﻿igneous 
  rocks 
  after 
  their 
  consolidation. 
  185 
  

  

  8. 
  Bearing 
  of 
  these 
  considerations 
  on 
  the 
  problem 
  of 
  the 
  Origin 
  

   of 
  Foliation 
  in 
  the 
  Metamorphic 
  Hocks. 
  

  

  It 
  is 
  scarcely 
  necessary 
  to 
  point 
  out 
  the 
  important 
  "bearing 
  of 
  this 
  

   principle 
  of 
  the 
  growth 
  of 
  crystals 
  — 
  and 
  especially 
  of 
  felspar-crystals 
  

   — 
  in 
  solid 
  rock-masses 
  upon 
  the 
  great 
  question 
  of 
  the 
  origin 
  of 
  folia- 
  

   tion 
  in 
  rocks, 
  a 
  question 
  in 
  the 
  discussion 
  of 
  which 
  the 
  Fellows 
  of 
  

   this 
  Society 
  have 
  taken 
  so 
  prominent 
  a 
  part. 
  

  

  That, 
  as 
  the 
  result 
  of 
  contact-metamorphism, 
  many 
  well-defined 
  

   mineral 
  species 
  are 
  developed 
  in 
  the 
  midst 
  of 
  solid 
  rocks, 
  the 
  crystals 
  

   growing 
  at 
  the 
  expense 
  of 
  and 
  deriving 
  their 
  materials 
  from 
  the 
  

   surrounding 
  detrital 
  fragments, 
  has 
  long 
  been 
  recognized. 
  These 
  

   phenomena 
  find 
  many 
  beautiful 
  illustrations 
  in 
  the 
  so-called 
  "spotted 
  

   schists." 
  I 
  hope 
  to 
  be 
  able 
  to 
  show, 
  on 
  a 
  future 
  occasion, 
  how 
  large 
  

   a 
  part 
  a 
  similar 
  action 
  plays 
  in 
  producing 
  the 
  characteristic 
  structures 
  

   of 
  many 
  fresh 
  and 
  apparently 
  unaltered 
  igneous 
  rocks. 
  

  

  The 
  founders 
  of 
  the 
  theory 
  of 
  dynamo-metamorphism 
  — 
  Scrope 
  

   and 
  Darwin 
  — 
  very 
  clearly 
  perceived 
  that 
  in 
  the 
  study 
  of 
  igneous 
  

   rock-masses 
  which 
  have 
  been 
  subjected 
  to 
  movements 
  and 
  internal 
  

   stresses, 
  we 
  find 
  alike 
  the 
  clearest 
  analogies 
  and 
  the 
  simplest 
  and 
  

   most 
  readily 
  studied 
  examples 
  of 
  the 
  processes 
  which 
  go 
  on 
  during 
  

   the 
  production 
  of 
  foliation 
  in 
  rocks. 
  Those 
  who 
  have 
  done 
  most 
  

   towards 
  establishing 
  the 
  theory 
  on 
  its 
  present 
  firm 
  basis, 
  by 
  tracing 
  

   with 
  the 
  aid 
  of 
  the 
  microscope 
  the 
  actual 
  changes 
  which 
  the 
  minerals 
  of 
  

   rocks 
  undergo 
  while 
  the 
  development 
  of 
  foliation 
  is 
  in 
  progress 
  — 
  and 
  

   I 
  especially 
  refer 
  to 
  the 
  beautiful 
  researches 
  of 
  Lossen 
  and 
  Lehmann 
  

   and 
  those 
  of 
  students 
  of 
  petrography 
  who, 
  like 
  Dr. 
  Reusch 
  in 
  

   Scandinavia, 
  Prof. 
  Williams 
  in 
  America, 
  and 
  Mr. 
  Teallin 
  this 
  country, 
  

   have 
  sought 
  to 
  follow 
  in 
  their 
  steps 
  — 
  have 
  skilfully 
  and 
  patiently 
  

   pursued 
  the 
  same 
  methods. 
  

  

  Charles 
  Darwin 
  was 
  able 
  to 
  show, 
  by 
  the 
  aid 
  of 
  a 
  pocket-lens 
  and 
  

   blowpipe 
  only, 
  that 
  in 
  a 
  noncrystalline 
  lava 
  in 
  the 
  Island 
  of 
  

   Ascension 
  which 
  had 
  been 
  subjected 
  to 
  powerful 
  internal 
  mechanical 
  

   stresses, 
  the 
  felspar, 
  augite, 
  and 
  magnetite 
  had 
  separated 
  from 
  the 
  

   glassy 
  mass 
  in 
  distinct 
  and 
  parallel 
  folia 
  *. 
  He 
  clearly 
  perceived, 
  

   what 
  some 
  in 
  more 
  recent 
  years 
  have 
  quite 
  failed 
  to 
  realize, 
  that 
  

   mechanical 
  force 
  'per 
  se 
  is 
  wholly 
  incompetent 
  to 
  produce 
  a 
  true 
  

   foliation 
  in 
  rocks 
  ; 
  but 
  that 
  the 
  mechanical 
  force 
  becomes 
  really 
  

   effective 
  by 
  determining 
  and 
  controlling 
  the 
  operations 
  of 
  chemical 
  

   affinity 
  and 
  crystallization; 
  and 
  it 
  is 
  by 
  these 
  that 
  the 
  metamor- 
  

   phoses 
  of 
  the 
  minerals 
  in 
  rocks 
  are 
  brought 
  about 
  which 
  result 
  in 
  

   the 
  development 
  of 
  the 
  foliated 
  structure. 
  

  

  The 
  more 
  carefully 
  these 
  metamorphoses 
  of 
  the 
  minerals 
  in 
  rocks 
  

   are 
  studied, 
  and 
  the 
  more 
  clearly 
  it 
  is 
  perceived 
  that 
  the 
  whole 
  

   structure 
  of 
  deep-seated 
  rocks 
  may 
  undergo 
  a 
  complete 
  transforma- 
  

   tion, 
  during 
  such 
  metamorphoses 
  of 
  their 
  constituent 
  minerals, 
  the 
  

   less 
  difficulty 
  will 
  be 
  felt 
  in 
  accepting 
  the 
  teachings 
  of 
  Scrope 
  and 
  

  

  * 
  Darwin's 
  ' 
  Volcanic 
  Islands,' 
  published 
  in 
  1844, 
  p. 
  56 
  (p. 
  64 
  of 
  reprint 
  issued 
  

   in 
  1876). 
  I 
  have 
  studied 
  Darwin's 
  original 
  specimens 
  with 
  the 
  microscope, 
  and 
  

   can 
  confirm 
  the 
  wonderful 
  accuracy 
  and 
  acumen 
  of 
  his 
  researches. 
  

  

  