  The 
  felspars, 
  of 
  small 
  size, 
  un- 
  

   striated 
  and 
  a 
  little 
  dirty, 
  are 
  in 
  corroded 
  forms 
  or 
  occasionally 
  

   idiomorphic. 
  The 
  mica 
  is 
  abundant, 
  and 
  is 
  usually 
  in 
  lath-shaped 
  

   sections. 
  

  

  The 
  changes 
  here 
  described 
  are 
  those 
  commonly 
  observed 
  in 
  the 
  

   conversion 
  of 
  granite 
  into 
  gneiss. 
  Felspar 
  is 
  reconstructed 
  in 
  small 
  

   granules 
  or 
  in 
  crystals, 
  and 
  white 
  mica 
  is 
  produced 
  out 
  of 
  it. 
  Secon- 
  

   dary 
  granular 
  quartz 
  is 
  also 
  formed. 
  

  

  Diorite 
  on 
  the 
  margin 
  of 
  the 
  shear-zone 
  shows 
  the 
  following 
  cha- 
  

   racters. 
  Hornblende 
  is 
  in 
  large 
  crystals 
  of 
  various 
  shades 
  of 
  green 
  

   and 
  brown. 
  Pleochroism 
  is 
  fairly 
  strong, 
  especially 
  in 
  sections 
  

   parallel 
  to 
  the 
  base, 
  in 
  which 
  the 
  double 
  system 
  of 
  cleavages 
  is 
  very 
  

   distinct. 
  The 
  sections 
  which 
  are 
  approximately 
  parallel 
  to 
  the 
  sides 
  

   of 
  the 
  prism 
  show 
  a 
  tendency 
  in 
  the 
  mineral 
  to 
  break 
  up 
  along 
  the 
  

   cleavage-planes, 
  giving 
  the 
  well-known 
  " 
  reedy 
  " 
  structure. 
  There 
  

   is 
  also 
  some 
  crushing 
  and 
  contortion 
  of 
  the 
  crystals. 
  A 
  little 
  chlo- 
  

   rite 
  is 
  formed 
  from 
  the 
  hornblende. 
  The 
  entire 
  quantity 
  of 
  felspar 
  

   is 
  very 
  small. 
  Some 
  of 
  it 
  shows 
  the 
  twinning 
  of 
  a 
  plagioclose, 
  but 
  

   a 
  large 
  proportion 
  is 
  untwinned. 
  It 
  is 
  mainly 
  replaced 
  by 
  epidote 
  

   and 
  white 
  mica. 
  Spicules 
  of 
  haematite 
  (or 
  limonite) 
  are 
  abundant, 
  

   and 
  often 
  occur 
  in 
  the 
  white 
  mica, 
  lying 
  parallel 
  with 
  the 
  lamellae. 
  

  

  Five 
  slides 
  (400-404) 
  were 
  taken, 
  a 
  few 
  inches 
  apart, 
  commen- 
  

   cing 
  outside 
  the 
  visible 
  zone, 
  and 
  proceeding 
  to 
  about 
  the 
  middle. 
  

   The 
  following 
  changes 
  were 
  noted 
  : 
  — 
  The 
  hornblende 
  becomes 
  more 
  

   and 
  more 
  " 
  reedy 
  " 
  and 
  broken 
  up, 
  and 
  in 
  one 
  of 
  the 
  later 
  slides 
  it 
  

   occurs 
  in 
  detached 
  spicules 
  (sections 
  parallel 
  to 
  prism), 
  in 
  minute 
  

   rhombic 
  forms 
  (sections 
  parallel 
  to 
  base) 
  and 
  in 
  various 
  irregular 
  

   fragments. 
  The 
  proportion 
  of 
  hornblende 
  diminishes 
  from 
  about 
  

   one 
  half 
  to 
  one 
  tenth 
  of 
  the 
  slide. 
  The 
  felspar 
  decreases 
  in 
  quan- 
  

   tity 
  till 
  in 
  the 
  last 
  of 
  the 
  series 
  it 
  cannot 
  with 
  certainty 
  be 
  detected, 
  

   and 
  in 
  the 
  last 
  but 
  one 
  it 
  occurs 
  only 
  in 
  a 
  small 
  number 
  of 
  minute 
  

   clear 
  crystals 
  and 
  a 
  few 
  scattered 
  granules 
  which 
  appear 
  to 
  be 
  the 
  

   eroded 
  remains 
  of 
  crystals. 
  It 
  is 
  sometimes 
  striated, 
  sometimes 
  

   not. 
  Its 
  place 
  is 
  taken 
  very 
  largely 
  by 
  white 
  mica. 
  In 
  the 
  first 
  

   slides 
  the 
  mica 
  is 
  seen 
  to 
  be 
  penetrating 
  the 
  felspar 
  from 
  the 
  side 
  or 
  

   wholly 
  enclosed 
  within 
  it, 
  and 
  it 
  shows 
  a 
  progressive 
  increase 
  

   through 
  the 
  series. 
  It 
  appears 
  mostly 
  in 
  lath-shaped 
  sections, 
  whose 
  

   longer 
  axes 
  lie 
  in 
  all 
  directions 
  in 
  the 
  earlier 
  slides 
  ; 
  but 
  in 
  the 
  later 
  

   and 
  more 
  schistose 
  specimens 
  they 
  tend 
  to 
  run 
  with 
  the 
  general 
  

   foliation. 
  As 
  the 
  abundant 
  appearance 
  of 
  white 
  mica 
  in 
  diorite 
  at 
  

   shear-zones 
  is 
  a 
  point 
  of 
  some 
  interest, 
  a 
  diagnosis 
  of 
  this 
  mineral 
  

   from 
  a 
  similar 
  rock-specimen 
  will 
  be 
  given 
  further 
  on 
  (Xo. 
  3^7, 
  

   p. 
  493). 
  

  

  Epidote 
  occurs 
  in 
  considerable 
  quantity 
  in 
  all 
  the 
  slides. 
  Much 
  

  

  