﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  ALPS. 
  \j\) 
  

  

  even 
  if 
  we 
  admit 
  an 
  igneous 
  origin 
  for 
  a 
  largor 
  proportion, 
  and 
  

   assign 
  a 
  more 
  important 
  role 
  to 
  dynamic 
  metamorphism, 
  it 
  is 
  very 
  

   difficult 
  to 
  explain 
  the 
  regular 
  banding 
  of 
  many 
  members, 
  occurring, 
  

   it 
  must 
  be 
  remembered, 
  not 
  locally, 
  but 
  in 
  thick 
  masses 
  over 
  exten- 
  

   sive 
  areas, 
  by 
  anything 
  but 
  an 
  original 
  structure 
  due 
  to 
  some 
  kind 
  

   of 
  stratification. 
  

  

  In 
  the 
  lowest 
  series, 
  the 
  question 
  of 
  origin 
  becomes 
  yet 
  more 
  

   difficult. 
  The 
  foliation 
  of 
  many 
  important 
  masses 
  is 
  very 
  probably 
  

   a 
  superinduced 
  structure, 
  due 
  to 
  subsequent 
  earth-movements. 
  The 
  

   mineral 
  banding 
  in 
  others 
  may 
  result 
  from 
  movements 
  in 
  a 
  plastic 
  

   mass 
  anterior 
  to 
  consolidation, 
  modified 
  also 
  by 
  subsequent 
  dis- 
  

   turbances 
  ; 
  so 
  that 
  an 
  igneous 
  origin 
  is 
  possible 
  for 
  many 
  rocks 
  

   which 
  are 
  lithologically 
  true 
  gneisses. 
  Still 
  the 
  universal 
  appli- 
  

   cation 
  of 
  this 
  hypotheis 
  would, 
  I 
  think, 
  sometimes 
  land 
  us 
  in 
  serious 
  

   difficulties, 
  and 
  I 
  find 
  it 
  difficult 
  to 
  account 
  for 
  certain 
  coarse 
  crys- 
  

   talline 
  limestones 
  which 
  appear 
  to 
  me 
  to 
  graduate, 
  though 
  quickly, 
  

   into 
  coarse 
  mica-schists 
  and 
  gneisses, 
  if 
  they 
  have 
  not 
  originated 
  in 
  

   some 
  kind 
  of 
  sedimentation 
  or 
  precipitation. 
  

  

  In 
  February 
  1886, 
  on 
  resigning 
  the 
  office 
  of 
  President, 
  I 
  laid 
  

   before 
  you 
  the 
  conclusions 
  in 
  regard 
  to 
  the 
  crystalline 
  schists, 
  at 
  

   which 
  I 
  had 
  arrived 
  after 
  some 
  years 
  of 
  special 
  study. 
  The 
  result 
  

   of 
  the 
  work 
  described 
  in 
  this 
  paper, 
  supplemented 
  by 
  visits 
  to 
  Corn- 
  

   wall, 
  Brittany, 
  Normandy, 
  and 
  the 
  Channel 
  Islands, 
  and 
  the 
  study 
  

   of 
  specimens 
  from 
  other 
  localities, 
  while 
  it 
  has 
  cleared 
  up 
  some 
  

   difficulties, 
  and 
  removed 
  some 
  minor 
  misconceptions, 
  has 
  confirmed 
  

   the 
  main 
  conclusions 
  which 
  I 
  then 
  expressed 
  as 
  to 
  the 
  Archaean 
  age 
  

   of 
  the 
  gneisses 
  and 
  schists 
  which 
  are 
  included 
  under 
  the 
  three 
  

   types 
  described 
  in 
  this 
  paper. 
  While 
  I 
  quite 
  admit 
  the 
  possibility 
  

   and, 
  in 
  some 
  cases, 
  the 
  probability 
  that 
  many 
  of 
  them, 
  especially 
  

   among 
  the 
  more 
  coarsely 
  crystalline, 
  are 
  modified 
  igneous 
  rocks, 
  

   I 
  am 
  of 
  opinion 
  that 
  where 
  the 
  modification 
  has 
  been 
  followed 
  by 
  

   complete 
  recrystallization, 
  it 
  goes 
  back, 
  as 
  a 
  rule, 
  to 
  Precambrian 
  

   ages 
  : 
  so 
  that, 
  even 
  if 
  the 
  rocks 
  have 
  become 
  schists 
  in 
  consequence 
  

   of 
  mechanical 
  stresses, 
  we 
  may 
  rightly 
  speak 
  of 
  them 
  as 
  Archaean 
  

   schists. 
  This, 
  at 
  any 
  rate, 
  is 
  the 
  result 
  of 
  my 
  studies 
  in 
  the 
  Alps 
  : 
  — 
  

   that 
  while 
  the 
  earth-movements 
  to 
  which 
  these 
  crystalline 
  rocks 
  

   have 
  been 
  subjected, 
  not 
  only 
  during 
  the 
  mountain-making 
  of 
  the 
  

   existing 
  chain, 
  but 
  also 
  in 
  the 
  interval 
  between 
  the 
  Trias 
  and 
  the 
  

   Carboniferous 
  periods, 
  and 
  probably 
  anterior 
  to 
  the 
  latter 
  (the 
  

   first, 
  at 
  the 
  present 
  day, 
  being 
  the 
  most 
  conspicuous), 
  have 
  modified 
  

   the 
  structures, 
  and 
  in 
  some 
  cases 
  have 
  blotted 
  out 
  all 
  record 
  of 
  the 
  

   previous 
  history 
  of 
  the 
  rock, 
  the 
  latter 
  is 
  an 
  occurrence 
  comparatively 
  

   rare 
  and 
  local. 
  To 
  learn 
  to 
  read 
  this 
  palimpsest 
  of 
  Nature's 
  stone 
  

   book 
  is, 
  no 
  doubt, 
  a 
  task 
  which 
  requires 
  long 
  and 
  laborious 
  work. 
  

   Study 
  of 
  the 
  rock-masses 
  in 
  the 
  field 
  over 
  extensive 
  districts, 
  study 
  

   of 
  carefully 
  selected 
  specimens 
  with 
  the 
  microscope 
  are 
  indispen- 
  

   sable. 
  We 
  must 
  remember 
  that, 
  like 
  those 
  who 
  are 
  engaged 
  in 
  deci- 
  

   phering 
  ancient 
  inscriptions 
  and 
  reconstructing 
  an 
  ancient 
  language, 
  

   we 
  are 
  feeling 
  our 
  way, 
  and 
  are 
  in 
  constant 
  danger, 
  owing 
  to 
  the 
  

   imperfection 
  of 
  our 
  data, 
  of 
  drawing 
  incorrect 
  conclusions 
  ; 
  neverthe- 
  

  

  h2 
  

  

  