﻿78 
  

  

  PEOF. 
  T. 
  G. 
  BONNET 
  ON 
  TWO 
  TEAVEESES 
  OF 
  THE 
  

  

  foliation, 
  but 
  with 
  little 
  or 
  no 
  success. 
  In 
  the 
  immediate 
  neigh- 
  

   bourhood 
  of 
  La 
  Grave 
  this 
  rock 
  is 
  a 
  dark 
  slate, 
  so 
  uniform 
  in 
  its 
  

   mineral 
  character 
  that 
  the 
  direction 
  of 
  bedding 
  is 
  not 
  easily 
  ascer- 
  

   tained. 
  Indeed 
  even 
  in 
  the 
  great 
  folds 
  opposite 
  La 
  Grave 
  the 
  beds 
  

   are 
  more 
  distinctly 
  seen 
  at 
  a 
  distance 
  than 
  when 
  on 
  the 
  spot. 
  The 
  

   cleavage, 
  of 
  course, 
  cuts 
  the 
  stratification 
  at 
  very 
  different 
  angles, 
  

   being 
  often 
  coincident 
  with 
  it, 
  though 
  sometimes 
  highly 
  inclined 
  to 
  

   it. 
  A 
  slight 
  satiny 
  sheen, 
  caused 
  by 
  extremely 
  minute 
  films 
  of 
  

   mica 
  (sericite?), 
  may 
  sometimes, 
  but 
  by 
  no 
  means 
  always, 
  be 
  observed, 
  

   and 
  of 
  this 
  the 
  best 
  examples 
  were 
  found 
  on 
  the 
  Alps 
  north-west 
  of 
  

   La 
  Grave, 
  some 
  1800 
  or 
  2000 
  feet 
  above 
  the 
  village. 
  Here 
  I 
  noticed 
  

   that 
  the 
  mica-flakes 
  became 
  distinctly 
  larger 
  in 
  immediate 
  proximity 
  

   to 
  a 
  quartz-vein, 
  when 
  it 
  had 
  also 
  been 
  squeezed 
  *. 
  Often, 
  however, 
  

   the 
  rock 
  is 
  quite 
  an 
  ordinary 
  slate, 
  as 
  in 
  the 
  case 
  of 
  a 
  specimen 
  taken 
  

   from 
  within 
  two 
  inches 
  of 
  a 
  junction 
  with 
  the 
  gneiss 
  in 
  the 
  glen 
  

   below 
  the 
  Meije 
  glacier 
  (fig. 
  2). 
  Here, 
  so 
  far 
  as 
  I 
  could 
  judge, 
  there 
  

  

  Fig. 
  2. 
  — 
  Junction 
  of 
  Gneiss 
  (A) 
  with 
  Jurassic 
  Slate 
  (B) 
  in 
  glen 
  

   beloiv 
  the 
  Meije 
  Glacier. 
  

  

  were 
  no 
  signs 
  of 
  faulting. 
  First 
  a 
  little 
  notch 
  about 
  an 
  inch 
  wide 
  

   filled 
  with 
  soil, 
  in 
  which 
  Alpine 
  plants 
  were 
  growing, 
  and 
  then 
  a 
  

  

  * 
  We 
  have 
  veins 
  of 
  quartz 
  and 
  occasionally 
  of 
  calcite, 
  sometimes 
  prior, 
  some- 
  

   times 
  posterior 
  to 
  the 
  cleavage. 
  This 
  effect 
  of 
  an 
  included 
  hard 
  mass 
  on 
  the 
  

   development 
  of 
  secondary 
  mica 
  in 
  a 
  rock 
  may 
  often 
  be 
  observed 
  in 
  disturbed 
  

   regions. 
  

  

  