﻿ME. 
  F. 
  EUTLEY 
  ON 
  FTJLGUEITES 
  FEOM 
  MONTE 
  VISO. 
  61 
  

  

  prisms, 
  seldom, 
  if 
  ever, 
  showing 
  any 
  distinct 
  terminal 
  faces 
  ; 
  while 
  

   much 
  of 
  it 
  seems 
  to 
  consist 
  of 
  minute 
  grains 
  and 
  extremely 
  small 
  

   prisms, 
  mixed 
  with 
  a 
  considerable 
  amount 
  of 
  epidote. 
  The 
  angle 
  

   of 
  the 
  prism 
  in 
  one 
  or 
  two 
  sections 
  of 
  glaucophane, 
  taken 
  transversely 
  

   to 
  the 
  vertical 
  axis, 
  was 
  similar 
  to 
  that 
  of 
  hornblende, 
  124° 
  to 
  125°. 
  

   In 
  orthopinacoidal 
  sections, 
  apparently 
  those 
  chiefly 
  seen 
  in 
  the 
  

   slides 
  which 
  have 
  been 
  prepared, 
  the 
  pleochroism 
  is 
  well 
  marked, 
  vi- 
  

   brations 
  parallel 
  to 
  e=pale 
  yellowish-green 
  to 
  greenish-blue, 
  while 
  

   those 
  parallel 
  to 
  h= 
  lavender 
  -blue, 
  the 
  absorption 
  being 
  c>6, 
  as 
  in 
  

   the 
  glaucophane 
  of 
  Ctakisan 
  in 
  Japan 
  described 
  by 
  Prof. 
  B. 
  Koto 
  *. 
  

  

  The 
  extinction-angle 
  in 
  a 
  clinopinacoidal 
  section, 
  measured 
  with 
  a 
  

   Bertrand's 
  stauroscope-ocular, 
  is 
  about 
  5° 
  only. 
  The 
  prisms 
  are 
  very 
  

   -commonly 
  traversed 
  by 
  transverse 
  fissures, 
  as 
  in 
  actinolite. 
  In 
  one 
  or 
  

   two 
  of 
  the 
  slides 
  a 
  little 
  diallage 
  occurs, 
  sometimes 
  enclosing 
  numerous 
  

   crystals 
  of 
  glaucophane, 
  so 
  that 
  the 
  extinction-angle 
  cannot 
  be 
  clearly 
  

   ascertained 
  ; 
  some 
  small 
  patches 
  of 
  diallage, 
  however, 
  appear 
  to 
  ex- 
  

   tinguish 
  at 
  an 
  angle 
  of 
  about 
  39° 
  to 
  the 
  direction 
  of 
  the 
  characteristic 
  

   separation-planes 
  or 
  cleavages 
  (i. 
  e. 
  to 
  oo 
  P 
  oo 
  or 
  100). 
  The 
  mineral 
  

   is 
  of 
  a 
  pale-greenish 
  tint, 
  exhibits 
  scarcely 
  a 
  trace 
  of 
  pleochroism, 
  

   and 
  shows 
  no 
  definite 
  crystallographic 
  boundaries. 
  

  

  In 
  most, 
  if 
  not 
  in 
  all, 
  of 
  the 
  sections 
  prepared 
  from 
  this 
  rock 
  

   numerous 
  little 
  crystals 
  are 
  present, 
  which 
  are 
  either 
  partially 
  or 
  

   wholly 
  opaque, 
  and 
  which, 
  where 
  they 
  transmit 
  light, 
  appear 
  of 
  a 
  

   more 
  or 
  less 
  deep 
  brown 
  colour. 
  Their 
  partial 
  or 
  entire 
  opacity 
  is 
  

   due 
  to 
  the 
  presence 
  of 
  a 
  decomposition-product, 
  which, 
  by 
  surface- 
  

   illumination, 
  is 
  seen 
  to 
  be 
  white. 
  Owing 
  to 
  the 
  prevalence 
  of 
  this 
  

   opaque 
  matter 
  the 
  character 
  of 
  the 
  pleochroism 
  cannot 
  be 
  made 
  out. 
  

  

  These 
  crystals 
  appear, 
  in 
  section, 
  mostly 
  as 
  very 
  acute 
  rhombs, 
  of 
  

   which 
  the 
  obtuse 
  angle 
  ranges 
  between 
  130° 
  and 
  140°. 
  The 
  mean 
  

   of 
  five 
  careful 
  measurements 
  gave 
  135° 
  36'. 
  It 
  seems 
  therefore 
  

   very 
  probable 
  that 
  these 
  crystals 
  are 
  sphene 
  in 
  an 
  advanced 
  stage 
  of 
  

   alteration. 
  One 
  measurement 
  gave 
  136° 
  15', 
  which 
  is 
  almost 
  the 
  

  

  angle 
  between 
  the 
  faces 
  | 
  P 
  2, 
  or 
  123 
  : 
  123 
  in 
  sphene, 
  while 
  several 
  

   others 
  gave 
  approximately 
  133°, 
  which 
  is 
  nearly 
  the 
  angle 
  of 
  the 
  

   prism. 
  It 
  would 
  be 
  unsafe, 
  however, 
  to 
  attach 
  much 
  importance 
  

   to 
  these 
  measurements, 
  since, 
  owing 
  to 
  the 
  opacity 
  of 
  the 
  crystals, 
  it 
  

   is 
  impossible 
  to 
  determine, 
  by 
  means 
  of 
  optical 
  characters, 
  in 
  what 
  

   directions 
  they 
  are 
  cut 
  f 
  . 
  The 
  epidote 
  present 
  in 
  the 
  rock 
  occurs 
  

   mostly 
  in 
  small, 
  irregularly 
  shaped 
  or 
  rounded 
  grains, 
  seldom 
  in 
  

   distinct 
  crystals. 
  

  

  A 
  very 
  striking 
  feature 
  in 
  sections 
  of 
  this 
  rock 
  is 
  the 
  presence 
  of 
  

   crystals 
  of 
  garnet, 
  which, 
  viewed 
  by 
  transmitted 
  light, 
  appear 
  nearly 
  

   colourless 
  or 
  of 
  pale 
  yellow 
  or 
  yellowish-green 
  tints. 
  They 
  are, 
  for 
  

   the 
  most 
  part, 
  of 
  very 
  irregular 
  form, 
  are 
  strongly 
  fissured, 
  and 
  seldom 
  

   show 
  any 
  definite 
  crystallographic 
  boundaries, 
  although 
  occasionally 
  

  

  * 
  " 
  A 
  Note 
  on 
  Glaucophane," 
  Journ. 
  Coll. 
  Sci. 
  Imp. 
  Univ. 
  Tokyd, 
  vol. 
  i. 
  

   pt.l,p.4. 
  

  

  t 
  Dr. 
  F. 
  H. 
  Hatch, 
  to 
  whom 
  I 
  have 
  since 
  submitted 
  the 
  sections, 
  is 
  also 
  in- 
  

   clined 
  to 
  regard 
  these 
  crystals 
  as 
  altered 
  sphene. 
  

  

  