﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  ALPS. 
  79 
  

  

  quartz-vein 
  of 
  about 
  the 
  same 
  thickness 
  separated 
  the 
  black 
  slate 
  * 
  

   and 
  the 
  grey 
  gneiss 
  (fig. 
  2), 
  and 
  I 
  believe 
  the 
  two 
  rocks 
  have 
  been 
  

   simply 
  doubled 
  up 
  together. 
  In 
  this 
  glen, 
  and 
  at 
  one 
  place 
  on 
  the 
  

   above-mentioned 
  alps 
  across 
  the 
  Romanche, 
  I 
  found 
  some 
  Belemnites 
  

   curiously 
  distorted 
  by 
  pressure. 
  

  

  The 
  road 
  from 
  La 
  Grave 
  to 
  the 
  Col 
  du 
  Lautaret 
  passes 
  over 
  dark 
  

   Jurassic 
  slates, 
  which 
  (about 
  9 
  kil. 
  from 
  La 
  Grave) 
  are 
  interbanded 
  

   with 
  an 
  impure 
  black 
  limestone 
  ; 
  the 
  former 
  rock 
  is 
  distinctly 
  

   cleaved 
  (parallel 
  with 
  its 
  bedding), 
  the 
  latter 
  practically 
  uncleaved. 
  

   On 
  the 
  descent 
  from 
  the 
  pass, 
  hard 
  grits 
  and 
  quartzites 
  occur, 
  re- 
  

   ferred 
  by 
  Prof. 
  Lory 
  in 
  part 
  to 
  the 
  Trias, 
  in 
  part 
  to 
  the 
  Carbon- 
  

   iferous. 
  At 
  these 
  I 
  only 
  glanced 
  in 
  passing, 
  as 
  there 
  was 
  little 
  

   specially 
  interesting 
  in 
  their 
  lithological 
  character. 
  I 
  need 
  only 
  

   add 
  that 
  a 
  moraine 
  near 
  Le 
  Casset, 
  which 
  represents 
  a 
  collection 
  

   from 
  the 
  eastern 
  part 
  of 
  the 
  northern 
  face 
  of 
  the 
  Crete 
  du 
  Glacier 
  

   Blanc, 
  consists 
  mainly 
  of 
  a 
  strong 
  gneiss, 
  like 
  that 
  described 
  as 
  

   occurring 
  on 
  the 
  Lautaret 
  road 
  below 
  La 
  Grave, 
  only 
  it 
  is 
  perhaps 
  a 
  

   little 
  greener 
  in 
  tint. 
  

  

  The 
  result, 
  then, 
  of 
  our 
  researches 
  in 
  this 
  part 
  of 
  the 
  section 
  may 
  

   be 
  summed 
  up 
  in 
  the 
  following 
  extract 
  from 
  my 
  diary 
  : 
  — 
  " 
  I 
  have 
  

   nowhere 
  seen 
  any 
  rock 
  that 
  reminds 
  me 
  either 
  of 
  the 
  Lepontine 
  

   gneiss 
  (Montalban 
  type), 
  or 
  of 
  the 
  Tremola 
  schists, 
  or 
  of 
  any 
  of 
  the 
  

   group 
  of 
  the 
  upper 
  schists 
  in 
  the 
  Central 
  Alps. 
  Everything 
  (excluding 
  

   the 
  Carboniferous 
  and 
  Mesozoic 
  rocks) 
  appears 
  to 
  be 
  either 
  granitoid 
  

   rock, 
  probably 
  of 
  igneous 
  origin, 
  or 
  else 
  a 
  strong 
  gneiss 
  of 
  Laurentian 
  

   type, 
  which 
  in 
  the 
  massif 
  south 
  of 
  La 
  Grave 
  does 
  not, 
  as 
  a 
  rule, 
  

   exhibit 
  marked 
  sheen 
  surfaces, 
  though 
  it 
  is 
  often 
  much 
  corrugated. 
  

   Some 
  of 
  the 
  bands 
  in 
  the 
  gneiss 
  and 
  some 
  varieties 
  of 
  the 
  rock 
  itself 
  

   are 
  rather 
  rich 
  in 
  mica 
  (mostly 
  dark), 
  others 
  comparatively 
  poor. 
  

   Green 
  rocks, 
  hornblendic 
  or 
  chloritic, 
  are 
  not 
  common, 
  though 
  there 
  

   is 
  often 
  a 
  coating 
  of 
  this 
  tint." 
  

  

  B. 
  From 
  Briangon 
  to 
  Turin. 
  

   (a) 
  Section 
  of 
  the 
  Mont 
  Genevre 
  Pass. 
  

  

  The 
  road 
  to 
  this 
  pass 
  runs 
  for 
  some 
  little 
  distance 
  up 
  the 
  Clairee 
  

   Valley 
  north 
  of 
  Briancon, 
  then 
  mounts 
  in 
  an 
  easterly 
  direction 
  up 
  

   the 
  slopes 
  of 
  a 
  glen. 
  The 
  rock 
  in 
  situ 
  here 
  is 
  a 
  Jurassic 
  limestone, 
  

   but 
  erratics 
  of 
  a 
  gabbro, 
  often 
  coarse, 
  occur 
  occasionally, 
  — 
  the 
  felspar 
  

   converted 
  into 
  saussurite, 
  the 
  dark 
  crystals 
  of 
  diallage 
  sometimes 
  

   partly 
  replaced 
  by 
  hornblende. 
  We 
  also 
  observed 
  pieces 
  of 
  a 
  dark 
  

   serpentine, 
  with 
  crystals 
  of 
  bronzite 
  or 
  bastite, 
  which 
  could 
  easily 
  be 
  

   matched 
  from 
  the 
  Grisons 
  or 
  the 
  Ligurian 
  Apennines. 
  Both 
  these 
  

   probably 
  come 
  from 
  a 
  massif 
  south 
  of 
  the 
  upper 
  part 
  of 
  the 
  pass. 
  

  

  * 
  I 
  have 
  examined 
  a 
  specimen 
  taken 
  about 
  2 
  inches 
  from 
  the 
  face 
  of 
  the 
  

   gneiss. 
  There 
  are 
  one 
  or 
  two 
  minute 
  constituents 
  of 
  minor 
  interest, 
  but 
  

   for 
  my 
  present 
  purpose 
  it 
  is 
  enough 
  to 
  say 
  that 
  it 
  is 
  a 
  common 
  type 
  of 
  slate, 
  

   with 
  less 
  evidence 
  of 
  the 
  development 
  of 
  secondary 
  minerals 
  than 
  I 
  have 
  often 
  

   seen 
  in 
  Palaeozoic 
  slates 
  in 
  regions 
  of 
  great 
  disturbance. 
  

  

  