﻿PELSTTES 
  IjST 
  THE 
  BALA 
  GROUP 
  OF 
  NORTH 
  WAEES. 
  291 
  

  

  contains 
  one 
  or 
  two 
  crystals 
  of 
  felspar, 
  one 
  showing 
  plagioclastic 
  

   twinning. 
  The 
  above 
  inicroliths 
  are 
  present 
  in 
  these 
  also, 
  together 
  

   with 
  a 
  speck 
  or 
  two 
  of 
  epidote 
  and 
  a 
  little 
  ferrite. 
  The 
  ground-mass 
  

   exhibits 
  the 
  usual 
  devitrified 
  structure, 
  which 
  also 
  gives 
  some 
  indica- 
  

   tion 
  of 
  a 
  nuidal 
  arrangement 
  *. 
  

  

  Another 
  specimen 
  taken 
  from 
  about 
  four 
  yards 
  lower 
  down 
  than 
  

   the 
  outcrop 
  of 
  the 
  first 
  nodular 
  part 
  differs 
  only 
  in 
  its 
  nuidal 
  structure 
  

   being 
  more 
  uniformly 
  banded 
  (fig. 
  6), 
  and 
  in 
  giving, 
  even 
  macroscopi- 
  

   cally, 
  indications 
  of 
  cleavage, 
  the 
  cracks, 
  under 
  the 
  microscope, 
  being- 
  

   seen 
  to 
  be 
  coated 
  with 
  the 
  green 
  mineral 
  already 
  described. 
  The 
  

   schist-like 
  rock 
  is 
  not 
  easily 
  cut 
  for 
  the 
  microscope; 
  but 
  Mr. 
  Cuttell 
  has 
  

   succeeded 
  in 
  preparing 
  for 
  me 
  a 
  fairly 
  thin 
  transverse 
  section 
  (fig. 
  3). 
  

   It 
  is 
  more 
  decomposed 
  than 
  the 
  other 
  specimen 
  ; 
  but 
  the 
  character 
  of 
  

   the 
  ground-mass 
  is 
  similar, 
  indications 
  of 
  fluidal 
  structure 
  can 
  still 
  

   be 
  seen, 
  and 
  the 
  cleavage 
  is 
  marked 
  by 
  irregular 
  wavy 
  cracks, 
  very 
  

   roughly 
  parallel 
  with 
  it. 
  These 
  are 
  coated 
  by 
  a 
  green 
  mineral, 
  

   resembling 
  that 
  already 
  described, 
  but 
  more 
  filmy. 
  It 
  is 
  very 
  difficult 
  

   to 
  isolate 
  one 
  of 
  the 
  folia 
  of 
  this 
  sufficiently 
  to 
  determine 
  its 
  optical 
  

   characters. 
  So 
  far 
  as 
  I 
  can 
  ascertain, 
  it 
  is 
  not 
  monoclinic 
  or 
  triclinic 
  ; 
  

   it 
  is 
  doubtless 
  a 
  hydrous 
  mineral, 
  probably 
  related 
  to 
  the 
  mica 
  group, 
  

   or 
  it 
  is 
  not 
  unlike 
  that 
  which 
  passes 
  under 
  the 
  name 
  of 
  sericite. 
  

   A 
  mineral 
  of 
  this 
  general 
  character 
  is 
  frequent 
  in 
  schistose 
  rocks. 
  

   The 
  slide 
  includes 
  two 
  or 
  three 
  of 
  the 
  knots 
  or 
  ' 
  eyes,' 
  which 
  give, 
  as 
  

   already 
  mentioned, 
  a 
  marked 
  character 
  to 
  the 
  rock. 
  These 
  are 
  clearly 
  

   vesicles, 
  now 
  filled 
  more 
  or 
  less 
  completely 
  with 
  crystalline 
  quartz 
  

   and 
  a 
  dusty 
  brown 
  mineral 
  (limonite, 
  with 
  perhaps 
  a 
  trace 
  of 
  manga- 
  

