﻿254 
  3T1SS 
  C. 
  A. 
  EAISIX 
  OX 
  SOU 
  

  

  maximnm 
  extinction. 
  The 
  twinning 
  seems 
  continuous, 
  with 
  only 
  

   a 
  very 
  slight 
  displacement 
  along 
  the 
  middle 
  of 
  the 
  vein, 
  as 
  if 
  infil- 
  

   tration 
  along 
  this 
  line 
  had 
  caused 
  a 
  pseudomorphism 
  of 
  the 
  neigh- 
  

   bouring 
  crystalline 
  structure, 
  although 
  it 
  is 
  not 
  easy 
  to 
  understand 
  

   the 
  exact 
  continuity 
  of 
  the 
  plagioclase 
  hands 
  over 
  the 
  vein. 
  Spheru- 
  

   lites, 
  in 
  this 
  and 
  in 
  other 
  slides, 
  seem 
  as 
  if 
  great 
  part 
  of 
  them 
  had 
  

   been 
  corroded 
  away, 
  leaving 
  scattered 
  quadrants 
  or 
  sectors, 
  with 
  a 
  

   suggestion 
  of 
  the 
  concentric 
  bands 
  still 
  traceable 
  in 
  the 
  matrix 
  

   beyond. 
  Elsewhere, 
  silicification 
  has 
  given 
  rise 
  to 
  granular 
  quartz, 
  

   formed 
  along 
  an 
  outer 
  ring 
  of 
  a 
  small 
  spherulite 
  transgressing 
  its 
  

   concentric 
  zones. 
  As 
  to 
  certain 
  other 
  structures, 
  we 
  may 
  doubt 
  

   whether 
  to 
  attribute 
  them 
  to 
  an 
  alteration 
  of 
  the 
  rock-mass 
  or 
  to 
  the 
  

   infilling 
  of 
  cavities 
  : 
  although 
  indubitable 
  amygdaloids 
  of 
  quartz 
  are 
  

   present 
  in 
  many 
  slides. 
  In 
  the 
  rock, 
  where 
  radial 
  tufts 
  are 
  developed 
  

   along 
  an 
  adjacent 
  stratum, 
  brownish 
  spherulites 
  appear 
  ragged 
  and 
  

   possibly 
  silicified 
  at 
  their 
  exterior, 
  rather 
  like 
  an 
  example 
  from 
  Lea 
  

   Rock*, 
  in 
  which, 
  however, 
  the 
  spherulite 
  rays 
  show 
  less 
  definite 
  

   crystalline 
  forms. 
  The 
  specimen 
  from 
  Pen-y-chain 
  beach 
  has 
  long 
  

   lath-shaped 
  sections 
  of 
  faded 
  crystals, 
  penetrating 
  the 
  spherulites. 
  

   and 
  imbedded 
  in 
  the 
  interspaces 
  : 
  but 
  they 
  are 
  masked, 
  in 
  polarized 
  

   light, 
  by 
  a 
  clear 
  granular 
  mosaic 
  of 
  secondary 
  formation. 
  These 
  

   crystals 
  may 
  project 
  with 
  simple 
  terminations, 
  or 
  may 
  cross 
  in 
  a 
  kind 
  

   of 
  trigonal 
  network. 
  It 
  is 
  possible 
  that 
  they 
  may 
  be 
  quartz 
  crystals, 
  

   but, 
  as 
  Professor 
  Bonney 
  pointed 
  out 
  to 
  me. 
  their 
  general 
  form 
  would 
  

   be 
  unusual 
  for 
  that 
  mineral, 
  and 
  they 
  are 
  most 
  probably 
  due 
  to 
  the 
  

   replacement 
  of 
  some 
  other 
  mineral, 
  possibly 
  of 
  a 
  zeolite 
  : 
  and 
  we 
  

   may 
  note 
  how 
  a 
  dark 
  deposit 
  is 
  aggregated 
  along 
  and 
  around 
  them, 
  

   which 
  proves 
  to 
  consist 
  of 
  very 
  minute 
  fluid-cavities 
  containing 
  

   bubbles. 
  These 
  might 
  well 
  have 
  originated 
  during 
  the 
  process 
  of 
  

   the 
  pseudomorphic 
  formation, 
  by 
  the 
  consequent 
  loss 
  of 
  water 
  from 
  

   the 
  zeolite. 
  The 
  arrangement 
  of 
  the 
  crystals, 
  both 
  here 
  and 
  simi- 
  

   larly 
  within 
  lenticular 
  spaces 
  in 
  the 
  neighbouring 
  layer, 
  is 
  suggestive 
  

   of 
  cavities, 
  in 
  which 
  the 
  first-formed 
  structures 
  grew 
  freely, 
  and 
  

   were 
  afterwards 
  almost 
  obliterated 
  by 
  a 
  complete 
  milling 
  of 
  quartz. 
  

   Among 
  the 
  Allport 
  Collection 
  in 
  the 
  British 
  Museum 
  (which, 
  by 
  

   the 
  kind 
  permission 
  of 
  the 
  authorities 
  of 
  the 
  Department, 
  I 
  was 
  

   allowed 
  to 
  examine) 
  is 
  a 
  slide 
  from 
  an 
  altered 
  pitchstone 
  of 
  Lawrence 
  

   Hill, 
  containing 
  somewhat 
  similar 
  structures, 
  which 
  seem 
  less 
  dubious 
  

   representatives 
  of 
  vesicles 
  f. 
  The 
  specimen 
  from 
  the 
  Lleyn 
  is, 
  

   however, 
  so 
  much 
  modified, 
  that 
  we 
  cannot 
  be 
  certain 
  under 
  what 
  

   circumstances 
  the 
  secondary 
  structure 
  was 
  produced 
  ; 
  and 
  the 
  inter- 
  

   spaces 
  occasionally 
  enclose 
  what 
  seem 
  to 
  be 
  altered 
  felspars, 
  pierced 
  

   by 
  the 
  lath-shaped 
  crystals. 
  Thus 
  these 
  formations 
  must 
  perhaps 
  

   be 
  left 
  as 
  doubtful 
  pseud-amygdals 
  : 
  but 
  they 
  would 
  still 
  be 
  an 
  

  

  * 
  I 
  was 
  fortunately 
  able 
  to 
  \isit 
  these 
  rocks, 
  and 
  to 
  receive 
  some 
  help 
  in 
  

   comparing 
  their 
  structures 
  from 
  Professor 
  Bonney, 
  during 
  the 
  Shropshire 
  

   excursion 
  of 
  the 
  class 
  from 
  University 
  College. 
  

  

  + 
  I 
  am 
  very 
  much 
  indebted 
  to 
  Mr. 
  T. 
  Danes, 
  F.G-.S.. 
  of 
  the 
  Mineralogies! 
  

   Department 
  of 
  the 
  British 
  Museum, 
  for 
  his 
  kindness 
  in 
  allowing 
  me 
  to 
  examine 
  

   many 
  slides 
  and 
  rock 
  specimens 
  in 
  the 
  penological 
  collection, 
  which 
  materially 
  

   helped 
  in 
  illustrating 
  these 
  and 
  other 
  doubtful 
  structures. 
  

  

  