﻿482 
  

  

  DR. 
  C. 
  CALLAWAY 
  OX 
  SECONDARY 
  3IIXERALS 
  IX 
  THE 
  

  

  Shear-zone 
  at 
  the 
  south 
  end 
  of 
  Swinyard's 
  Hill 
  (Fig. 
  2). 
  — 
  

   This 
  section 
  is 
  seen 
  at 
  the 
  eastern 
  end 
  of 
  the 
  large 
  quarry 
  on 
  the 
  

   road 
  through 
  the 
  hollow 
  between 
  Swinyard's 
  Hill 
  and 
  Midsummer 
  

   Hill. 
  The 
  face 
  of 
  rock 
  represented 
  is 
  20 
  feet 
  in 
  length 
  from 
  south- 
  

   east 
  to 
  north-west. 
  In 
  the 
  lofty 
  and 
  precipitous 
  slope 
  rising 
  

   from 
  it 
  to 
  the 
  north 
  we 
  see 
  granite-veins, 
  several 
  feet 
  in 
  thickness, 
  

   ascending 
  amidst 
  black 
  diorite, 
  and 
  still 
  higher 
  up 
  the 
  ridge 
  the 
  

   granite 
  swells 
  out 
  into 
  masses, 
  one 
  of 
  which 
  furnished 
  me 
  with 
  the 
  

   gradation 
  into 
  muscovite-gneiss 
  described 
  in 
  my 
  last 
  paper 
  *. 
  The 
  

   veins 
  above 
  the 
  quarry 
  decrease 
  in 
  size 
  towards 
  our 
  section, 
  where 
  

   they 
  range 
  between 
  the 
  breadth 
  of 
  a 
  yard 
  or 
  so 
  and 
  the 
  thinness 
  of 
  

  

  Fig. 
  2. 
  — 
  Section 
  of 
  Shear-zone 
  at 
  south 
  end 
  of 
  Swinyard's 
  Hill. 
  

  

  (Scale 
  6 
  feet 
  to 
  1 
  inch.) 
  

  

  S.E. 
  

  

  ZOXE 
  OF 
  BlOTITE 
  GNEISS 
  

   With 
  numerous 
  granite-veins. 
  

  

  With 
  a 
  few 
  granite-veins. 
  

  

  a 
  wafer. 
  Their 
  number 
  is 
  countless. 
  One 
  large 
  vein 
  (not 
  repre- 
  

   sented) 
  reaches 
  to 
  the 
  top 
  of 
  the 
  low 
  cliff 
  : 
  bnt 
  the 
  others 
  terminate 
  

   upwards. 
  The 
  thick 
  veins 
  usually 
  give 
  off 
  several 
  branches 
  which 
  

   rise 
  almost 
  vertically 
  in 
  straight 
  narrow 
  tongues 
  ending 
  in 
  a 
  sharp 
  

   point, 
  so 
  as 
  to 
  resemble 
  long 
  spikes. 
  This 
  effect 
  is 
  obviously 
  pro- 
  

   duced 
  by 
  pressure. 
  Some 
  of 
  the 
  thin 
  veins 
  terminate 
  downwards 
  

   also 
  in 
  a 
  point. 
  The 
  veins 
  represented 
  in 
  the 
  fignre 
  are 
  drawn 
  

   with 
  approximate 
  accuracy 
  ; 
  but 
  most 
  of 
  the 
  remainder 
  are 
  of 
  such 
  

   extreme 
  tenuity 
  and 
  so 
  closely 
  interlaced 
  that 
  the 
  attempt 
  to 
  copy 
  

   them 
  was 
  abandoned. 
  

  

  The 
  thin 
  veins 
  of 
  granite 
  frequently 
  differ 
  from 
  the 
  masses 
  and 
  

   larger 
  veins 
  in 
  their 
  greater 
  acidity. 
  In 
  the 
  field-examination 
  of 
  a 
  

   plexus 
  of 
  veins 
  we 
  often 
  see 
  that 
  the 
  thinner 
  ones 
  are 
  partially 
  

   silicified, 
  and 
  sometimes 
  they 
  are 
  converted 
  into 
  pure 
  quartz. 
  The 
  

   nature 
  of 
  the 
  process, 
  as 
  seen 
  under 
  the 
  microscope, 
  will 
  be 
  described 
  

   further 
  on 
  (p. 
  498). 
  In 
  other 
  cases 
  the 
  enclosing 
  diorite 
  has 
  trans- 
  

   ferred 
  some 
  of 
  its 
  basic 
  matter 
  to 
  the 
  granite, 
  the 
  orthoclase 
  of 
  the 
  

   vein 
  being 
  penetrated 
  to 
  a 
  greater 
  or 
  less 
  distance 
  by 
  crystals 
  of 
  

   striated 
  felspars. 
  

  

  We 
  return 
  to 
  our 
  section. 
  The 
  right 
  (south-eastern) 
  end 
  of 
  the 
  

   low 
  vertical 
  face 
  of 
  the 
  rock 
  consists 
  of 
  the 
  " 
  medium-black 
  " 
  diorite 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  sliii. 
  p. 
  528. 
  

  

  