﻿CKYSTALLINE 
  ROCKS 
  OP 
  THE 
  ALPS. 
  73 
  

  

  Alps 
  of 
  Dauphine, 
  being 
  the 
  more 
  open 
  of 
  the 
  two 
  valleys. 
  So 
  far 
  

   as 
  we 
  could 
  see, 
  the 
  wall 
  of 
  cliffs 
  on 
  the 
  north 
  side 
  of 
  the 
  former 
  

   river 
  consists 
  of 
  gneiss 
  similar 
  to 
  that 
  described 
  above, 
  except 
  that 
  

   as 
  we 
  approached 
  the 
  entrance 
  of 
  the 
  gorge 
  the 
  rocks 
  became 
  slightly 
  

   greener 
  in 
  colour, 
  and 
  blocks 
  of 
  a 
  greenish 
  chloritic 
  rock 
  (possibly 
  

   a 
  modified 
  diabase) 
  occurred 
  occasionally 
  among 
  the 
  screes. 
  But 
  

   close 
  to 
  the 
  entrance 
  is 
  a 
  fine 
  crag 
  of 
  a 
  granitoid 
  rock 
  *, 
  and 
  about 
  

   a 
  furlong 
  further, 
  on 
  the 
  left 
  bank 
  of 
  the 
  Eomanche 
  (the 
  road 
  having 
  

   crossed 
  the 
  river), 
  we 
  find 
  a 
  little 
  of 
  a 
  similar 
  rock, 
  apparently 
  in- 
  

   trusive 
  in 
  a 
  greenish 
  banded 
  gneiss, 
  like 
  that 
  on 
  the 
  opposite 
  bank. 
  

   The 
  structure 
  of 
  this 
  rock 
  strikes 
  a 
  little 
  W. 
  of 
  NVW., 
  dipping 
  at 
  

   about 
  45° 
  on 
  the 
  N.E. 
  side. 
  

  

  The 
  road 
  now 
  ascends 
  on 
  the 
  left 
  bank 
  of 
  the 
  Eomanche 
  (which 
  

   dashes 
  along 
  at 
  the 
  bottom 
  of 
  a 
  gorge), 
  passing 
  over 
  banded 
  gneiss 
  

   varying 
  from 
  coarse 
  to 
  fine, 
  occasionally 
  distinctly 
  micaceous, 
  and 
  

   more 
  usually 
  resembling 
  the 
  green 
  variety 
  mentioned 
  above. 
  The 
  

   strike 
  of 
  the 
  structure 
  continues 
  practically 
  unchanged 
  ; 
  the 
  dip 
  

   varies, 
  ranging 
  commonly 
  from 
  about 
  35° 
  to 
  60°. 
  The 
  rock 
  some- 
  

   times 
  assumes 
  a 
  " 
  slabby 
  " 
  structure, 
  with 
  the 
  surfaces 
  parallel 
  to 
  the 
  

   mineral 
  bands. 
  The 
  rock 
  has, 
  no 
  doubt, 
  been 
  modified 
  by 
  pressure, 
  

   though 
  to 
  what 
  extent, 
  in 
  the 
  present 
  stage 
  of 
  our 
  knowledge, 
  can 
  

   hardly 
  be 
  determined; 
  but 
  it 
  produced 
  on 
  my 
  mind 
  the 
  impression 
  

   that 
  we 
  had 
  before 
  us 
  a 
  very 
  ancient 
  type 
  of 
  rock, 
  which, 
  anterior 
  to 
  

   the 
  last 
  modification, 
  was 
  gneissoid 
  rather 
  than 
  granitoid. 
  After 
  a 
  time 
  

   the 
  dip 
  steepens 
  till 
  it 
  is 
  almost 
  vertical, 
  and 
  then 
  the 
  rock 
  becomes 
  

   greener 
  and 
  more 
  chloritic. 
  Then, 
  about 
  7 
  kil. 
  from 
  Bourg 
  d'Oisans, 
  

   a 
  cultivated 
  slope 
  of 
  debris 
  conceals 
  the 
  section 
  for 
  some 
  distance 
  ; 
  

   after 
  this 
  the 
  road 
  traverses 
  a 
  rather 
  fissile 
  gneiss, 
  striking 
  rather 
  

   E. 
  of 
  N., 
  and 
  dipping 
  eastwards 
  at 
  60° 
  at 
  least. 
  To 
  this 
  succeeds 
  

   a 
  grey 
  granitoid 
  rock, 
  with 
  " 
  sheen 
  " 
  surfaces, 
  which 
  dip 
  roughly 
  E. 
  

   at 
  about 
  45° 
  ; 
  then 
  comes, 
  after 
  another 
  interruption, 
  a 
  very 
  fissile 
  

   rock, 
  with 
  a 
  similar 
  strike, 
  reminding 
  me 
  of 
  a 
  common 
  "crush 
  

   type 
  " 
  in 
  the 
  Alps 
  (e. 
  g. 
  one 
  on 
  the 
  St. 
  Gothard, 
  just 
  above 
  Amsteg). 
  

   Beyond 
  this 
  we 
  come 
  upon 
  a 
  sharp 
  infold 
  of 
  Carboniferous 
  rock. 
  

   This 
  narrow 
  trough, 
  well 
  shown 
  on 
  Prof. 
  Lory's 
  map, 
  runs 
  almost 
  

   due 
  north 
  for 
  about 
  10 
  miles 
  from 
  near 
  Venose, 
  in 
  the 
  Veneon 
  valley, 
  

   to 
  the 
  west 
  flank 
  of 
  the 
  Grandes-Rousses 
  peaks. 
  

  

  This 
  Palaeozoic 
  rock 
  is 
  a 
  carbonaceous 
  f 
  slaty 
  roCk, 
  containing 
  

   small 
  fragmental 
  scales 
  of 
  silvery 
  mica 
  (Appendix, 
  p. 
  102). 
  It 
  is 
  not 
  

   more 
  metamorphosed 
  than 
  any 
  ordinary 
  slate, 
  and 
  can 
  be 
  readily 
  

   distinguished 
  by 
  both 
  colour 
  and 
  texture 
  from 
  the 
  crushed 
  crystal- 
  

   line 
  series, 
  though, 
  as 
  the 
  latter 
  is 
  at 
  last 
  almost 
  pulverized, 
  and 
  a 
  

  

  * 
  Microscopic 
  examination 
  shows 
  the 
  rock 
  to 
  consist 
  of 
  quartz, 
  orthoclase, 
  a 
  

   very 
  little 
  microcline, 
  plagioclase, 
  some 
  more 
  or 
  less 
  altered 
  biotite, 
  with 
  a 
  few 
  

   small 
  zircons 
  and 
  other 
  microlithic 
  minerals. 
  It 
  has 
  evidently 
  undergone 
  con- 
  

   siderable 
  crushing, 
  followed 
  by 
  cementation, 
  but 
  it 
  retains 
  in 
  part 
  the 
  charac- 
  

   teristic 
  structure 
  of 
  a 
  true 
  granite. 
  It 
  is 
  a 
  variety 
  of 
  the 
  "protogine" 
  of 
  the 
  

   region, 
  and 
  a 
  very 
  similar 
  rock 
  forms 
  the 
  two 
  highest 
  peaks 
  of 
  the 
  Grandes 
  

  

  t 
  A 
  little 
  anthracite 
  is 
  worked 
  among 
  the 
  mountains 
  near 
  the 
  north 
  end 
  of 
  

   the 
  trough. 
  

  

  