﻿292 
  

  

  PROF. 
  To 
  G. 
  BOXtfEY 
  Otf 
  SOME 
  NODULAR 
  

  

  (excepting 
  that 
  with 
  the 
  very 
  large 
  pebble-like 
  concretions) 
  it 
  will 
  

   save 
  time 
  to 
  include 
  the 
  specimens 
  from 
  the 
  Lledr 
  valley 
  which 
  are 
  

   similar 
  in 
  character. 
  Of 
  these 
  two 
  rocks 
  I 
  have 
  had 
  a 
  series 
  of 
  

   slides 
  prepared. 
  The 
  rock 
  has 
  now 
  lost 
  the 
  schisty 
  structure, 
  or 
  at 
  

   any 
  rate 
  retains 
  no 
  more 
  than 
  gives 
  it 
  at 
  the 
  first 
  glance 
  an 
  ashy 
  

   aspect 
  *. 
  In 
  these 
  the 
  ground-mass 
  exhibits 
  more 
  or 
  less 
  clearly 
  a 
  

   fiuidal 
  structure 
  and 
  is 
  devitrified. 
  In 
  it 
  there 
  have 
  been 
  vesicles 
  : 
  

   these 
  (as 
  described 
  above) 
  are 
  more 
  or 
  less 
  filled 
  with 
  chalcedonic 
  

   quartz 
  ; 
  this 
  on 
  the 
  inner 
  side 
  of 
  the 
  layer 
  frequently 
  terminates 
  in 
  

   well-marked 
  pyramids 
  ; 
  the 
  brown 
  and 
  the 
  green 
  minerals 
  appear 
  to 
  

   have 
  been 
  deposited 
  subsequently, 
  the 
  former 
  being 
  mainly 
  an 
  impure 
  

   chalybite 
  with 
  some 
  limonite 
  t, 
  the 
  latter 
  a 
  felted 
  mass 
  of 
  a 
  chloritic 
  

   mineral, 
  which 
  towards 
  the 
  margin 
  exhibits 
  beautiful 
  vermicular 
  

   concretions. 
  Owing 
  to 
  the 
  smallness 
  of 
  the 
  individual 
  scales 
  the 
  

   results 
  of 
  optical 
  tests 
  are 
  not 
  quite 
  satisfactory 
  to 
  me 
  ; 
  but 
  I 
  think 
  

   it 
  probably 
  rhombohedral, 
  and 
  it 
  agrees 
  generally 
  with 
  the 
  mineral 
  

   which 
  Prof. 
  Hcddle 
  and 
  Prof. 
  A. 
  Geikie 
  X 
  recognize 
  as 
  dclessitc 
  §. 
  I 
  

   have 
  already 
  noticed 
  the 
  peculiar 
  ovoid 
  or 
  shuttle-like 
  form 
  assumed 
  by 
  

   these 
  masses 
  ; 
  this 
  is 
  due 
  in 
  part 
  to 
  the 
  bending 
  of 
  the 
  rude 
  cleavage- 
  

   surfaces 
  round 
  these 
  harder 
  amygdaloids 
  (as 
  round 
  included 
  frag- 
  

   ments) 
  ; 
  but 
  I 
  doubt 
  whether 
  this 
  is 
  the 
  only 
  reason 
  of 
  the 
  structure. 
  

   One 
  slide, 
  cut 
  transversely 
  to 
  the 
  longer 
  axis 
  of 
  a 
  nodule, 
  shows 
  it 
  to 
  

   be 
  bounded 
  by 
  a 
  well-defined 
  ring 
  indicated 
  by 
  a 
  dark 
  line. 
  Under 
  

   the 
  microscope 
  we 
  sec 
  that 
  the 
  form 
  of 
  the 
  inner 
  surface 
  of 
  the 
  cavity 
  

   is 
  less 
  regular. 
  It 
  is 
  almost 
  wholly 
  filled 
  with 
  chalcedonic 
  quartz. 
  

