﻿338 
  

  

  W. 
  R. 
  HUDLESTON 
  ON 
  SOME 
  

  

  slightly 
  corichoidal 
  in 
  fracture. 
  Largely 
  charged 
  with 
  small 
  crystals 
  

   of 
  pyrite, 
  chiefly 
  in 
  cakes. 
  Contains 
  crystals 
  of 
  a 
  somewhat 
  vi- 
  

   treous 
  felspar 
  in 
  moderate 
  amount, 
  together 
  with 
  some 
  augite. 
  

   Structure 
  microporphyritic. 
  

  

  In 
  thin 
  sections 
  the 
  general 
  mass 
  appears 
  of 
  a 
  brownish 
  black 
  

   colour, 
  very 
  opaque 
  and 
  granular, 
  but 
  relieved 
  by 
  numerous 
  small 
  

   prisms 
  of 
  triclinic 
  felspars, 
  more 
  than 
  in 
  any 
  of 
  the 
  other 
  specimens. 
  

   In 
  this 
  case 
  the 
  granular 
  or 
  felsitic 
  texture 
  and 
  the 
  microcrystalline 
  

   or 
  doleritic 
  texture 
  occur 
  in 
  almost 
  equal 
  amount. 
  It 
  would 
  seem 
  

   therefore 
  to 
  be 
  a 
  well-balanced 
  trachydolerite. 
  

  

  This 
  being 
  less 
  porphyritic 
  than 
  the 
  two 
  previous 
  specimens, 
  the 
  

   large 
  felspars 
  are 
  not 
  so 
  numerous, 
  and 
  are, 
  on 
  the 
  whole, 
  in 
  worse 
  

   condition. 
  The 
  inclusions 
  of 
  ground-mass 
  in 
  the 
  crystalline 
  matter 
  

   are 
  less 
  well 
  contrasted 
  ; 
  and 
  in 
  some 
  cases 
  a 
  certain 
  amount 
  of 
  

   granulation 
  has 
  supervened. 
  Still 
  the 
  crystals 
  polarize 
  fairly 
  well, 
  

   though 
  the 
  triclinic 
  character 
  is 
  not 
  so 
  strongly 
  marked, 
  and 
  there 
  

   are 
  appearances 
  which 
  would 
  lead 
  one 
  to 
  suspect 
  interlamination 
  

   or 
  mixing 
  of 
  orthoclase. 
  A 
  crystal 
  or 
  two 
  of 
  sanidine 
  may 
  be 
  noted. 
  

  

  Tho 
  augites 
  are 
  mostly 
  small, 
  and 
  not 
  very 
  numerous. 
  Besides 
  

   one 
  or 
  two 
  characteristic 
  forms, 
  frequently 
  much 
  rounded 
  at 
  the 
  

   angles, 
  there 
  is 
  one 
  large 
  triangular 
  piece 
  with 
  well-defined 
  edges 
  

   which 
  evidently 
  belongs 
  to 
  this 
  variety 
  of 
  pyroxene. 
  Pyrite 
  and, 
  

   perhaps, 
  magnetite 
  occur 
  sporadically 
  in 
  small 
  crystalline 
  forms 
  ; 
  

   and, 
  besides 
  this, 
  the 
  opacity 
  of 
  the 
  base 
  is 
  much 
  increased 
  by 
  nu- 
  

   merous 
  small 
  dots 
  of 
  ferrite, 
  which 
  may 
  include 
  both 
  the 
  previously 
  

   mentioned 
  species. 
  

  

  No. 
  5. 
  "Cartago 
  Rock." 
  Specific 
  gravity 
  2-54? 
  A 
  trachytic 
  

   rock 
  ; 
  matrix 
  close, 
  and 
  of 
  a 
  grey 
  colour, 
  rather 
  prone 
  to 
  fracture, 
  

   and 
  studded 
  with 
  felspars, 
  mostly 
  glassy 
  and 
  brittle. 
  The 
  specimen 
  

   examined 
  was 
  very 
  full 
  of 
  air, 
  owing 
  to 
  cracks 
  and 
  to 
  the 
  cavities 
  

   resulting 
  from 
  the 
  fractured 
  crystals 
  of 
  felspar. 
  

  

  In 
  thin 
  section 
  the 
  ground-mass 
  appears 
  greyish 
  : 
  it 
  is 
  highly 
  fel- 
  

   sitic 
  in 
  texture, 
  there 
  being 
  but 
  little 
  trace 
  of 
  microcrystalline 
  

   matter. 
  Hence 
  this 
  specimen 
  scarcely 
  comes 
  within 
  the 
  group 
  of 
  

   trachydolerites, 
  but 
  approaches 
  the 
  acidic 
  trachytes 
  more 
  than 
  any 
  

   of 
  these 
  lavas. 
  The 
  ground-mass 
  occupies 
  a 
  large 
  proportion 
  of 
  the 
  

   slice. 
  

  

  Most 
  of 
  the 
  felspars, 
  even 
  in 
  this 
  specimen, 
  are 
  triclinic, 
  and 
  are 
  

   probably 
  mainly 
  oligoclase, 
  but 
  with 
  some 
  which 
  I 
  take 
  to 
  be 
  sani- 
  

   dine. 
  The 
  inclusions 
  are 
  small 
  and 
  mostly 
  granulated, 
  like 
  the 
  base, 
  

   but 
  occasionally 
  in 
  a 
  state 
  of 
  glass. 
  As 
  a 
  whole 
  they 
  are 
  in 
  good 
  

   condition, 
  and 
  polarize 
  with 
  great 
  brilliancy. 
  Many 
  of 
  the 
  felspars 
  

   break 
  out 
  on 
  grinding, 
  so 
  that 
  it 
  has 
  not 
  been 
  easy 
  to 
  select 
  a 
  slice 
  

   sufficiently 
  free 
  from 
  flaws. 
  

  

  The 
  augites 
  are 
  fairly 
  numerous 
  ; 
  one 
  long 
  greenish 
  crystal 
  is 
  

   more 
  dichroic 
  than 
  the 
  rest, 
  and 
  may 
  be 
  slightly 
  altered. 
  Several 
  

   of 
  the 
  crystals, 
  as 
  usual, 
  have 
  their 
  angles 
  much 
  rounded, 
  present- 
  

   ing 
  in 
  one 
  or 
  two 
  instances 
  a 
  pyriform 
  shape. 
  The 
  majority 
  are 
  

   fresh, 
  and 
  polarize 
  well. 
  The 
  iron 
  mineral 
  is 
  mostly 
  magnetite, 
  

   often 
  in 
  well-defined 
  octahedra, 
  but 
  in 
  some 
  cases 
  having 
  a 
  tendency 
  

  

  