﻿ON 
  SOME 
  NODULAR 
  EELSITES 
  IN 
  THE 
  BALA 
  GEOUP, 
  N. 
  WALES. 
  289 
  

  

  30. 
  On 
  some 
  Nodttlae 
  Pelsites 
  in 
  the 
  Bala 
  Geoep 
  of 
  Noeth 
  Wales. 
  

   By 
  Prof. 
  T. 
  G. 
  Bonnet, 
  M.A., 
  P.R.S., 
  Sec. 
  G.S., 
  Fellow 
  of 
  St. 
  

   John's 
  College, 
  Cambridge. 
  (Read 
  April 
  5, 
  1882.) 
  

  

  [Plate 
  X.] 
  

  

  The 
  great 
  group 
  of 
  felsites 
  associated 
  contemporaneously 
  with 
  sedi- 
  

   mentary 
  deposits 
  of 
  Bala 
  age 
  has 
  been 
  admirably 
  described 
  by 
  Sir 
  

   A. 
  Ramsay 
  in 
  the 
  Memoirs 
  of 
  the 
  Geological 
  Survey*, 
  and 
  its 
  extent 
  

   is 
  indicated 
  on 
  their 
  maps 
  f. 
  Some 
  peculiar 
  structures 
  exhibited 
  here 
  

   and 
  there 
  in 
  these 
  ancient 
  lava-flows 
  have, 
  I 
  believe, 
  remained 
  

   hitherto 
  undescribed, 
  except 
  by 
  an 
  incidental 
  notice 
  in 
  that 
  volume 
  

   (pp. 
  93, 
  94), 
  and 
  in 
  the 
  Catalogue 
  of 
  Rock-specimens 
  in 
  the 
  Jermyn- 
  

   Street 
  Museum. 
  During 
  the 
  last 
  two 
  years 
  I 
  have 
  devoted 
  some 
  

   time 
  to 
  the 
  study 
  of 
  their 
  microscopic 
  structure, 
  the 
  results 
  of 
  which 
  

   are 
  perhaps 
  of 
  sufficient 
  interest 
  to 
  be 
  laid 
  before 
  the 
  Society. 
  

  

  The 
  lavas 
  of 
  this 
  northern 
  border 
  of 
  "Wales, 
  from 
  the 
  peak 
  of 
  

   Snowdon 
  to 
  the 
  valley 
  of 
  the 
  Conway, 
  are, 
  so 
  far 
  as 
  I 
  know 
  them, 
  

   generally 
  compact, 
  with 
  a 
  more 
  or 
  less 
  conchoidal 
  fracture, 
  exhi- 
  

   biting 
  sometimes 
  a 
  distinctly 
  fluidal 
  structure, 
  and 
  of 
  a 
  pale 
  grey 
  

   colour 
  inclining 
  to 
  green 
  J. 
  Small 
  scattered 
  felspar 
  crystals 
  are 
  not 
  

   uncommon, 
  and 
  even 
  minute 
  grains 
  of 
  quartz 
  ; 
  but, 
  as 
  a 
  rule, 
  these 
  

   lavas 
  are 
  by 
  no 
  means 
  distinctly 
  porphyritic. 
  Their 
  general 
  character, 
  

   together 
  with 
  the 
  special 
  structures 
  which 
  I 
  propose 
  to 
  describe, 
  is 
  

   well 
  exhibited 
  in 
  more 
  than 
  one 
  locality 
  in 
  the 
  neighbourhood 
  of 
  

   Bettws-y-Coed. 
  The 
  highroad 
  from 
  the 
  latter 
  place 
  to 
  Pentre 
  

   Yoelas, 
  on 
  approaching 
  the 
  deep 
  glen 
  in 
  which 
  the 
  falls 
  of 
  the 
  Conway 
  

   are 
  concealed, 
  runs 
  at 
  a 
  considerable 
  distance 
  above 
  the 
  river 
  on 
  the 
  

   right 
  bank 
  of 
  the 
  valley. 
  Close 
  to 
  a 
  little 
  public 
  house 
  called 
  the 
  

   Conway 
  Palls 
  Inn 
  we 
  find 
  the 
  following 
  section. 
  The 
  rock 
  on 
  

   either 
  side 
  of 
  the 
  house 
  is 
  a 
  compact 
  felsite. 
  This 
  continues 
  for 
  

   some 
  distance 
  as 
  we 
  ascend 
  the 
  road, 
  and 
  then 
  becomes 
  pretty 
  dis- 
  

   tinctly 
  cleaved, 
  showing 
  here 
  and 
  there 
  a 
  rather 
  flattened 
  nodule 
  

   or 
  spheroid 
  about 
  \ 
  inch 
  in 
  diameter, 
  the 
  interior 
  of 
  which 
  is 
  often 
  

   hollow. 
  To 
  this 
  succeeds 
  a 
  peculiar 
  rock, 
  which 
  at 
  the 
  first 
  glance 
  

   might 
  easily 
  be 
  taken 
  for 
  a 
  true 
  schist, 
  cropping 
  out 
  so 
  as 
  to 
  form 
  

   a 
  little 
  crag 
  or 
  reef 
  on 
  the 
  left 
  hand-side 
  of 
  the 
  road. 
  It 
  consists 
  

   of 
  wavy 
  laminae 
  of 
  a 
  whitish 
  material 
  (apparently 
  minute 
  quartz 
  and 
  

  

  * 
  The 
  Geology 
  of 
  North 
  Wales 
  (Memoirs, 
  vol. 
  iii.). 
  

  

  t 
  They 
  are 
  a 
  little 
  below 
  the 
  horizon 
  of 
  the 
  Bala 
  limestone. 
  Por 
  stratigraphical 
  

   details 
  I 
  refer 
  the 
  reader 
  to 
  the 
  excellent 
  description 
  in 
  the 
  memoir, 
  ch. 
  xiv. 
  

  

  \ 
  Several 
  of 
  the 
  Snowdonian 
  felsites 
  with 
  the 
  associated 
  ash-beds 
  are 
  carefully 
  

   described 
  by 
  the 
  late 
  Mr. 
  Ward 
  in 
  his 
  paper 
  " 
  On 
  the 
  Microscopic 
  Structure 
  of 
  

   Ancient 
  and 
  Modern 
  Volcanic 
  Rocks 
  " 
  (Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxi. 
  p. 
  401). 
  

   I 
  venture, 
  however, 
  to 
  think 
  that 
  prolonged 
  study 
  would 
  have 
  enabled 
  him 
  to 
  

   speak 
  more 
  confidently 
  as 
  to 
  the 
  distinction 
  between 
  the 
  lava-flows 
  and 
  the 
  

   liner 
  ash-beds. 
  Mr. 
  Rutley 
  also 
  has 
  recently 
  described 
  and 
  figured 
  some 
  peculiar 
  

   varieties 
  from 
  the 
  Snowdon 
  district 
  (Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxvii. 
  p. 
  403). 
  

  

  