﻿428 
  

  

  MESSES. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  V. 
  JENNINGS 
  

  

  (fig. 
  4) 
  will 
  convey 
  some 
  idea 
  of 
  the 
  characters 
  of 
  this 
  deposit. 
  The 
  

   compact 
  grey 
  matrix 
  of 
  the 
  tuff, 
  with 
  its 
  abundant 
  felspars, 
  makes 
  

   it 
  resemble 
  to 
  the 
  eye 
  an 
  intrusive 
  felstone 
  with 
  inclusions. 
  

  

  The 
  larger 
  fragments 
  of 
  lava 
  thrown 
  into 
  these 
  tuffs 
  serve, 
  far 
  

   better 
  than 
  the 
  easily 
  altered 
  and 
  more 
  permeable 
  matrix, 
  to 
  record 
  

   the 
  nature 
  of 
  the 
  rocks 
  that 
  were 
  formed 
  at 
  the 
  centre 
  of 
  eruption. 
  

  

  Pig. 
  3. 
  — 
  Tuff 
  with 
  Felspar-crystals 
  and 
  fragments 
  of 
  Slate 
  and 
  

   Vesicular 
  Glass. 
  East 
  flank 
  of 
  Tyrau 
  Mawr. 
  (x 
  8.) 
  

  

  Fig. 
  4. 
  — 
  Joint-surface 
  of 
  block 
  of 
  Tuff, 
  showing 
  included 
  Slate- 
  

   fragments. 
  North 
  slope 
  of 
  Tyrau 
  Mawr. 
  (Size 
  7 
  feet 
  by 
  

   4 
  feet 
  6 
  inches.) 
  

  

  The 
  absence 
  of 
  lava-flows 
  in 
  this 
  locality 
  leads 
  one 
  to 
  depend 
  upon 
  

   less 
  satisfactory 
  evidence 
  ; 
  but 
  there 
  is 
  here 
  no 
  reason 
  to 
  suppose 
  

   that 
  the 
  abundant 
  scoriaceous 
  or 
  compacter 
  masses 
  in 
  the 
  tuffs 
  were 
  

   torn 
  from 
  some 
  more 
  ancient 
  and 
  deep-seated 
  deposits. 
  

  

  