﻿426 
  MESSRS. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  Y. 
  JENNINGS 
  

  

  In 
  places, 
  as 
  on 
  the 
  ridge 
  of 
  Mynydd-y-Gader, 
  where 
  the 
  alteration 
  

   due 
  to 
  the 
  dolerites 
  is 
  displayed 
  with 
  admirable 
  clearness, 
  the 
  ejected 
  

   blocks 
  standing 
  out 
  from 
  their 
  hornstone 
  matrix 
  sufficiently 
  indicate 
  

   the 
  original 
  character 
  of 
  the 
  rock. 
  

  

  Again 
  and 
  again 
  the 
  black 
  slates 
  assert 
  themselves, 
  the 
  muddy 
  

   sediments 
  of 
  more 
  quiet 
  waters, 
  to 
  be 
  covered 
  in 
  their 
  turn 
  by 
  the 
  

   products 
  of 
  explosive 
  action. 
  The 
  " 
  pisolitic 
  ironstone 
  " 
  of 
  the 
  

   Llyn-Aran 
  area 
  occurs 
  as 
  a 
  hard 
  black 
  band 
  among 
  the 
  higher 
  of 
  

   these 
  normal 
  sediments. 
  A 
  handsome 
  specimen 
  of 
  the 
  rock, 
  kindly 
  

   sent 
  us 
  by 
  Mr. 
  Jones 
  of 
  Gwernan 
  Yilla, 
  shows 
  numerous 
  clayey 
  

   concretions 
  an 
  inch 
  or 
  so 
  in 
  length, 
  and 
  contains 
  a 
  large 
  quantity 
  of 
  

   disseminated 
  pyrrhotite. 
  This 
  magnetic 
  pyrites 
  itself 
  forms 
  in 
  

   places 
  concentrically-built 
  ovoid 
  concretions. 
  

  

  A 
  microscopic 
  section, 
  cut 
  from 
  a 
  specimen 
  obtained 
  by 
  us 
  nearer 
  

   the 
  Cross 
  Poxes 
  Inn, 
  reveals 
  the 
  characteristic 
  " 
  pisolitic" 
  structure 
  

   in 
  a 
  very 
  pleasing 
  manner. 
  But 
  for 
  traditional 
  usage, 
  the 
  rock 
  might 
  

   be 
  preferably 
  termed 
  " 
  oolitic 
  ;" 
  for 
  the 
  grains, 
  distinctly 
  seen 
  on 
  the 
  

   surface, 
  are 
  barely 
  a 
  millimetre 
  in 
  diameter. 
  "With 
  transmitted 
  light 
  

   they 
  appear 
  green, 
  with 
  a 
  well-developed 
  concentric 
  structure. 
  Here 
  

   and 
  there 
  a 
  shell-fragment 
  or 
  other 
  foreign 
  body 
  is 
  seen, 
  round 
  

   which 
  the 
  grain 
  has 
  formed. 
  The 
  bulk 
  of 
  the 
  iron 
  is 
  now 
  represented 
  

   by 
  little 
  cubes 
  of 
  magnetite, 
  which 
  have 
  separated 
  out 
  most 
  exten- 
  

   sively 
  in 
  the 
  interstitial 
  matter, 
  but 
  which 
  also 
  mark 
  certain 
  of 
  the 
  

   concentric 
  coats 
  of 
  the 
  grains. 
  Minute 
  nests 
  of 
  a 
  fibrous 
  green 
  

   mineral 
  also 
  occur 
  throughout 
  the 
  matrix. 
  

  

  The 
  majority 
  of 
  the 
  grains 
  are 
  ellipsoidal 
  rather 
  than 
  spherical, 
  

   and 
  in 
  such 
  cases, 
  on 
  rotating 
  the 
  section 
  between 
  crossed 
  nicols, 
  

   the 
  arms 
  of 
  the 
  characteristic 
  dark 
  cross 
  are 
  seen 
  to 
  approach 
  or 
  to 
  

   recede 
  from 
  one 
  another. 
  Prom 
  this 
  we 
  conclude 
  that 
  the 
  crystalline 
  

   particles, 
  whether 
  original 
  or 
  pseudomorphic, 
  have 
  been 
  added 
  so 
  

   that 
  one 
  direction 
  of 
  extinction 
  lies 
  tangentially, 
  the 
  other 
  therefore, 
  

   in 
  the 
  vast 
  majority 
  of 
  cases, 
  not 
  pointing 
  directly 
  towards 
  the 
  

   centre 
  of 
  the 
  ellipsoid 
  * 
  In 
  this 
  respect 
  the 
  grains 
  resemble 
  those 
  

   of 
  the 
  pisolite 
  of 
  Carlsbad 
  and 
  of 
  oolitic 
  limestones 
  now 
  in 
  course 
  of 
  

   formation 
  f 
  ; 
  but 
  we 
  hesitate 
  to 
  express 
  an 
  opinion 
  as 
  to 
  the 
  chemical 
  

   constitution 
  of 
  the 
  green 
  mineral 
  of 
  which 
  they 
  are 
  now 
  composed. 
  

   The 
  use 
  of 
  the 
  quartz-wedge 
  shows 
  that 
  the 
  tangential 
  direction 
  is 
  

   that 
  of 
  minimum 
  elasticity; 
  and 
  the 
  feeble 
  pleochroism 
  gives 
  a 
  

   bluish-green 
  tint 
  when 
  this 
  direction 
  is 
  parallel 
  to 
  the 
  shorter 
  

   diagonal 
  of 
  the 
  nicol, 
  and 
  a 
  pale 
  yellow-green 
  when 
  rotated 
  through 
  

   90°. 
  If, 
  moreover, 
  a 
  polished 
  surface 
  of 
  the 
  rock 
  is 
  etched 
  with 
  

   strong 
  hydrochloric 
  acid, 
  the 
  greatest 
  solution 
  occurs 
  where 
  the 
  

   magnetite 
  is 
  developed, 
  and 
  the 
  least 
  where 
  the 
  green 
  substance 
  

   predominates. 
  After 
  boiling 
  small 
  fragments 
  of 
  the 
  rock 
  in 
  hydro- 
  

  

  * 
  Of 
  course 
  the 
  particles 
  that 
  have 
  been 
  added 
  along 
  the 
  axes 
  of 
  the 
  elliptical 
  

   section 
  have 
  one 
  direction 
  of 
  elasticity 
  pointing 
  towards 
  the 
  centre 
  ; 
  and 
  it 
  

   follows 
  that 
  when 
  the 
  axes 
  of 
  the 
  section 
  are 
  placed 
  parallel 
  to 
  the 
  diagonals 
  

   of 
  the 
  nicols 
  the 
  dark 
  cross 
  becomes 
  normal 
  and 
  rectangular. 
  

  

  t 
  See 
  Sorby, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxv. 
  (1879), 
  Proc. 
  p. 
  74. 
  

  

  