﻿630 
  MR. 
  F. 
  EUTLEY 
  ON 
  TACHYLYTE 
  EROH 
  

  

  from 
  this 
  specimen 
  the 
  chief 
  constituents 
  are 
  decomposed 
  felspar 
  

   (labradorite 
  ?) 
  and 
  olivine 
  in 
  irregularly 
  shaped 
  grains, 
  sometimes 
  

   containing 
  fluid 
  lacunae 
  with 
  bubbles. 
  These 
  enclosures 
  often 
  

   take 
  the 
  form 
  of 
  negative 
  crystals, 
  whose 
  outlines 
  are 
  rhombs 
  

   or 
  rhomboids, 
  although 
  they 
  occasionally 
  appear 
  in 
  approximately 
  

   spherical 
  or 
  ellipsoidal 
  forms. 
  Limonite 
  and 
  pyrites 
  are 
  also 
  present, 
  

   and 
  there 
  is 
  a 
  small 
  amount 
  of 
  interstitial 
  glass. 
  

  

  A 
  section 
  taken 
  from 
  a 
  still 
  finer-grained 
  chip 
  from 
  one 
  of 
  these 
  

   veins, 
  also 
  of 
  a 
  pale 
  grey 
  or 
  greyish-white 
  tint, 
  shows 
  a 
  confused 
  

   felted 
  mass 
  of 
  very 
  small 
  crystals, 
  which, 
  here 
  and 
  there, 
  have 
  a 
  

   tendency 
  to 
  form 
  little 
  divergent 
  or 
  radiating 
  groups. 
  These, 
  for 
  

   the 
  most 
  part, 
  seem 
  to 
  be 
  minute 
  decomposed 
  felspars, 
  while 
  

   occasionally 
  somewhat 
  larger 
  cr3 
  T 
  stals 
  of 
  felspar 
  may 
  be 
  seen, 
  but 
  

   they 
  do 
  not 
  occur 
  in 
  sufficient 
  number 
  to 
  give 
  a 
  micro- 
  porphyritic 
  

   character 
  to 
  the 
  rock. 
  In 
  transmitted 
  light 
  the 
  section 
  shows 
  a 
  

   profusion 
  of 
  minute 
  opaque 
  specks, 
  which, 
  by 
  surface-illumination, 
  

   appear 
  white, 
  and 
  are 
  probably 
  kaolin. 
  Limonite 
  and 
  a 
  few 
  specks 
  

   of 
  pyrites 
  are 
  also 
  present, 
  together 
  with 
  a 
  little 
  calcite. 
  

  

  In 
  the 
  thinner 
  parts 
  of 
  the 
  section 
  a 
  certain 
  amount 
  of 
  interstitial 
  

   isotropic 
  matter 
  may 
  be 
  detected, 
  and 
  this 
  is 
  probably 
  the 
  original 
  

   glassy 
  basis. 
  Its 
  presence, 
  however, 
  cannot 
  be 
  satisfactorily 
  made 
  

   out 
  in 
  the 
  thicker 
  portions 
  of 
  the 
  section, 
  although 
  it 
  doubtless 
  

   exists 
  there, 
  owing 
  to 
  the 
  anisotropic 
  bodies 
  with 
  which 
  it 
  is 
  

   associated. 
  There 
  is, 
  in 
  fact, 
  no 
  evidence 
  to 
  2^rove 
  that 
  the 
  vein 
  

   solidified 
  wholly 
  as 
  a 
  glass, 
  and 
  I 
  am 
  inclined 
  to 
  think 
  that 
  it 
  

   merely 
  had 
  a 
  glassy 
  basis 
  or 
  residuum, 
  although 
  the 
  marginal 
  

   portions 
  were 
  once 
  quite 
  vitreous. 
  

  

  This 
  is 
  the 
  only 
  point 
  in 
  which 
  I 
  differ 
  from 
  the 
  opinions 
  

   expressed 
  by 
  Messrs. 
  Young 
  and 
  Corse 
  Glen 
  in 
  the 
  admirable 
  paper 
  

   already 
  cited, 
  and 
  to 
  which 
  I 
  am 
  so 
  largely 
  indebted 
  for 
  infor- 
  

   mation. 
  

  

  The 
  devitrification 
  both 
  of 
  the 
  colourless 
  and 
  the 
  coloured 
  glassy 
  

   bands 
  forming 
  the 
  margins 
  of 
  these 
  veins 
  has 
  been 
  caused 
  in 
  great 
  

   part 
  by 
  the 
  development 
  of 
  globulites 
  *, 
  accompanied 
  by 
  a 
  separation 
  

   of 
  what 
  has 
  apparently 
  been 
  titaniferous 
  magnetite 
  or 
  ilmenite, 
  now 
  

   altered 
  into 
  leucoxene 
  ; 
  but 
  in 
  the 
  band 
  (c) 
  next 
  to 
  the 
  shale 
  the 
  

   globulites 
  are 
  much 
  more 
  densely 
  massed, 
  and 
  appear 
  to 
  be 
  of 
  a 
  

   deeper 
  colour, 
  an 
  appearance 
  which 
  is 
  perhaps 
  due 
  in 
  some 
  degree 
  

   to 
  the 
  fact 
  that 
  they 
  are 
  so 
  massed, 
  perfect 
  opacity 
  resulting 
  in 
  

   many 
  places 
  from 
  this 
  cause. 
  It 
  may 
  be 
  that 
  some 
  of 
  the 
  larger 
  

   brownish 
  granules 
  are 
  spessartine, 
  as 
  they 
  closely 
  resemble 
  those 
  

   seen 
  in 
  sections 
  of 
  the 
  hone-stone 
  or 
  coticule 
  from 
  Viel 
  Salm 
  

   in 
  Belgium, 
  described- 
  by 
  Prof. 
  Kenardt, 
  and 
  the 
  occurrence 
  of 
  

   0*14 
  per 
  cent, 
  of 
  manganese 
  protoxide 
  in 
  Mr. 
  Holland's 
  analysis 
  

   of 
  the 
  rock 
  tends 
  to 
  support 
  this 
  assumption. 
  The 
  main 
  point 
  of 
  

  

  * 
  This 
  devitrification 
  seems 
  to 
  accord 
  to 
  some 
  extent 
  with 
  that 
  met 
  with 
  in 
  a 
  

   tachylyte 
  described 
  by 
  Dr. 
  P. 
  IS". 
  Wenjukoff, 
  " 
  Spharolith 
  Tachylyt 
  von 
  Sichota 
  

   Alin 
  im 
  Ussurgebiet," 
  Bull. 
  Soc. 
  Beige 
  de 
  Geol. 
  t. 
  i. 
  p. 
  367 
  (1887). 
  

  

  t 
  " 
  Memoire 
  sur 
  la 
  structure 
  et 
  la 
  composition 
  mineralogique 
  du 
  Coticule," 
  

   Mem. 
  Acad. 
  E. 
  Sci. 
  Belg. 
  t. 
  xli. 
  Bruxelles, 
  1877. 
  

  

  