﻿Vol. 
  53.] 
  

  

  SOME 
  OF 
  THE 
  GNEISSES 
  OF 
  ANGLESEY. 
  

  

  355 
  

  

  Diorite-blocks 
  in 
  the 
  Grey 
  Gneiss. 
  

  

  The 
  eruptive 
  origin 
  of 
  the 
  gneiss 
  is 
  also 
  shown 
  by 
  another 
  kind 
  

   of 
  evidence. 
  In 
  the 
  Llangaffo 
  section 
  a 
  number 
  of 
  irregular 
  blocks 
  

   are 
  enclosed 
  in 
  the 
  grey 
  gneiss 
  (fig. 
  2). 
  They 
  consist 
  largely 
  of 
  

  

  Fig. 
  2. 
  — 
  Blocks 
  of 
  altered 
  and 
  squeezed 
  diorite 
  in 
  grey 
  gneiss. 
  

  

  [At 
  a 
  and 
  b 
  portions 
  of 
  the 
  contorted 
  gneiss 
  are 
  represented 
  

  

  chlorite 
  and 
  other 
  decomposition-products, 
  but 
  there 
  is 
  no 
  doubt 
  

   that 
  they 
  were 
  originally 
  diorite. 
  The 
  shape 
  of 
  the 
  masses 
  indi- 
  

   cates 
  that 
  they 
  have 
  been 
  compressed 
  by 
  a 
  force 
  acting 
  vertically. 
  

   The 
  direction 
  of 
  the 
  foliation 
  in 
  the 
  grey 
  gneiss 
  is 
  determined 
  by 
  

   the 
  dark 
  blocks. 
  It 
  bends 
  in 
  between 
  them, 
  and 
  curves 
  outward 
  

   where 
  they 
  project, 
  just 
  as 
  if 
  a 
  high-dipping 
  foliation 
  had 
  been 
  

   contorted 
  by 
  the 
  vertical 
  pressure. 
  

  

  (2) 
  Gneiss 
  of 
  Secondary 
  Injection. 
  

  

  Excellent 
  examples 
  of 
  this 
  kind 
  of 
  gneiss 
  are 
  seen 
  in 
  the 
  Llangaffo 
  

   cutting. 
  Near 
  the 
  eastern 
  end 
  is 
  exposed 
  a 
  mass 
  of 
  dark 
  rock 
  

   about 
  25 
  yards 
  wide, 
  with 
  grey 
  gneiss 
  on 
  each 
  side 
  of 
  it 
  (fig. 
  3, 
  p. 
  356). 
  

   In 
  the 
  centre 
  the 
  dark 
  mass 
  is 
  normal 
  diorite, 
  but 
  towards 
  both 
  

   margins 
  it 
  graduates 
  into 
  a 
  chloritic 
  micaceous 
  gneiss. 
  The 
  eastern 
  

   margin 
  is 
  nearly 
  straight, 
  hading 
  at 
  a 
  high 
  angle 
  to 
  the 
  west, 
  and 
  

   the 
  grey 
  gneiss 
  in 
  contact 
  has 
  the 
  foliation 
  coinciding 
  in 
  direction 
  

  

  2b2 
  

  

  