﻿430 
  MESSES. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  V. 
  JENNINGS 
  

  

  which 
  was 
  probably 
  mainly 
  composed 
  of 
  comminuted 
  glassy 
  particles, 
  

   and 
  yielded 
  easily 
  to 
  metamorphic 
  agents 
  during 
  the 
  intrusion 
  of 
  

   the 
  dolerite 
  below. 
  

  

  As 
  might 
  be 
  expected, 
  this 
  fine-grained 
  ash 
  has 
  not 
  undergone 
  

   actual 
  fusion, 
  although 
  it 
  has 
  been 
  converted 
  into 
  a 
  hornstone 
  or 
  

   porcellanite, 
  in 
  which 
  the 
  original 
  nature 
  of 
  the 
  constituents 
  is 
  no 
  

   longer 
  determinable 
  even 
  under 
  the 
  higher 
  powers 
  of 
  the 
  microscope. 
  

   The 
  imbedded 
  quartz-grains 
  have 
  since 
  become 
  enlarged, 
  the 
  added 
  

   silica 
  being, 
  as 
  usual, 
  in 
  optical 
  continuity 
  with 
  that 
  of 
  the 
  original 
  

   grain, 
  and 
  in 
  some 
  cases 
  spreading 
  outward 
  in 
  short 
  processes 
  into 
  

   the 
  microgranular 
  material 
  around. 
  The 
  specific 
  gravity 
  of 
  a 
  com- 
  

   pact 
  specimen 
  of 
  this 
  altered 
  rock 
  is 
  2-64:. 
  

  

  These 
  examples 
  will 
  suffice 
  to 
  show 
  that 
  the 
  great 
  series 
  of 
  tuffs 
  

   and 
  ashes 
  contains 
  beds 
  of 
  fairly 
  acid 
  characters, 
  particularly 
  as 
  the 
  

   higher 
  members 
  are 
  approached, 
  though 
  some 
  reversions 
  occasionally 
  

   occur 
  towards 
  the 
  andesitic 
  type 
  of 
  Penrhyn-gwyn. 
  We 
  confess, 
  

   however, 
  that 
  we 
  were 
  unprepared 
  to 
  find 
  among 
  the 
  still 
  later 
  beds 
  

   of 
  the 
  Tal-y-llyn 
  valley 
  such 
  perfect 
  examples 
  of 
  devitrified 
  glassy 
  

   lavas 
  as 
  occur 
  upon 
  the 
  cliffs 
  of 
  Craig-y-Llam. 
  The 
  igneous 
  rocks 
  

   are 
  here 
  mapped 
  as 
  " 
  interbedded 
  felspathic 
  trap," 
  but 
  include 
  much 
  

   flinty 
  felsitic 
  ash, 
  the 
  fine 
  stratification 
  of 
  which 
  resembles, 
  in 
  hand- 
  

   specimens, 
  the 
  fluidal 
  structure 
  of 
  a 
  rhyolite. 
  Some 
  of 
  the 
  imbedded 
  

   fragments 
  show 
  under 
  the 
  microscope 
  a 
  perlitic 
  structure 
  of 
  the 
  most 
  

  

  Fig. 
  5. 
  — 
  Compressed 
  Tuff, 
  containing 
  fragments 
  of 
  Vesicular 
  and 
  

   Perlitic 
  Lavas, 
  and 
  showing 
  round 
  many 
  of 
  these 
  a 
  zone 
  of 
  crushed 
  

   and 
  drawn-out 
  material. 
  Craig-y-Llam. 
  ( 
  X 
  8.) 
  

  

  complete 
  and 
  delicate 
  kind, 
  the 
  cracks 
  being 
  occupied 
  by 
  a 
  green 
  

   pleochroic 
  alteration 
  -product 
  (fig. 
  5). 
  Others 
  exhibit 
  remains 
  of 
  sphe- 
  

   rulites 
  in 
  a 
  similarly 
  perlitic 
  matrix. 
  A 
  specimen 
  of 
  the 
  lavas 
  of 
  this 
  

   horizon 
  contains 
  altered 
  spherulites 
  measuring 
  half 
  an 
  inch 
  across, 
  the 
  

   fluidal 
  structure 
  passing 
  through 
  them 
  ; 
  they 
  are 
  partly 
  decomposed 
  

   and 
  hollowed, 
  with 
  infillings 
  of 
  granular 
  quartz. 
  The 
  surrounding 
  

   perlitic 
  matrix 
  has 
  been 
  much 
  deformed 
  by 
  pressure, 
  and 
  lines 
  of 
  

  

  