﻿476 
  DE. 
  C. 
  CALLAWAY 
  OX 
  SECONDARY 
  HINEEALS 
  IX 
  THE 
  

  

  to 
  Mr. 
  J. 
  J. 
  H. 
  Teall 
  for 
  several 
  valuable 
  suggestions, 
  and 
  his 
  

   admirable 
  monograph 
  * 
  has 
  been 
  of 
  much 
  use. 
  It 
  is 
  satisfactory 
  

   to 
  note 
  that 
  Mr. 
  Teall 
  has 
  recognized 
  t 
  at 
  the 
  Lizard 
  certain 
  

   " 
  banded 
  gneisses 
  " 
  to 
  which 
  he 
  assigns 
  an 
  origin 
  similar 
  to 
  that 
  of 
  

   the 
  " 
  injection- 
  schists 
  " 
  of 
  my 
  last 
  paper 
  ; 
  but 
  I 
  understand 
  that 
  he 
  

   expresses 
  no 
  decision 
  on 
  the 
  genesis 
  of 
  the 
  granite-veins. 
  Prof. 
  

   Bonney 
  has 
  also 
  favoured 
  me 
  with 
  his 
  opinion 
  on 
  certain 
  points. 
  

  

  I. 
  Stetjcttjeal 
  and 
  Mineral 
  Changes 
  peodttced 
  at 
  Sheae-zones. 
  

   A. 
  Structure 
  of 
  a 
  Shear-zone. 
  

  

  As 
  the 
  production 
  of 
  new 
  minerals 
  is 
  frequently 
  connected 
  with 
  

   a 
  differential 
  movement 
  in 
  rock-masses, 
  it 
  is 
  necessary 
  to 
  describe 
  

   the 
  nature 
  of 
  the 
  shearing 
  and 
  the 
  distribution 
  of 
  the 
  sheared 
  rocks. 
  

   At 
  Malvern, 
  foliation 
  (which 
  in 
  this 
  district 
  is 
  ordinarily 
  the 
  result 
  

   of 
  shearing) 
  occurs 
  in 
  bands 
  of 
  irregular 
  breadth 
  situated 
  at 
  irre- 
  

   gular 
  intervals. 
  The 
  zones 
  may 
  be 
  a 
  few 
  inches 
  or 
  many 
  yards 
  

   broad, 
  and 
  the 
  intervals 
  between 
  them 
  may 
  vary 
  from 
  a 
  few 
  feet 
  to 
  

   several 
  furlongs. 
  As 
  the 
  strike 
  of 
  the 
  zones 
  is 
  usually 
  transverse 
  to 
  

   the 
  geographical 
  axis, 
  the 
  length 
  can 
  rarely 
  be 
  ascertained; 
  but 
  

   in 
  the 
  Ragged 
  Stone 
  Hill 
  a 
  shear-zone 
  which 
  coincides 
  in 
  strike 
  

   with 
  one 
  of 
  the 
  spurs 
  can 
  be 
  traced 
  for 
  about 
  half 
  a 
  mile. 
  The 
  

   gneissic 
  structure 
  usually 
  shades 
  off 
  on 
  each 
  side 
  of 
  the 
  zone 
  into 
  

   the 
  ordinary 
  igneous 
  masses, 
  and 
  within 
  the 
  zone 
  itself 
  the 
  meta- 
  

   morphism 
  varies 
  in 
  intensity, 
  bands 
  of 
  maximum 
  shearing 
  alternating 
  

   with 
  rock 
  in 
  which 
  the 
  original 
  structure 
  of 
  the 
  diorite 
  or 
  the 
  gra- 
  

   nite 
  has 
  only 
  partially 
  been 
  destroyed. 
  The 
  schistosity 
  may 
  exist 
  

   in 
  granite, 
  diorite, 
  or 
  felsite 
  ; 
  but 
  the 
  most 
  interesting 
  phenomena 
  

   occur 
  where 
  diorite 
  is 
  interlaced 
  with 
  granite 
  veins. 
  These 
  foliated 
  

   bands 
  may 
  be 
  described 
  as 
  " 
  shear-zones.'" 
  Their 
  most 
  frequent 
  

   strike 
  is 
  to 
  the 
  north-west. 
  

  

  The 
  proofs 
  of 
  mechanical 
  force 
  resulting 
  in 
  shearing 
  are 
  numerous 
  

   and 
  clear. 
  Hornblende-crystals 
  are 
  drawn 
  out 
  into 
  ribands, 
  which 
  

   are 
  sometimes 
  bent 
  round 
  crystals 
  of 
  felspar. 
  Felspars 
  are 
  occa- 
  

   sionally 
  bent, 
  but 
  more 
  frequently 
  they 
  are 
  broken 
  ; 
  sometimes 
  the 
  

   fragments 
  are 
  partially 
  rounded, 
  so 
  that 
  when 
  immersed 
  in 
  chlorite 
  

   and 
  arranged 
  in 
  layers 
  they 
  present 
  a 
  curious 
  mimicry 
  of 
  the 
  coarse 
  

   seams 
  in 
  a 
  sedimentary 
  rock. 
  The 
  effects 
  of 
  pressure 
  increase 
  in 
  

   intensity 
  towards 
  the 
  shear-zone, 
  the 
  crystals 
  of 
  hornblende 
  being 
  

   progressively 
  elongated, 
  the 
  felspars, 
  when 
  not 
  broken, 
  becoming 
  

   somewhat 
  flattened, 
  and 
  folia 
  of 
  secondary 
  quartz 
  increasing 
  in 
  length 
  

   and 
  tenuity. 
  Sometimes 
  a 
  crystal 
  of 
  hornblende 
  breaks 
  up 
  along 
  

   the 
  cleavage-planes 
  into 
  numerous 
  fragments, 
  which 
  are 
  drawn 
  

   away 
  from 
  each 
  other 
  along 
  the 
  direction 
  of 
  foliation, 
  and 
  tail 
  out 
  

   at 
  the 
  ends 
  in 
  a 
  few 
  widely 
  separated 
  particles. 
  The 
  interspaces 
  

   are 
  filled 
  with 
  other 
  minerals 
  : 
  but 
  the 
  scattered 
  parts 
  of 
  the 
  

   broken 
  crystals 
  often 
  extinguish 
  simultaneously. 
  " 
  Strain-shadows 
  " 
  

   in 
  crystals 
  are 
  very 
  frequent. 
  

  

  * 
  British 
  Petrography, 
  1888. 
  

  

  t 
  British 
  Association 
  Beport, 
  1887, 
  p. 
  707. 
  

  

  