﻿t>4 
  ME. 
  F. 
  EETLEY 
  ON 
  FULGTJEITES 
  FEOM 
  MOXTE 
  VISO. 
  

  

  PL 
  III., 
  as 
  seen 
  under 
  a 
  magnifying-power 
  of 
  250 
  linear. 
  Fig. 
  5, 
  

   PI. 
  III., 
  shows 
  a 
  few 
  margarites, 
  selected 
  from 
  different 
  fragments, 
  

   while 
  fig. 
  6 
  gives 
  a 
  fair 
  idea 
  of 
  the 
  vesicular 
  character 
  of 
  the 
  

   fulgurite-glass, 
  as 
  seen 
  in 
  a 
  fragment 
  taken 
  from 
  the 
  interior 
  of 
  one 
  

   of 
  the 
  lightning- 
  tubes 
  and 
  magnified 
  120 
  linear. 
  Pig. 
  7 
  represents 
  

   a 
  microlith 
  imbedded 
  in 
  a 
  fragment 
  of 
  fulgurite-glass. 
  This 
  

   microlith 
  shows 
  distinct 
  double 
  refraction, 
  and 
  the 
  direction 
  of 
  

   maximum 
  extinction 
  makes 
  an 
  angle 
  of 
  about 
  16° 
  to 
  17° 
  with 
  the 
  

   longest 
  axis. 
  

  

  In 
  a 
  thin 
  section 
  of 
  excessively 
  vesicular 
  fulgurite-glass 
  taken 
  

   from 
  the 
  specimen 
  represented 
  in 
  fig. 
  1, 
  PI. 
  III., 
  there 
  are 
  some 
  good 
  

   examples 
  of 
  microliths 
  which 
  exhibit 
  very 
  distinct 
  double 
  refraction 
  ; 
  

   some 
  of 
  them 
  appear 
  to 
  show 
  parallel, 
  and 
  others 
  oblique 
  extinctions, 
  

   the 
  latter 
  giving 
  an 
  extinction-angle 
  of 
  15° 
  to 
  20° 
  with 
  the 
  longest 
  axis. 
  

   That 
  both 
  those 
  showing 
  parallel 
  and 
  those 
  giving 
  oblique 
  extinctions 
  

   are 
  identical 
  seems 
  highly 
  probable, 
  their 
  apparent 
  differences 
  in 
  this 
  

   respect 
  being 
  most 
  likely 
  due 
  to 
  the 
  positions 
  in 
  which 
  they 
  lie 
  in 
  

   the 
  section. 
  That 
  some 
  of 
  them 
  are 
  clinometric 
  is 
  certain. 
  That 
  

   they 
  are 
  all 
  clinometric 
  is 
  probable. 
  In 
  this 
  section 
  globulites 
  are 
  

   also 
  plentiful. 
  They 
  occur 
  either 
  singly 
  or 
  in 
  lines 
  (margarites) 
  

   composed 
  of 
  several 
  globulites, 
  occasionally 
  five 
  in 
  number. 
  Besides 
  

   these 
  linear 
  groupings 
  other 
  arrangements 
  are 
  also 
  seen 
  — 
  groups 
  of 
  

   three 
  globulites 
  massed 
  trefoilwise, 
  as 
  in 
  fig. 
  8 
  b, 
  PI. 
  III., 
  are 
  not 
  

   uncommon, 
  while 
  less 
  frequently 
  cruciform 
  groupings, 
  similar 
  to 
  

   that 
  represented 
  under 
  a 
  in 
  the 
  same 
  figure, 
  may 
  be 
  seen. 
  The 
  

   fulgurite-glass, 
  when 
  viewed 
  by 
  transmitted 
  light, 
  ranges 
  from 
  a 
  

   tolerably 
  deep 
  coffee-colour 
  to 
  pale 
  yellowish-brown 
  or 
  almost 
  

   colourless, 
  according 
  to 
  the 
  thickness. 
  The 
  vesicles 
  in 
  the 
  glass 
  are, 
  

   as 
  a 
  rule, 
  perfectly 
  spherical, 
  and 
  in 
  some 
  instances 
  so 
  numerous 
  

   and 
  closely 
  packed 
  that 
  the 
  glass 
  is 
  quite 
  spongy 
  in 
  character. 
  

  

  Where 
  the 
  fulgurite-glass 
  comes 
  in 
  contact 
  with 
  the 
  rock 
  the 
  

   latter 
  appears 
  to 
  have 
  undergone 
  no 
  appreciable 
  alteration 
  beyond 
  

   the 
  development 
  of 
  a 
  very 
  narrow 
  band 
  of 
  opaque, 
  white 
  matter, 
  

   resembling 
  the 
  substance 
  of 
  the 
  altered 
  crystals 
  of 
  sphene 
  which 
  the 
  

   rock 
  contains. 
  The 
  band 
  is 
  exceedingly 
  variable 
  in 
  thickness, 
  and 
  

   occasionally 
  thins 
  almost 
  entirely 
  away, 
  as 
  shown 
  in 
  the 
  dark 
  

   irregular 
  belt 
  which 
  crosses 
  fig. 
  2, 
  where 
  the 
  upper 
  portion 
  of 
  the 
  

   drawing 
  represents 
  the 
  crust 
  of 
  vesicular 
  fulgurite-glass 
  lining 
  one 
  of 
  

   the 
  lightning-tubes, 
  while 
  the 
  lower 
  portion 
  shows 
  the 
  rock 
  ( 
  x 
  120 
  

   linear). 
  In 
  one 
  section 
  a 
  minute, 
  rounded 
  grain 
  of 
  schist, 
  containing 
  

   a 
  fragment 
  of 
  a 
  strongly 
  depolarizing 
  crystal, 
  probably 
  epidote, 
  

   appears 
  to 
  have 
  been 
  taken 
  up 
  in 
  the 
  fulgurite-glass. 
  

  

  As 
  regards 
  the 
  opaque 
  white 
  band, 
  already 
  mentioned 
  as 
  occur- 
  

   ring 
  at 
  the 
  junction 
  of 
  the 
  glass 
  with 
  the 
  rock, 
  I 
  am 
  much 
  more 
  

   disposed 
  to 
  think 
  that 
  it 
  is 
  due 
  to 
  the 
  presence 
  of 
  altered 
  sphene 
  than 
  

   to 
  any 
  alteration 
  of 
  the 
  rock-surface 
  induced 
  by 
  the 
  action 
  of 
  

   lightning 
  ; 
  for 
  a 
  line 
  drawn 
  through 
  other 
  parts 
  of 
  the 
  section 
  might 
  

   easily 
  pass 
  through 
  crystals 
  and 
  segregations 
  of 
  similar 
  white 
  matter 
  

   of 
  like 
  extent 
  and 
  continuity. 
  If 
  it 
  were 
  an 
  alteration-product 
  

   engendered 
  suddenly 
  by 
  an 
  intense 
  heat 
  we 
  should 
  rather 
  expect 
  

  

  