﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  ALPS. 
  77 
  

  

  the 
  valley 
  descending 
  from 
  the 
  Breche 
  gneiss 
  may 
  often 
  be 
  seen 
  in 
  

   situ. 
  This 
  is 
  a 
  strong 
  grey 
  gneiss 
  with 
  a 
  fairly 
  well-defined 
  banding 
  

   of 
  lighter 
  and 
  darker 
  rock, 
  the 
  colour 
  being 
  due 
  to 
  the 
  comparative 
  

   absence 
  or 
  presence 
  of 
  a 
  ferro-magnesian 
  mica. 
  The 
  bands 
  are 
  often 
  

   from 
  a 
  quarter 
  to 
  half 
  an 
  inch 
  thick, 
  and 
  the 
  rock 
  usually 
  breaks 
  

   without 
  difficulty 
  across 
  them. 
  It 
  closely 
  resembles 
  in 
  all 
  respects 
  

   many 
  of 
  the 
  Laurentian 
  gneisses 
  of 
  Canada 
  and 
  the 
  Hebridean 
  (mica) 
  

   gneisses 
  of 
  North-west 
  Scotland 
  (Appendix, 
  p. 
  102). 
  Probably 
  it 
  

   has 
  been 
  somewhat 
  modified 
  by 
  pressure 
  followed 
  by 
  recementation 
  ; 
  

   but 
  I 
  cannot 
  attribute 
  the 
  mineral-banding 
  to 
  this 
  cause, 
  and 
  believe 
  

   that 
  whatever 
  may 
  be 
  its 
  true 
  explanation, 
  the 
  rock 
  prior 
  to 
  the 
  

   deposition 
  of 
  the 
  Jurassics 
  was 
  a 
  banded 
  gneiss 
  of 
  Laurentian 
  aspect. 
  

   Among 
  the 
  debris 
  fragments 
  of 
  granitoid 
  rock 
  are 
  commoner 
  than 
  

   those 
  of 
  the 
  gneiss. 
  More 
  than 
  one 
  variety 
  is 
  present, 
  the 
  com- 
  

   monest 
  being 
  a 
  moderately 
  coarse-grained 
  rock, 
  often 
  in 
  good 
  pre- 
  

   servation, 
  with 
  a 
  slightly 
  pink-coloured 
  felspar 
  and 
  with 
  green 
  spots 
  ; 
  

   indeed 
  it 
  resembles 
  some 
  of 
  the 
  St. 
  David's 
  (so-called) 
  Dimetian 
  *. 
  It 
  

   exhibits 
  but 
  little 
  sign 
  of 
  mechanical 
  disturbance. 
  The 
  other 
  (less 
  

   common) 
  is 
  a 
  coarser 
  rock, 
  occasionally 
  slightly 
  porphyritic, 
  with 
  

   some 
  of 
  its 
  felspar 
  of 
  a 
  pinkish, 
  some 
  of 
  a 
  greenish 
  hue. 
  This 
  also 
  

   appears 
  to 
  be 
  a 
  granite, 
  though 
  it 
  is 
  a 
  little 
  modified 
  by 
  pressure, 
  

   which 
  may 
  be 
  a 
  record 
  of 
  the 
  great 
  upheaval. 
  

  

  In 
  the 
  glen 
  of 
  the 
  Romanche 
  the 
  crystalline 
  rock 
  rises 
  sharply 
  

   from 
  beneath 
  the 
  slate 
  in 
  the 
  face 
  of 
  a 
  steep 
  wall, 
  which 
  forms 
  a 
  

   kind 
  of 
  step 
  in 
  the 
  level. 
  This 
  too 
  is 
  a 
  gneiss 
  of 
  the 
  type 
  already 
  

   described, 
  and 
  among 
  the 
  debris 
  brought 
  down 
  from 
  the 
  crags 
  around 
  

   the 
  upper 
  glens 
  we 
  find 
  many 
  representatives 
  of 
  a 
  coarse 
  gneissoid 
  

   rock, 
  varying 
  from 
  a 
  moderately 
  fine 
  grey 
  rock 
  with 
  but 
  little 
  mica, 
  

   to 
  a 
  rather 
  coarser 
  rock 
  with 
  dark 
  bands, 
  which 
  contain 
  a 
  consider- 
  

   able 
  amount 
  of 
  black 
  mica. 
  In 
  the 
  latter, 
  the 
  bands 
  often 
  exhibit 
  

   contortion, 
  and 
  though 
  I 
  recognize 
  the 
  effects 
  of 
  pressure, 
  this 
  

   appears 
  to 
  me 
  to 
  have 
  affected 
  a 
  rock 
  which 
  had 
  already 
  acquired 
  a 
  

   banded 
  structure. 
  Here 
  also 
  the 
  rock 
  can 
  commonly 
  be 
  broken 
  

   across 
  the 
  mineral 
  bands. 
  Blocks 
  of 
  granitic 
  rock 
  are 
  also 
  abun- 
  

   dant, 
  varieties 
  of 
  the 
  types 
  already 
  mentioned; 
  but 
  here 
  the 
  coarser 
  

   and 
  somewhat 
  porphyritic 
  rock 
  is 
  commoner 
  than 
  in 
  the 
  glen 
  of 
  

   the 
  Meije 
  glacier. 
  These 
  granitoid 
  rocks 
  are 
  the 
  typical 
  protogine 
  of 
  

   the 
  region 
  : 
  the 
  felspar 
  varies 
  from 
  a 
  tolerably 
  distinct 
  red 
  to 
  almost 
  

   colourless 
  ; 
  the 
  brown 
  mica 
  is 
  converted 
  more 
  or 
  less 
  perfectly 
  into 
  

   a 
  green 
  mineral, 
  often 
  called 
  chlorite, 
  and 
  a 
  greenish 
  film 
  usually 
  

   coats 
  the 
  joint-faces. 
  

  

  I 
  also 
  examined 
  the 
  slaty 
  rocks 
  of 
  the 
  Jurassic 
  series 
  with 
  con- 
  

   siderable 
  attention, 
  in 
  the 
  hopes 
  of 
  finding 
  indications 
  of 
  incipient 
  

  

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

   plagioclase 
  (in 
  part 
  at 
  least 
  oligoclase), 
  and 
  some 
  biotite, 
  which 
  has 
  generally- 
  

   been 
  replaced 
  by 
  a 
  green 
  mineral, 
  with 
  separation 
  of 
  black 
  iron 
  oxide 
  along 
  the 
  

   cleavage-planes. 
  There 
  are 
  also 
  one 
  or 
  two 
  original 
  grains 
  of 
  magnetite. 
  The 
  

   rock 
  exhibits 
  some 
  indications 
  of 
  mechanical 
  disturbance 
  subsequent 
  to 
  its 
  first 
  

   consolidation 
  ; 
  but 
  the 
  structures 
  characteristic 
  of 
  a 
  rock 
  of 
  igneous 
  origin 
  are 
  

   generally 
  well 
  retained, 
  so 
  that 
  the 
  rock 
  is 
  beyond 
  doubt 
  a 
  true 
  granite, 
  perhaps 
  

   rather 
  richer 
  in 
  plagioclase 
  than 
  in 
  orthoclase. 
  

  

  