   nese). 
  As 
  the 
  cleavage-cracks 
  bend 
  round 
  these, 
  we 
  may, 
  I 
  think, 
  

   affirm 
  without 
  hesitation 
  that 
  we 
  have 
  here 
  a 
  vesicular 
  rhyolitic 
  lava 
  

   which, 
  after 
  the 
  cavities 
  had 
  been 
  more 
  or 
  less 
  filled 
  with 
  infiltrated 
  

   chalceclonic 
  quartz 
  andferrite, 
  assumed 
  under 
  pressure 
  a 
  rude 
  cleavage- 
  

   structure, 
  the 
  cracks 
  of 
  which 
  by 
  further 
  infiltration 
  were 
  coated 
  

   with 
  the 
  micaceous 
  mineral. 
  The 
  above-described 
  schistose 
  aspect 
  

   of 
  the 
  rock, 
  macroscopic 
  as 
  well 
  as 
  microscopic, 
  shows 
  that 
  it 
  would 
  

   not 
  be 
  prudent 
  to 
  reject 
  absolutely, 
  without 
  any 
  qualification, 
  the 
  

   view 
  which 
  regards 
  foliation 
  f 
  as 
  a 
  process 
  subsequent 
  to 
  and 
  influenced 
  

   by 
  cleavage 
  ; 
  it 
  suggests 
  also 
  that 
  in 
  some 
  cases 
  lavas 
  and 
  tuffs 
  may 
  

   have 
  been 
  the 
  rocks 
  from 
  which 
  certain 
  schists 
  have 
  been 
  produced. 
  

  

  In 
  describing 
  the 
  more 
  distinctly 
  spheroidal 
  masses 
  which 
  succeed 
  

  

  * 
  The 
  felsite 
  from 
  the 
  Conway 
  Falls, 
  which 
  in 
  places 
  exhibits 
  a 
  tolerably 
  well- 
  

   marked 
  columnar 
  structure, 
  is 
  a 
  slightly 
  greener 
  rock, 
  but 
  does 
  not 
  materially 
  

   differ 
  from 
  the 
  above, 
  except 
  that 
  the 
  chloritic 
  mineral 
  is 
  a 
  little 
  more 
  abundant, 
  

   and 
  the 
  rock 
  apparently 
  a 
  little 
  more 
  decomposed. 
  The 
  general 
  character 
  of 
  

   these 
  rocks 
  is 
  so 
  well 
  rendered 
  in 
  fig. 
  2, 
  plate 
  xxi. 
  vol. 
  xxxvii. 
  of 
  the 
  Quarterly 
  

   Journal, 
  illustrating 
  Mr. 
  F. 
  Ridley's 
  excellent 
  paper 
  on 
  some 
  rocks 
  of 
  about 
  the 
  

   same 
  age 
  nearer 
  Snowdon, 
  that 
  I 
  have 
  thought 
  it 
  needless 
  to 
  give 
  illustrations. 
  

   The 
  penologist 
  will 
  readily 
  understand 
  the 
  merely 
  varietal 
  differences 
  from 
  this 
  

   as 
  a 
  type. 
  

  

  t 
  See 
  also 
  Dr. 
  Sorby's 
  Presidential 
  address, 
  Q. 
  J. 
  G. 
  S. 
  vol. 
  xxxvi. 
  Some 
  cases, 
  

   however, 
  of 
  " 
  cleavage 
  foliation 
  " 
  I 
  am 
  myself 
  disposed 
  to 
  regard 
  as 
  the 
  result 
  of 
  

   pressure 
  during 
  the 
  process 
  of 
  mineral 
  change, 
  i. 
  c. 
  as 
  rather 
  subsequent 
  to 
  it 
  

   than 
  anterior, 
  inasmuch 
  as 
  the 
  structure 
  is 
  produced 
  when 
  the 
  crystals 
  which 
  occur 
  

   in 
  the 
  foliated 
  rock 
  are 
  being 
  formed, 
  and 
  so 
  in 
  the 
  last 
  stage 
  of 
  the 
  development, 
  

  

  