﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  ALPS. 
  71 
  

  

  especially 
  in 
  the 
  neighbourhood 
  of 
  the 
  Italian 
  plain), 
  together 
  with 
  

   green 
  schists, 
  more 
  or 
  less 
  serpentinons 
  in 
  character, 
  and, 
  occasion- 
  

   ally, 
  of 
  true 
  sedimentary 
  rocks. 
  

  

  Thus 
  our 
  first 
  traverse 
  was 
  divided 
  into 
  two 
  well-marked 
  portions 
  : 
  

   the 
  one 
  the 
  crystalline 
  series 
  cut 
  by 
  the 
  Romanche 
  between 
  Yizille 
  

   and 
  the 
  Col 
  du 
  Lautaret, 
  the 
  other 
  the 
  Franco-Italian 
  range 
  between 
  

   Briangon 
  and 
  the 
  plain 
  of 
  Piedmont. 
  

  

  The 
  former 
  of 
  these, 
  as 
  above 
  stated, 
  admits 
  of 
  further 
  subdivi- 
  

   sion 
  into 
  the 
  Belledonne 
  range, 
  the 
  Grandes-Rousses 
  range, 
  and 
  the 
  

   Dauphine 
  massif. 
  I 
  may 
  add 
  that, 
  in 
  addition 
  to 
  my 
  general 
  ac- 
  

   quaintance 
  with 
  all 
  this 
  region, 
  I 
  have 
  been 
  able 
  to 
  refresh 
  my 
  

   memory 
  and 
  enlarge 
  my 
  knowledge 
  by 
  the 
  examination 
  of 
  a 
  most 
  

   interesting 
  series 
  of 
  specimens 
  from 
  its 
  higher 
  peaks 
  and 
  passes, 
  

   collected 
  by 
  the 
  Rev. 
  W. 
  H. 
  B. 
  Coolidge, 
  Eellow 
  of 
  Magdalen 
  College, 
  

   Oxford, 
  and 
  presented 
  by 
  him 
  to 
  the 
  Alpine 
  Club. 
  

  

  (a) 
  From 
  Vizille 
  to 
  Bourg 
  oVOisans 
  : 
  Belledonne 
  Range. 
  

  

  After 
  considerable 
  hesitation 
  I 
  decided 
  to 
  examine 
  the 
  natural 
  

   section 
  which 
  is 
  made 
  by 
  the 
  Romanche 
  in 
  cutting 
  through 
  this 
  chain. 
  

   Although 
  its 
  course 
  makes 
  a 
  very 
  oblique 
  angle 
  with 
  the 
  general 
  

   trend 
  of 
  the 
  chain, 
  and, 
  from 
  a 
  variety 
  of 
  causes, 
  I 
  was 
  not 
  able 
  to 
  

   examine 
  this 
  district 
  so 
  carefully 
  as 
  the 
  rest, 
  I 
  believe 
  that 
  I 
  

   succeeded 
  in 
  ascertaining 
  the 
  dominant 
  types 
  of 
  rock. 
  The 
  first 
  crys- 
  

   talline 
  rock 
  seen 
  is 
  at 
  the 
  Octroi 
  boundary, 
  a 
  short 
  distance 
  from 
  

   Yizille. 
  It 
  is 
  a 
  hard, 
  darkish 
  brown 
  mica-schist, 
  often 
  evidently 
  

   considerably 
  corrugated, 
  not 
  very 
  fissile, 
  or 
  markedly 
  bedded 
  or 
  

   foliated, 
  and 
  rather 
  irregularly 
  jointed 
  (Appendix, 
  p. 
  100). 
  Rock 
  of 
  

   this 
  type 
  continues 
  for 
  some 
  distance; 
  then 
  a 
  little 
  beyond 
  Sechilienne 
  

   it 
  is 
  succeeded 
  by 
  a 
  greener 
  and 
  more 
  distinctly 
  bedded 
  micaceous 
  

   and 
  somewhat 
  fine-grained 
  rock, 
  the 
  lines 
  of 
  mineral 
  banding 
  and 
  

   foliation 
  being 
  approximately 
  horizontal 
  ; 
  but 
  as 
  this 
  has 
  probably 
  

   been 
  modified 
  by 
  subsequent 
  pressure, 
  it 
  is 
  difficult 
  to 
  pronounce 
  

   on 
  its 
  true 
  nature. 
  West 
  of 
  Les 
  Clavaux 
  the 
  rock 
  becomes 
  more 
  

   like 
  a 
  fine-grained 
  granite, 
  having 
  a 
  slightly 
  gritty 
  aspect 
  on 
  the 
  

   weathered 
  surface 
  *. 
  There 
  is 
  also 
  a 
  little 
  of 
  a 
  " 
  porphyroid," 
  no 
  

   doubt 
  an 
  intrusive 
  felstone, 
  modified 
  by 
  subsequent 
  pressure 
  f. 
  

   Below 
  Riouperoux 
  fallen 
  blocks 
  of 
  a 
  darkish, 
  massive, 
  banded 
  gneiss 
  

   lie 
  by 
  the 
  roadside, 
  and 
  east 
  of 
  this 
  similar 
  blocks 
  of 
  a 
  dark 
  green 
  

   rock, 
  seemingly 
  hornblendic, 
  and 
  of 
  a 
  gabbro. 
  The 
  latter 
  rock 
  appears 
  

   to 
  be 
  intrusive, 
  and 
  is 
  the 
  usual 
  Alpine 
  gabbro 
  ; 
  but 
  it 
  exhibits 
  the 
  

   common 
  modification, 
  the 
  felspar 
  passing 
  to 
  a 
  kind 
  of 
  saussurite, 
  the 
  

   diallage 
  to 
  fibrous 
  hornblende, 
  and 
  occasionally 
  a 
  foliation, 
  due 
  

   possibly 
  to 
  pressure. 
  

  

  * 
  Microscopic 
  examination 
  shows 
  this 
  rock 
  to 
  have 
  been 
  so 
  much 
  modified 
  

   by 
  subsequent 
  pressure 
  that 
  the 
  original 
  structure 
  has 
  been 
  almost 
  obliterated. 
  

   So 
  far, 
  however, 
  as 
  it 
  can 
  be 
  discerned, 
  it 
  appears 
  that 
  of 
  a 
  true 
  granite 
  rather 
  

   than 
  of 
  a 
  granitoid 
  gneiss. 
  The 
  minerals 
  are 
  chiefly 
  quartz, 
  felspar, 
  and 
  a 
  

   chloritic 
  mineral 
  which 
  often 
  occupies 
  the 
  cracks, 
  and 
  probably 
  indicates 
  the 
  

   former 
  presence 
  of 
  biotite. 
  

  

  f 
  These 
  modified 
  dykes 
  or 
  masses 
  of 
  felstone 
  occur 
  not 
  seldom 
  in 
  the 
  crys- 
  

   talline 
  districts 
  of 
  the 
  Alps. 
  

  

  