﻿296 
  PEG!. 
  T. 
  G. 
  BONNET 
  ON 
  SOME 
  NODULAR 
  

  

  (3) 
  That 
  the 
  cavities 
  are 
  then 
  filled, 
  wholly 
  or 
  partially, 
  by 
  in- 
  

   filtrated 
  minerals 
  in 
  the 
  usual 
  way. 
  

  

  (4) 
  That 
  the 
  nodules 
  thus 
  rendered 
  more 
  solid 
  (and 
  in 
  other 
  cases 
  

   from 
  the 
  effect 
  of 
  their 
  form, 
  aided 
  perhaps 
  by 
  extremely 
  minute 
  

   differences 
  in 
  texture 
  due 
  to 
  the 
  disturbance 
  of 
  equilibrium 
  in 
  

   cooling) 
  produce 
  the 
  usual 
  distortion 
  of 
  the 
  cleavage-planes 
  when 
  the 
  

   whole 
  mass 
  is 
  compressed. 
  

  

  (5) 
  That, 
  in 
  some 
  cases, 
  further 
  infiltration 
  takes 
  place 
  along 
  the 
  

   the 
  cleavage-planes, 
  giving 
  rise 
  to 
  the 
  schisty 
  aspect. 
  

  

  At 
  what 
  period 
  the 
  devitrified 
  structure 
  was 
  produced 
  it 
  is 
  difficult 
  

   to 
  say. 
  I 
  should 
  not 
  wish 
  to 
  preclude 
  myself 
  from 
  supposing 
  that 
  in 
  

   some 
  cases 
  it 
  may 
  have 
  been 
  set 
  up 
  during 
  the 
  cooling 
  of 
  the 
  

   lava. 
  It 
  may 
  be 
  well 
  in 
  conclusion 
  to 
  call 
  attention 
  to 
  the 
  occurrence 
  

   of 
  two 
  great 
  outbursts 
  of 
  rhyolitic 
  lava 
  in 
  approximately 
  the 
  same 
  

   area 
  of 
  Xorth 
  Wales, 
  separated 
  by 
  so 
  vast 
  an 
  interval 
  of 
  time 
  as 
  

   that 
  which 
  divides 
  the 
  Bala 
  from 
  the 
  later 
  Pre-Cambrian 
  period. 
  

  

  EXPLANATION 
  OF 
  PLATE 
  X. 
  

  

  Fig. 
  1. 
  Felsite 
  from 
  close 
  to 
  the 
  Conway 
  Falls 
  Inn, 
  illustrating 
  the 
  normal 
  

  

  condition 
  of 
  the 
  rock 
  (p. 
  290), 
  x20. 
  

   2. 
  The 
  dark-coloured 
  felsite 
  from 
  the 
  western 
  end 
  of 
  Conway 
  Mountain 
  

  

  (p. 
  295, 
  note), 
  x20. 
  

   o. 
  The 
  schistose 
  felsite 
  from 
  the 
  road 
  -side 
  above 
  the 
  Conway 
  Falls 
  Inn 
  

  

  (p. 
  291), 
  X20. 
  

   4. 
  Slide 
  cut 
  from 
  the 
  interior 
  of 
  one 
  of 
  the 
  large 
  pebble-like 
  nodules, 
  

  

  nearly 
  3 
  inches 
  in 
  diameter, 
  in 
  the 
  felsite 
  rather 
  beyond 
  the 
  schistose 
  

  

  rock 
  of 
  last 
  figure 
  (p. 
  29o), 
  X 
  50. 
  

   ."). 
  Part 
  of 
  a 
  nodule, 
  showing 
  portion 
  of 
  a 
  cavity, 
  a 
  boundary 
  crack 
  defined 
  

  

  by 
  a 
  dull 
  green 
  mineral 
  and 
  the 
  subsequent 
  chalcedonic 
  infiltration, 
  

  

  a 
  few 
  yards 
  below 
  the 
  schistose 
  felsite 
  above 
  Conway 
  Falls 
  Inn 
  

  

  (p. 
  292), 
  X20. 
  

   6. 
  Felsite, 
  about 
  four 
  yards 
  lower 
  down 
  the 
  road 
  than 
  the 
  outcrop 
  of 
  the 
  

  

  first 
  nodular 
  felsite 
  (fig. 
  5), 
  from 
  the 
  road-side 
  above 
  the 
  Conway 
  

  

  Falls 
  Inn 
  (p. 
  291), 
  X20. 
  

  

  Discussion. 
  

  

  Prof. 
  Blake 
  asked 
  why, 
  if 
  the 
  cracks 
  were 
  formed 
  in 
  the 
  felsites 
  

   in 
  the 
  way 
  described, 
  similar 
  cracks 
  are 
  not 
  found 
  in 
  other 
  amyg- 
  

   daloidal 
  rocks. 
  

  

  Eev. 
  E. 
  Hill 
  asked 
  if 
  there 
  were 
  any 
  criteria 
  by 
  which 
  these 
  

   altered 
  felsites 
  could 
  be 
  distinguished 
  from 
  true 
  schists 
  and 
  gneisses. 
  

  

  The 
  Author 
  stated 
  in 
  reply 
  to 
  Mr. 
  Blake 
  that 
  the 
  conditions 
  of 
  

   rock-formation 
  were 
  so 
  various 
  that 
  it 
  was 
  impossible 
  to 
  explain 
  

  

  