﻿216 
  

  

  MR. 
  W. 
  P. 
  D. 
  STEBBING 
  ON 
  TWO 
  BOULDERS 
  OF 
  [May 
  1 
  897, 
  

  

  grains 
  of 
  a 
  mineral 
  having 
  a 
  very 
  slight 
  earth 
  -brown 
  tint 
  in 
  

   ordinary 
  light, 
  and 
  appearing 
  (with 
  crossed 
  nicols) 
  as 
  an 
  aggregate 
  

   of 
  minute 
  specks 
  inclining 
  to 
  flaky 
  or 
  fibrous, 
  which 
  bave 
  weak 
  

   depolarizing 
  power 
  : 
  probably 
  a 
  pinite 
  (? 
  after 
  cordierite). 
  A 
  very 
  

   little 
  of 
  rather 
  altered 
  biotite 
  and 
  iron 
  oxide, 
  also 
  zircon, 
  and 
  (?) 
  

   rutile, 
  both 
  small 
  and 
  rare. 
  The 
  structure 
  of 
  the 
  rock 
  is 
  not 
  that 
  

   of 
  a 
  normal 
  granite, 
  but 
  more 
  nearly 
  resembles 
  one 
  of 
  those 
  which 
  

   are 
  often 
  banded, 
  and 
  commonly 
  occur 
  in 
  a 
  complex 
  of 
  the 
  older 
  

   Archaean, 
  such 
  as 
  the 
  so-called 
  Laurentian 
  or 
  Hebridean 
  (see 
  ng. 
  1). 
  

   'Boulder 
  B. 
  — 
  The 
  constituents 
  in 
  this 
  specimen 
  differ, 
  in 
  that 
  much 
  

   of 
  the 
  felspar 
  is 
  ordinary 
  plagioclase, 
  and 
  orthoclase 
  is 
  more 
  certainly 
  

   present: 
  the 
  pinitic 
  mineral 
  is 
  doubtfully 
  present; 
  there 
  is 
  very 
  

  

  Fig. 
  1. 
  — 
  Microscopic 
  section 
  of 
  Boulder 
  A. 
  

  

  X30. 
  

  

  The 
  dark 
  grain 
  at 
  the 
  edge, 
  about 
  S.S.W. 
  of 
  the 
  centre, 
  is 
  blackened 
  biotite, 
  

   and 
  there 
  are 
  some 
  small 
  films 
  to 
  the 
  N.W. 
  of 
  it. 
  The 
  lighter 
  grains 
  are 
  

   quartz 
  ; 
  the 
  rest 
  is 
  felspar. 
  A 
  trace 
  of 
  minute 
  micrographic 
  structure 
  is 
  

   visible. 
  The 
  rock 
  is 
  cracked, 
  but 
  the 
  peculiarly 
  formed 
  clear 
  space 
  

   between 
  S.E. 
  and 
  E.S.E. 
  is 
  caused 
  by 
  a 
  breakage 
  in 
  the 
  slice. 
  

  

  little 
  biotite, 
  but 
  much 
  white 
  mica. 
  A 
  little 
  of 
  this 
  may 
  be 
  

   bleached 
  biotite, 
  and 
  in 
  one 
  or 
  two 
  cases 
  I 
  think 
  that 
  small 
  flakes 
  

   of 
  biotite 
  are 
  interlaminated. 
  There 
  is 
  a 
  small 
  elongated 
  cluster 
  

   of 
  impure 
  red 
  garnets, 
  pierced 
  longitudinally 
  by 
  a 
  thin 
  tongue 
  of 
  

   quartz 
  (see 
  fig. 
  2). 
  In 
  this 
  rock 
  signs 
  of 
  mechanical 
  action 
  are 
  

   clear. 
  The 
  mica 
  is 
  sometimes 
  bent 
  or 
  'rucked,' 
  the 
  quartz 
  and 
  

  

  