﻿

  FELSITES 
  IN 
  THE 
  BALA 
  GROUP 
  OE 
  NORTH 
  WALES. 
  295 
  

  

  A 
  well-preserved 
  flinty 
  pale-grey 
  felsite 
  with 
  nodular 
  inclusions 
  

   of 
  quartz, 
  in 
  places 
  dark 
  green, 
  with 
  admixture 
  of 
  a 
  chloritic 
  

   mineral, 
  may 
  be 
  seen 
  in 
  loose 
  blocks 
  by 
  the 
  road-side 
  between 
  Pen- 
  

   maenmawr 
  and 
  Conway. 
  Under 
  the 
  microscope 
  it 
  shows 
  a 
  wavy 
  

   fluidal 
  structure, 
  like 
  many 
  of 
  the 
  above, 
  with 
  a 
  few 
  small 
  felspar 
  

   crystals, 
  most 
  being 
  orthoclase. 
  The 
  inclusions 
  are 
  vesicular 
  cavities 
  

   filled 
  with 
  quartz, 
  and 
  vermicular 
  chlorite, 
  like 
  that 
  described 
  above, 
  

   With 
  a 
  few 
  tufted 
  groups 
  on 
  the 
  edge 
  of 
  the 
  cavities 
  of 
  w 
  r 
  hat 
  appears 
  

   to 
  be 
  a 
  colourless 
  mica, 
  which 
  also 
  is 
  occasionally 
  seen 
  in 
  the 
  above- 
  

   described 
  specimens. 
  I 
  do 
  not 
  know 
  the 
  locality 
  where 
  this 
  parti- 
  

   cular 
  variety 
  of 
  felsite 
  occurs 
  in 
  situ 
  ; 
  but 
  doubtless 
  it 
  is 
  one 
  of 
  those 
  

   belonging 
  to 
  the 
  Bala 
  group. 
  

  

  From 
  the 
  above 
  descriptions 
  we 
  are, 
  I 
  think, 
  justified 
  in 
  the 
  fol- 
  

   lowing 
  conclusions 
  : 
  — 
  

  

  (1) 
  That 
  these 
  Bala 
  felsites 
  are 
  a 
  group 
  of 
  ancient 
  rhyolitic 
  lavas, 
  

   exhibiting 
  vesicular, 
  slaggy, 
  fluidal, 
  and 
  other 
  characteristic 
  struc- 
  

   tures, 
  and 
  in 
  no 
  way 
  (except 
  as 
  regards 
  the 
  effects 
  of 
  their 
  great 
  age) 
  

   different 
  from 
  those 
  of 
  modern 
  date. 
  

  

  (2) 
  That 
  the 
  nodular 
  or 
  spheroidal 
  structure 
  has 
  been 
  produced 
  

   in 
  two 
  ways 
  : 
  — 
  

  

  (a) 
  By 
  simple 
  contraction 
  and 
  roughly 
  concentric 
  cracking 
  of 
  the 
  

   mass 
  in 
  cooling, 
  being 
  thus 
  intermediate 
  between 
  the 
  perlitic 
  structure 
  

   common 
  in 
  glassy 
  acid 
  lavas, 
  and 
  the 
  spheroidal 
  structure 
  common 
  

   in 
  basalt, 
  which, 
  so 
  far 
  as 
  I 
  know, 
  is 
  rather 
  rarer 
  in 
  the 
  former. 
  

  

  (6) 
  By 
  similar 
  contraction 
  in 
  cooling, 
  which 
  is 
  determined 
  by 
  

   the 
  presence 
  of 
  a 
  cavity, 
  and 
  produced 
  as 
  follows 
  : 
  — 
  "When 
  the 
  cavity 
  

   is 
  first 
  formed, 
  we 
  may 
  regard 
  the 
  whole 
  viscid 
  mass 
  in 
  the 
  neigh- 
  

   bourhood 
  as 
  in 
  a 
  state 
  of 
  equilibrium 
  between 
  the 
  various 
  forces 
  

   acting 
  on 
  the 
  cooling 
  lava 
  (contraction 
  &c.) 
  and 
  the 
  pressure 
  of 
  the 
  

   gaseous 
  contents 
  of 
  the 
  cavity. 
  As 
  cooling 
  proceeds 
  (uniformly 
  

   suppose) 
  the 
  volume 
  of 
  the 
  latter 
  diminishes 
  rapidly, 
  and 
  its 
  pres- 
  

   sure 
  against 
  the 
  walls 
  of 
  the 
  cavity 
  decreases. 
  The 
  various 
  forces 
  

   are 
  no 
  longer 
  in 
  equilibrium, 
  and 
  the 
  contractile 
  strain 
  will 
  be 
  re- 
  

   lieved 
  by 
  the 
  formation 
  of 
  a 
  crack, 
  roughly 
  concentric 
  with 
  the 
  cavity, 
  

   which, 
  as 
  we 
  might 
  expect, 
  is 
  more 
  regular 
  than 
  it 
  in 
  form 
  *. 
  

  

  glassy 
  residuum) 
  crowded 
  with 
  niicroliths, 
  many 
  of 
  elongated 
  form, 
  which, 
  from 
  

   their 
  very 
  small 
  extinction-angles 
  with 
  the 
  longer 
  edge, 
  I 
  should 
  infer 
  to 
  be 
  

   oligoclase 
  ; 
  but 
  there 
  is 
  some 
  orthoclase. 
  Some 
  longer 
  felspar 
  crystals 
  are 
  more 
  

   or 
  less 
  replaced 
  by 
  a 
  filmy 
  transparent 
  mineral, 
  giving 
  brilliant 
  colours 
  with 
  

   polarized 
  light, 
  neither 
  monoclinic 
  nor 
  triclinic. 
  There 
  is 
  a 
  grain 
  or 
  two 
  of 
  

   ilmenite 
  (fig. 
  2). 
  This 
  rock 
  passes 
  into 
  the 
  compact 
  greenish-grey 
  felsite 
  of 
  Pen- 
  

   maenbach, 
  which 
  exhibits 
  a 
  somewhat 
  similar 
  but 
  less 
  definite 
  structure, 
  and 
  is 
  

   more 
  decomposed. 
  It 
  is 
  marked 
  on 
  the 
  Survey 
  map 
  as 
  intrusive. 
  I 
  had 
  not 
  

   time 
  to 
  work 
  out 
  its 
  stratigraphical 
  relations 
  ; 
  but 
  petrologically 
  it 
  has 
  rather 
  the 
  

   characters 
  of 
  a 
  flow. 
  

  

  * 
  I 
  do 
  not 
  remember 
  to 
  have 
  noticed 
  an 
  instance 
  of 
  the 
  presence 
  of 
  cavities 
  in 
  

   the 
  spheroids 
  of 
  a 
  basic 
  rock 
  ; 
  but 
  Prof. 
  A. 
  H. 
  Green 
  has 
  mentioned 
  to 
  me 
  an 
  

   instance 
  observed 
  by 
  him 
  on 
  the 
  coast 
  of 
  Newfoundland, 
  where 
  cavities 
  partly 
  

   filled 
  with 
  calcite, 
  quartz, 
  and 
  epidote, 
  occur 
  in 
  the 
  middle 
  of 
  large 
  rude 
  sphe- 
  

   roids 
  in 
  a 
  dark 
  trap, 
  probably 
  a 
  diabase. 
  

  

  