﻿NODTJLAK 
  FELSTONES 
  OP 
  THE 
  1LEYN. 
  253 
  

  

  model 
  of 
  the 
  devitrified 
  sheet 
  of 
  glass 
  described 
  by 
  Professor 
  

   Bonney 
  *, 
  and 
  showing 
  similarly 
  a 
  shadowy 
  transverse 
  banding. 
  A 
  

   felspar 
  in 
  another 
  slide 
  includes 
  a 
  roughly 
  spherulitic 
  arrangement 
  

   of 
  micrographic 
  growth, 
  somewhat 
  similar 
  to 
  the 
  aggregation 
  of 
  

   fucoid-like 
  pyroxene, 
  illustrated 
  by 
  Mr. 
  Allport 
  f 
  ; 
  but 
  here 
  at 
  the 
  

   centre 
  of 
  the 
  radial 
  group 
  is 
  a 
  rounded 
  quartz-grain. 
  

  

  The 
  micrographic 
  and 
  spherulitic 
  growth 
  may 
  thus 
  develop 
  within 
  

   felspar 
  crystals, 
  or 
  subsequently 
  to 
  the 
  formation 
  of 
  perlitic 
  lines. 
  

   There 
  may 
  be, 
  similarly, 
  a 
  somewhat 
  late 
  origin 
  £ 
  for 
  the 
  larger 
  

   spherulites 
  of 
  nodular 
  character. 
  In 
  them 
  the 
  radial 
  fibres 
  are 
  

   linear 
  and 
  close, 
  with 
  a 
  kind 
  of 
  pectinated 
  arrangement, 
  and 
  traverse 
  

   usually 
  a 
  mass 
  of 
  large 
  secondary 
  granules. 
  These 
  sometimes 
  are 
  

   elongated 
  radially, 
  as 
  if 
  they 
  may 
  have 
  been 
  constrained 
  in 
  their 
  

   formation 
  by 
  an 
  earlier 
  fibrous 
  structure, 
  which, 
  however, 
  is 
  not 
  

   obliterated. 
  

  

  The 
  micrographic 
  structures 
  seem 
  not 
  without 
  relation 
  to 
  a 
  

   silicification 
  of 
  the 
  rocks, 
  as 
  has 
  been 
  suggested 
  in 
  connexion 
  with 
  

   some 
  of 
  the 
  altered 
  felspars. 
  Other 
  illustrations 
  of 
  such 
  change 
  

   occur 
  in 
  a 
  rock 
  from 
  Careg-y-def 
  aid, 
  on 
  an 
  exposed 
  surface 
  of 
  which 
  

   are 
  small 
  superficial 
  bosses 
  ; 
  these 
  Professor 
  Bonney 
  suggests 
  to 
  me 
  

   are 
  possibly 
  a 
  form 
  of 
  beekite. 
  The 
  microscope 
  slide 
  exhibits 
  

   granular 
  quartz, 
  replacing 
  porphyritie 
  crystals, 
  deposited 
  along 
  the 
  

   even 
  lines 
  of 
  flow, 
  and 
  within 
  transverse 
  veins, 
  which 
  interrupt 
  

   without 
  displacing 
  the 
  lamination, 
  as 
  if 
  the 
  rock 
  had 
  undergone 
  

   a 
  quiet 
  continuous 
  permeation 
  by 
  silica. 
  One 
  vein, 
  about 
  *04 
  mm. 
  

   broad, 
  crosses 
  a 
  small 
  felspar 
  (fig. 
  4). 
  With 
  polarized 
  light 
  the 
  

  

  Pig. 
  4. 
  — 
  Felspar 
  Crystal 
  traversed 
  by 
  a 
  vein 
  through 
  which 
  the 
  plagio- 
  

   clase 
  bands 
  are 
  continued. 
  Silicified 
  Felsite 
  west 
  of 
  Careg-y- 
  

   def 
  aid. 
  (Enlarged 
  110 
  diameters.) 
  

  

  plagioclase 
  structure 
  is 
  traceable 
  in 
  the 
  crystal 
  and 
  across 
  the 
  vein, 
  

   which, 
  however, 
  is 
  still 
  distinguishable, 
  except 
  at 
  the 
  positions 
  of 
  

  

  * 
  Pres. 
  Addr. 
  Geol. 
  Soc. 
  1885, 
  p. 
  66. 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxx. 
  pi. 
  xxxiv. 
  fig. 
  28. 
  

  

  \ 
  See 
  Prof. 
  Bonney, 
  Geol. 
  Mag. 
  dec. 
  ii. 
  vol. 
  iv. 
  1877, 
  p. 
  509. 
  

  

  