﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  ALPS. 
  103 
  

  

  the 
  calcite), 
  but 
  the 
  structure 
  being 
  truly 
  crystalline. 
  I 
  have 
  

   examined 
  specimens 
  from 
  the 
  following 
  localities 
  : 
  — 
  («) 
  by 
  the 
  road 
  to 
  

   Oulx, 
  about 
  hall 
  a 
  mile 
  below 
  Cesanne 
  ; 
  (6) 
  associated 
  with 
  serpentine 
  

   west 
  side 
  of 
  Col 
  de 
  Sestrieres 
  ; 
  (c) 
  succeeding 
  the 
  serpentine 
  ; 
  

   {d) 
  harder 
  band 
  in 
  micaceous 
  schist 
  ; 
  (<?) 
  contorted 
  schist 
  at 
  sketch 
  

   (p. 
  80) 
  ; 
  (/) 
  the 
  dark 
  schist 
  below 
  Champlain 
  du 
  Col. 
  b-f 
  are 
  taken 
  

   in 
  order 
  on 
  the 
  ascent 
  of 
  the 
  west 
  side 
  of 
  the 
  Col 
  de 
  Sestrieres. 
  

  

  Their 
  chief 
  constituents 
  (excepting 
  6) 
  are 
  quartz, 
  calcite, 
  and 
  mica, 
  

   the 
  last 
  sometimes 
  brown, 
  more 
  often 
  white, 
  rendered 
  in 
  places 
  

   almost 
  opaque 
  by 
  black 
  carbonaceous 
  dust, 
  which 
  has 
  a 
  streaky, 
  

   more 
  or 
  less 
  banded 
  arrangement, 
  and 
  is 
  commonly 
  associated 
  with 
  

   the 
  mica, 
  (d) 
  contains 
  a 
  fair 
  amount 
  of 
  dolomite 
  and 
  not 
  much 
  

   mica. 
  All 
  the 
  slides 
  indicate 
  that 
  the 
  rocks 
  have 
  been 
  subjected 
  to 
  

   considerable 
  pressure, 
  followed 
  by 
  a 
  certain 
  amount 
  of 
  mineral 
  re- 
  

   arrangement 
  ; 
  but 
  the 
  form 
  and 
  mode 
  of 
  association 
  of 
  almost 
  all 
  the 
  

   grains 
  of 
  quartz 
  and 
  calcite 
  (or 
  dolomite), 
  and 
  of 
  the 
  mica-flakes, 
  is 
  

   such 
  as 
  to 
  convince 
  me 
  that 
  the 
  rocks, 
  prior 
  to 
  the 
  disturbances, 
  

   were 
  true 
  crystalline 
  schists. 
  Microscopic 
  study 
  also 
  has 
  confirmed 
  

   the 
  opinion 
  formed 
  in 
  the 
  field, 
  that 
  the 
  foliation 
  was 
  a 
  record 
  of 
  

   stratification, 
  and 
  that 
  the 
  rocks 
  are 
  altered 
  sediments. 
  (6) 
  differs 
  

   from 
  the 
  rest. 
  Its 
  constituents 
  are 
  extremely 
  minute, 
  and 
  the 
  diffi- 
  

   culty 
  of 
  examining 
  them 
  is 
  increased 
  by 
  their 
  feeble 
  action 
  on 
  

   polarized 
  light. 
  The 
  only 
  distinct 
  minerals, 
  except 
  a 
  few 
  granules 
  

   of 
  iron-oxide, 
  are 
  some 
  fairly 
  thick 
  scales 
  of 
  a 
  chlorite 
  (average 
  

   about 
  -005" 
  in 
  the 
  longer 
  diameter) 
  of 
  a 
  pale 
  green 
  colour 
  and 
  very 
  

   weak 
  depolarizing 
  action. 
  These 
  are 
  thinly 
  scattered 
  through 
  a 
  

   ground-mass, 
  which 
  appears 
  to 
  consist 
  of 
  minute 
  chalcedonic 
  silica 
  

   (perhaps 
  sometimes 
  even 
  of 
  opal) 
  and 
  plates 
  or 
  belonites 
  in 
  great 
  

   abundance 
  of 
  a 
  very 
  pale 
  greenish-brown 
  mineral. 
  Evidently 
  the 
  

   rocks 
  have 
  been 
  greatly 
  compressed. 
  This 
  is 
  indicated 
  by 
  the 
  con- 
  

   tortion 
  of 
  some 
  tiny 
  chalcedonic 
  veins 
  and 
  the 
  presence 
  of 
  a 
  rude 
  

   cleavage. 
  It 
  might 
  be 
  either 
  an 
  ordinary 
  sediment 
  greatly 
  affected 
  

   by 
  pressure, 
  or 
  a 
  rather 
  fine-grained 
  schist 
  similarly 
  modified. 
  

   Further 
  we 
  know 
  not 
  how 
  much 
  alteration 
  may 
  be 
  due 
  to 
  the 
  prox- 
  

   imity 
  of 
  the 
  serpentine. 
  Hence 
  I 
  can 
  merely 
  state 
  the 
  facts 
  and 
  

   say 
  that, 
  with 
  my 
  present 
  knowledge, 
  I 
  cannot 
  venture 
  to 
  draw 
  any 
  

   inference 
  from 
  this 
  specimen. 
  Crush 
  a 
  fine-grained 
  schist, 
  and 
  crush 
  

   a 
  fine-grained 
  mudstone, 
  and 
  you 
  sometimes 
  come 
  locally 
  to 
  the 
  same 
  

   result, 
  the 
  difficulty 
  of 
  separation, 
  be 
  it 
  noted, 
  being 
  chiefly 
  due 
  

   to 
  the 
  minuteness 
  of 
  the 
  constituents. 
  

  

  (e) 
  Gneisses 
  of 
  the 
  lower 
  part 
  of 
  Val 
  Chisone 
  (p. 
  83). 
  

  

  These 
  are 
  grouped 
  together 
  as 
  having 
  much 
  in 
  common. 
  The 
  

   specimens 
  examined 
  are 
  from 
  (a) 
  near 
  La 
  Mean 
  (?), 
  Commune 
  de 
  

   Bolle, 
  (b) 
  Quarry 
  1| 
  kil. 
  above 
  La 
  Perouse, 
  (c) 
  Large 
  quarry 
  about 
  

   300 
  yards 
  south 
  of 
  San 
  Germano. 
  The 
  chief 
  constituents 
  are 
  quartz, 
  

   felspar, 
  biotite, 
  more 
  or 
  less 
  epidote, 
  and 
  some 
  iron-oxide, 
  with 
  

   garnet 
  and 
  zoisite 
  as 
  accessories. 
  They 
  agree 
  generally 
  in 
  the 
  fresh 
  

   condition 
  of 
  the 
  constituent 
  minerals, 
  and 
  in 
  the 
  absence 
  of 
  signs 
  of 
  

  

  