﻿1 
  02 
  PEOF. 
  T. 
  G. 
  BONNET 
  ON 
  TWO 
  TEAVEESES 
  OF 
  THE 
  

  

  believe 
  it 
  (and 
  this 
  agrees 
  with 
  the 
  impression 
  formed 
  in 
  the 
  field) 
  

   to 
  have 
  been 
  a 
  gneiss 
  of 
  the 
  Laurentian 
  type. 
  

  

  (6) 
  Gneiss, 
  4 
  Ml. 
  West 
  of 
  La 
  Grave 
  (p. 
  75). 
  — 
  The 
  constituents 
  are 
  

   as 
  above 
  described, 
  except 
  that 
  there 
  is 
  little 
  or 
  no 
  epidote, 
  but 
  a 
  few 
  

   very 
  small, 
  almost 
  colourless 
  garnets 
  are 
  present. 
  The 
  mica, 
  how- 
  

   ever, 
  occurs 
  in 
  larger 
  flakes, 
  and 
  evidently 
  is 
  mostly 
  biotite 
  more 
  or 
  

   less 
  altered. 
  The 
  structure 
  of 
  the 
  rock 
  has 
  been 
  modified 
  by 
  pressure, 
  

   but 
  the 
  original 
  one 
  is 
  retained 
  more 
  distinctly 
  than 
  in 
  the 
  former 
  

   case. 
  It 
  is 
  that 
  of 
  a 
  very 
  old 
  gneiss, 
  not 
  of 
  a 
  normal 
  granite. 
  

  

  (c) 
  Gneiss 
  from 
  glen 
  below 
  the 
  Meije 
  Glacier 
  (p. 
  77). 
  — 
  The 
  differ- 
  

   ences 
  from 
  the 
  last 
  described 
  are 
  only 
  varietal. 
  

  

  (y) 
  Carboniferous 
  Series 
  near 
  Le 
  Freney. 
  

  

  (a) 
  Matrix 
  of 
  the 
  Conglomerate, 
  east 
  side 
  of 
  village 
  (p. 
  74). 
  — 
  This 
  

   is 
  mainly 
  composed 
  of 
  small 
  fragments, 
  which 
  are 
  cemented 
  by 
  a 
  

   minutely 
  flaky 
  or 
  granular 
  material 
  of 
  paler 
  dusty 
  grey 
  colour 
  ; 
  pro- 
  

   bably 
  the 
  latter 
  largely 
  consists 
  of 
  mica, 
  produced 
  by 
  the 
  decompo- 
  

   sition 
  of 
  pulverized 
  felspar. 
  Here 
  and 
  there 
  is 
  a 
  fair-sized 
  flake 
  of 
  

   white 
  mica. 
  The 
  fragments 
  occasionally 
  contain 
  felspars 
  of 
  fair 
  size, 
  

   about 
  "05" 
  long, 
  but 
  the 
  matrix 
  of 
  these, 
  and 
  the 
  whole 
  of 
  the 
  rest, 
  

   consists 
  of 
  a 
  mottled 
  mosaic 
  of 
  quartz, 
  apparently 
  traversed 
  by 
  thin 
  

   wavy 
  bands, 
  chiefly 
  of 
  the 
  above-mentioned 
  microscopic 
  mica. 
  Their 
  

   structures 
  suggest 
  a 
  derivation 
  from 
  a 
  gneiss 
  or 
  schist 
  that 
  has 
  been 
  

   greatly 
  crushed 
  and 
  sheared. 
  As 
  these 
  structure-lines 
  point 
  in 
  all 
  

   directions 
  in 
  the 
  slide*, 
  it 
  is 
  difficult 
  to 
  refer 
  them 
  to 
  Post-Carbon- 
  

   iferous 
  movements. 
  The 
  neighbouring 
  rock, 
  however, 
  when 
  in 
  its 
  

   normal 
  conditions, 
  appears 
  to 
  be 
  the 
  ordinary 
  coarse 
  crystalline 
  rock 
  

   described 
  in 
  Sect. 
  I. 
  It 
  would 
  seem 
  therefore 
  as 
  if 
  the 
  fragments 
  

   had 
  not 
  been 
  derived 
  from 
  the 
  immediate 
  vicinity, 
  and 
  their 
  struc- 
  

   ture 
  was 
  a 
  record 
  of 
  Pre-Carboniferous 
  disturbances. 
  

  

  (b) 
  The 
  Slate 
  west 
  of 
  the 
  village 
  (p. 
  73) 
  consists 
  of 
  fragmental 
  

   quartz, 
  commonly 
  very 
  angular 
  ; 
  felspar, 
  more 
  or 
  less 
  decomposed 
  ; 
  

   white 
  mica, 
  with 
  (rarely) 
  a 
  little 
  brown, 
  and 
  tourmaline 
  (very 
  rare). 
  

   The 
  rude 
  cleavage- 
  lines 
  are 
  darkened 
  by 
  the 
  deposit 
  of 
  a 
  carbonaceous 
  

   substance, 
  which 
  is 
  more 
  or 
  less 
  disseminated 
  throughout 
  the 
  slide. 
  

   We 
  have 
  evidently 
  here 
  a 
  rather 
  fine 
  detritus 
  (the 
  fragments 
  rarely 
  

   exceed 
  -01" 
  in 
  diameter), 
  derived 
  from 
  a 
  crystalline 
  series. 
  The 
  

   mineral 
  changes 
  in 
  the 
  rock, 
  since 
  Carboniferous 
  times, 
  appear 
  to 
  me 
  

   to 
  have 
  been 
  extremely 
  slight, 
  yet, 
  as 
  stated 
  above 
  (p. 
  73), 
  it 
  is 
  com- 
  

   pletely 
  infolded 
  between 
  huge 
  masses 
  of 
  crystalline 
  rock, 
  and 
  must 
  

   have 
  been 
  affected 
  by 
  Pre-Jurassic 
  as 
  well 
  as 
  by 
  Tertiary 
  disturb- 
  

   ances. 
  

  

  (3) 
  Calc-mica-schists 
  East 
  of 
  the 
  Watershed 
  of 
  the 
  Cottian 
  Alps 
  

   (pp. 
  80-82). 
  

  

  As 
  stated 
  in 
  the 
  foregoing 
  paper, 
  these 
  rocks 
  belong 
  to 
  one 
  

   general 
  type, 
  the 
  individual 
  constituents 
  being 
  small 
  grains 
  or 
  

   flakes, 
  generally 
  less 
  than 
  -01" 
  in 
  diameter 
  (except 
  in 
  the 
  cases 
  of 
  

   * 
  This 
  is 
  also 
  true 
  of 
  the 
  larger 
  fragments. 
  

  

  