﻿432 
  MESSES. 
  G. 
  A. 
  J. 
  COLE 
  AXE 
  A. 
  V. 
  JEXXIXGS 
  

  

  It 
  is 
  obvious 
  from 
  the 
  first 
  examination 
  in 
  the 
  field 
  that 
  these 
  

   intrusive 
  sheets, 
  which 
  one 
  may 
  comprehensively 
  class 
  as 
  diabases 
  *, 
  

   have 
  undergone 
  considerable 
  alteration. 
  Calcite 
  is 
  very 
  freely 
  

   developed 
  ; 
  while 
  quartz-veins 
  are 
  conspicuous 
  in 
  the 
  large 
  quarry 
  

   near 
  Bron-y-Gader, 
  where 
  they 
  were 
  formed 
  prior 
  to 
  the 
  infil- 
  

   tration 
  or 
  formation 
  of 
  the 
  carbonate 
  of 
  lime. 
  In 
  this 
  quarry 
  

   Air. 
  Aildn 
  observed 
  epidote, 
  which 
  he 
  describes 
  as 
  " 
  fibres 
  and 
  

   curved 
  crystals 
  of 
  pale 
  wine-yellow 
  thallite." 
  While 
  small 
  epidotes 
  

   are 
  certainly 
  abundant, 
  the 
  larger 
  acicular 
  crystals 
  collected 
  by 
  

   ourselves 
  prove 
  to 
  be 
  actinolite 
  and 
  tremolite 
  ; 
  and 
  we 
  have 
  noticed 
  

   also 
  greenish 
  asbestos 
  in 
  the 
  clefts 
  on 
  the 
  northern 
  face 
  of 
  Mynydd- 
  

   y-Gader. 
  

  

  Under 
  the 
  microscope 
  the 
  original 
  characters 
  of 
  these 
  diabases 
  

   can 
  be 
  determined 
  with 
  some 
  accuracy, 
  despite 
  the 
  wide 
  develop- 
  

   ment 
  of 
  secondary 
  products. 
  Thus 
  the 
  coarser 
  varieties, 
  like 
  the 
  

   rock 
  of 
  Bwlch-yr-hendref 
  on 
  the 
  west 
  and 
  that 
  of 
  Bron-y-Gader 
  

   on 
  the 
  east, 
  were 
  clearly 
  dolerites 
  approaching 
  gabbro, 
  with 
  

   a 
  well-developed 
  ophitic 
  structure. 
  The 
  mass 
  at 
  Bron-y-Gader 
  

   contains 
  stout 
  porphyritic 
  felspars 
  and 
  pale 
  secondary 
  horn- 
  

   blende. 
  The 
  rock 
  rising 
  like 
  a 
  weathered 
  dyke 
  on 
  the 
  south 
  

   shore 
  of 
  the 
  larger 
  Crogenen 
  Lake 
  is 
  particularly 
  rich 
  in 
  plagio- 
  

   clastic 
  felspar, 
  both 
  as 
  porphyritic 
  crystals 
  and 
  as 
  a 
  well-marked 
  

   meshwork 
  in 
  the 
  ground-mass 
  : 
  granular 
  altered 
  pyroxene 
  occurs 
  

   also 
  interstitially, 
  in 
  the 
  midst 
  of 
  what 
  probably 
  was 
  once 
  residual 
  

   glass. 
  

  

  The 
  handsome 
  rock 
  of 
  the 
  north 
  front 
  of 
  Alynydd-y-Gader 
  is, 
  

   again, 
  an 
  ophitic 
  dolerite 
  of 
  medium 
  grain. 
  The 
  pale 
  brown 
  augite 
  

   seems 
  to 
  have 
  suffered 
  little, 
  and 
  the 
  included 
  felspars 
  are 
  also 
  fairly 
  

   fresh. 
  While 
  the 
  structure 
  of 
  some 
  of 
  these 
  rocks 
  points 
  towards 
  

   a 
  basic 
  composition, 
  we 
  have 
  failed 
  to 
  detect 
  pseudomorphs 
  after 
  

   olivine, 
  and 
  believe 
  that 
  the 
  iron 
  originally 
  separated 
  out 
  was 
  even 
  

   smaller 
  in 
  amount 
  than 
  it 
  appears 
  at 
  present. 
  

  

  The 
  characteristic 
  decomposition-product 
  of 
  these 
  basic 
  rocks 
  is 
  

   an 
  epidote 
  (colourless 
  in 
  thin 
  sections) 
  which 
  occurs 
  in 
  every 
  slide 
  

   associated 
  with 
  chloritic 
  areas. 
  Its 
  abundance, 
  coupled 
  with 
  that 
  

   of 
  the 
  all-pervading 
  calcite, 
  implies 
  an 
  immense 
  withdrawal 
  of 
  lime 
  

   from 
  the 
  original 
  constituents. 
  In 
  fact, 
  alumina 
  and 
  lime, 
  rather 
  

   than 
  magnesia 
  and 
  iron, 
  must 
  have 
  been 
  the 
  prevailing 
  bases 
  during 
  

   these 
  eruptions 
  : 
  and 
  the 
  same 
  evidence 
  is 
  afforded 
  by 
  the 
  very 
  tough 
  

   and 
  compact 
  " 
  greenstones 
  " 
  either 
  north 
  or 
  south 
  of 
  the 
  old 
  Towyn 
  

   Koad, 
  which 
  are 
  merely 
  fine-grained 
  variations 
  of 
  the 
  types 
  already 
  

   cited. 
  

  

  On 
  the 
  whole 
  these 
  diabases 
  contain 
  surprisingly 
  little 
  secondary 
  

   quartz 
  : 
  but 
  in 
  one 
  instance, 
  that 
  of 
  a 
  pale 
  grey 
  rock 
  close 
  under 
  

   the 
  cliff 
  of 
  Tyrau 
  Mawr, 
  this 
  mineral 
  occurs 
  in 
  the 
  most 
  striking 
  

   manner, 
  so 
  that 
  the 
  mass 
  at 
  first 
  suggests 
  a 
  porphyritic 
  elvan. 
  The 
  

   microscope 
  relegates 
  this 
  rock 
  also 
  to 
  the 
  andesites 
  ; 
  but 
  the 
  general 
  

  

  * 
  We 
  use 
  this 
  term 
  in 
  the 
  wide 
  sense, 
  aud 
  as 
  an 
  international 
  equivalent 
  of 
  

   the 
  loosely-applied 
  English 
  " 
  greenstone.'' 
  

  

  