﻿72 
  PEOF. 
  T. 
  G. 
  BONNET 
  ON 
  TWO 
  TEAVEESES 
  OF 
  THE 
  

  

  After 
  this 
  a 
  more 
  micaceous 
  and 
  banded 
  rock 
  is 
  seen 
  in 
  situ, 
  

   "West 
  of 
  Livet 
  comes 
  a 
  fairly 
  strong 
  mica-schist 
  containing 
  many 
  tiny 
  

   garnets, 
  and 
  traversed 
  by 
  numerous 
  quartz- 
  veins 
  (Appendix, 
  p. 
  101). 
  

   In 
  it 
  cleavage-foliation 
  has 
  been 
  developed 
  at 
  an 
  angle 
  of 
  about 
  70° 
  

   with 
  the 
  horizon, 
  without, 
  however, 
  materially 
  interfering 
  with 
  the 
  

   cohesion 
  of 
  the 
  mass. 
  It 
  is 
  later 
  in 
  date 
  than 
  these 
  veins, 
  for 
  they 
  

   have 
  been 
  bent 
  into 
  serpentine 
  curves, 
  which 
  are 
  thickened 
  at 
  the 
  

   crests 
  of 
  folds 
  and 
  attenuated 
  at 
  the 
  sides, 
  just 
  like 
  bands 
  of 
  sand 
  

   in 
  clay 
  under 
  pressure, 
  illustrating 
  in 
  a 
  very 
  interesting 
  way 
  the 
  

   principle 
  of 
  solution 
  and 
  deposition 
  indicated 
  by 
  Dr. 
  Sorby. 
  

  

  Beyond 
  this 
  place 
  the 
  crags 
  recede 
  from 
  the 
  road 
  as 
  the 
  Combe 
  de 
  

   Gavet 
  opens 
  out 
  ; 
  but 
  the 
  great 
  infold 
  of 
  Jurassic 
  rock, 
  out 
  of 
  which 
  

   has 
  been 
  excavated 
  the 
  singular 
  plain 
  around 
  the 
  confluence 
  of 
  the 
  

   Olle 
  and 
  the 
  Yeneon 
  with 
  the 
  Eomanche, 
  affords 
  a 
  wonderful 
  spec- 
  

   tacle. 
  The 
  strata 
  here 
  are 
  very 
  distinctly 
  bedded, 
  not 
  unlike 
  parts 
  

   of 
  the 
  Lower 
  Lias 
  of 
  England, 
  and 
  thus, 
  by 
  structure 
  as 
  well 
  as 
  

   tint, 
  are 
  distinguished 
  at 
  a 
  glance 
  from 
  the 
  underlying 
  crystalline 
  

   masses. 
  Cleavage 
  is 
  well 
  defined 
  and 
  is 
  not 
  far 
  from 
  the 
  vertical 
  ; 
  

   it 
  cuts 
  the 
  bedding 
  at 
  various, 
  but 
  generally 
  high, 
  angles, 
  because 
  

   that 
  structure 
  not 
  unfrequently 
  is 
  almost 
  horizontal. 
  

  

  (/3) 
  Bourg 
  d'Oisans 
  to 
  Le 
  Dauphin 
  : 
  Grandes-Bousses 
  Range 
  (fig. 
  1).. 
  

  

  Eoughly 
  to 
  the 
  north 
  of 
  Bourg 
  d'Oisans 
  the 
  crystalline 
  rock 
  reap- 
  

   pears 
  from 
  beneath 
  the 
  stratified. 
  On 
  the 
  south 
  side 
  of 
  the 
  valley 
  

   the 
  former 
  rises 
  sharply 
  up 
  from 
  beneath 
  the 
  latter 
  to 
  form 
  a 
  peak, 
  

   perhaps 
  8000 
  feet 
  above 
  the 
  sea 
  ; 
  on 
  the 
  north 
  side 
  an 
  " 
  epaulette 
  " 
  

   of 
  the 
  latter 
  rests 
  on 
  a 
  kind 
  of 
  shoulder 
  of 
  the 
  former, 
  so 
  that 
  the 
  

   peaks 
  of 
  the 
  Grandes 
  Bousses 
  are 
  almost 
  enclosed 
  by 
  the 
  Mesozoic 
  

   rocks. 
  North 
  of 
  the 
  town 
  a 
  small 
  mountain-road 
  gives 
  a 
  good 
  

   opportunity 
  of 
  studying 
  the 
  relations 
  of 
  the 
  two 
  series, 
  for 
  it 
  zigzags 
  

   over 
  their 
  junction, 
  which, 
  however, 
  is 
  only 
  perfectly 
  exposed 
  in 
  

   one 
  or 
  two 
  places. 
  

  

  The 
  crystalline 
  rock 
  is 
  a 
  rather 
  fine-grained 
  and 
  somewhat 
  massive 
  

   gneiss, 
  exhibiting 
  distinct 
  light 
  and 
  dark 
  bands, 
  which 
  are 
  often 
  

   about 
  ^ 
  inch 
  thick. 
  It 
  is 
  not 
  fissile, 
  for 
  it 
  breaks 
  readily 
  across 
  

   this 
  structure, 
  which 
  has 
  a 
  strike 
  between 
  NJN.W. 
  and 
  N.W., 
  and 
  a 
  

   dip 
  of 
  from 
  75° 
  to 
  80° 
  on 
  the 
  eastern 
  side. 
  Microscopic 
  examination 
  

   (Appendix, 
  p. 
  101) 
  indicates 
  that 
  the 
  rock, 
  after 
  crystallization, 
  

   has 
  been 
  modified 
  by 
  presssure 
  ; 
  but 
  I 
  am 
  very 
  doubtful 
  whether 
  the 
  

   banded 
  structure 
  can 
  be 
  attributed 
  to 
  this, 
  and 
  think 
  that 
  the 
  rock 
  

   when 
  thus 
  affected 
  must 
  have 
  been 
  already 
  a 
  banded 
  gneiss 
  re- 
  

   sembling 
  those 
  of 
  the 
  Laurentian 
  series. 
  The 
  base 
  of 
  the 
  Jurassic 
  

   series 
  is 
  a 
  hard 
  quartzose 
  grit, 
  which 
  has 
  apparently 
  derived 
  its 
  

   materials 
  from 
  the 
  underlying 
  rock. 
  Here 
  and 
  there 
  it 
  becomes 
  

   conglomeratic, 
  the 
  fragments 
  seeming 
  to 
  be 
  identical 
  with 
  the 
  

   underlying 
  gneiss. 
  

  

  About 
  two 
  miles 
  from 
  Bourg 
  d'Oisans 
  the 
  Eomanche 
  issues 
  from 
  

   a 
  narrow 
  gorge 
  ; 
  the 
  valley 
  of 
  the 
  Yeneon, 
  which 
  drains 
  the 
  interior 
  

   of 
  the 
  great 
  horseshoe 
  of 
  high 
  peaks 
  forming 
  the 
  massif 
  of 
  the 
  High- 
  

  

  