   The 
  dark 
  hue 
  cuts 
  through 
  the 
  matrix, 
  the 
  outer 
  edge 
  is 
  sharp 
  ; 
  it 
  

   is 
  defined 
  by 
  minute 
  scales 
  of 
  the 
  green 
  mineral 
  and 
  of 
  ferrite, 
  as 
  if 
  

   infiltration 
  had 
  taken 
  place 
  along 
  a 
  crack 
  inwards 
  ; 
  and 
  a 
  vein 
  of 
  

   chalcedonic 
  quartz 
  cuts 
  through 
  it 
  and 
  communicates 
  with 
  that 
  in 
  

   the 
  interior 
  of 
  the 
  vesicle 
  in 
  a 
  way 
  which 
  suggests 
  that 
  the 
  crack 
  

   defining 
  the 
  ring 
  was 
  anterior 
  to 
  the 
  filling 
  of 
  the 
  vesicle 
  and 
  to 
  the 
  

   cleavage 
  of 
  the 
  rock 
  || 
  (see 
  fig. 
  5). 
  

  

  * 
  This 
  peculiarity 
  is 
  noticed 
  by 
  the 
  Officers 
  of 
  the 
  Survey 
  (Geology 
  of 
  

   North 
  Wales, 
  p. 
  93), 
  who 
  speak 
  of 
  it 
  as 
  " 
  a 
  very 
  singular 
  rock, 
  having 
  a 
  fel- 
  

   spathic 
  base, 
  sometimes 
  hard 
  and 
  compact, 
  and 
  sometimes 
  flaky 
  ; 
  in 
  the 
  one 
  

   case 
  like 
  a 
  trap, 
  in 
  the 
  other 
  like 
  an 
  ash." 
  To 
  indicate 
  the 
  accuracy 
  of 
  this 
  

   description, 
  I 
  may 
  mention 
  that 
  until 
  I 
  saw 
  it 
  through 
  the 
  microscope, 
  I 
  thought 
  

   the 
  latter 
  variety 
  more 
  probably 
  a 
  peculiar 
  form 
  of 
  an 
  ashy 
  rock. 
  

  

  t 
  Through 
  the 
  kindness 
  of 
  Prof. 
  Williamson 
  a 
  qualitative 
  analysis 
  of 
  this 
  

   mineral 
  has 
  been 
  made 
  for 
  me 
  at 
  the 
  University 
  College 
  Laboratory 
  by 
  Mr. 
  F. 
  

   H. 
  Hatch. 
  He 
  reports: 
  — 
  " 
  Soluble 
  in 
  HC1. 
  ; 
  insoluble 
  in 
  H 
  2 
  0. 
  Metals 
  present 
  : 
  

   (1) 
  Iron 
  in 
  proportion 
  of 
  ferrous 
  to 
  ferric 
  =3 
  : 
  1, 
  (2) 
  Manganese, 
  (3) 
  Calcium, 
  (4) 
  

   Magnesium. 
  Acid 
  : 
  Carbonic 
  acid 
  only." 
  It 
  is 
  therefore, 
  as 
  I 
  anticipated 
  from 
  

   the 
  microscopic 
  examination, 
  an 
  impure 
  chalybite. 
  

  

  I 
  Heddle, 
  " 
  Chloritic 
  Minerals," 
  Trans. 
  Eoy. 
  Soc. 
  Ed. 
  vol. 
  xxix 
  p. 
  55. 
  

  

  § 
  Little 
  fragments 
  of 
  the 
  material 
  containing 
  it 
  (always, 
  no 
  doubt, 
  associated 
  

   with 
  silica) 
  are 
  almost 
  exactly 
  like 
  fragments 
  of 
  greenish 
  flint. 
  They 
  are, 
  

   however, 
  easily 
  scratched, 
  having 
  a 
  hardness 
  of 
  about 
  3, 
  and 
  are 
  slightly 
  unctuous 
  

   to 
  the 
  touch. 
  

  

  || 
  One 
  of 
  the 
  varieties 
  of 
  felsite 
  from 
  the 
  Lledr 
  valley 
  is 
  rather 
  slaggy- 
  

   looking, 
  and 
  has 
  small 
  irregular 
  cavities 
  nearly 
  or 
  quite 
  filled 
  with 
  a 
  brown 
  

   ferruginous 
  mineral, 
  showing 
  a 
  slight 
  approach 
  to 
  the 
  schisty 
  rock 
  of 
  the 
  Con- 
  

   wav 
  valley. 
  

  

  