﻿502 
  DR. 
  C. 
  CALLAWAY 
  ON 
  SECONDARY 
  MINERALS 
  IN 
  THE 
  

  

  end 
  they 
  were 
  mainly 
  micaceous 
  schists 
  and 
  bedded 
  quartzites, 
  

   which 
  he 
  regarded 
  as 
  altered 
  sedimentary 
  rocks. 
  There 
  is 
  a 
  difference 
  

   too 
  in 
  the 
  specific 
  gravity 
  of 
  the 
  minerals 
  as 
  we 
  go 
  from 
  north 
  

   to 
  sonth, 
  the 
  percentage 
  of 
  those 
  of 
  higher 
  density 
  being 
  greater 
  

   in 
  the 
  northern 
  parts 
  of 
  the 
  range. 
  There 
  was 
  nothing 
  very 
  remark- 
  

   able 
  in 
  the 
  biotite-gneisses 
  of 
  Swinyard's 
  Hill. 
  Tn 
  cut 
  sections 
  he 
  

   had 
  not 
  seen 
  much 
  evidence 
  of 
  mechanical 
  deformation 
  of 
  eruptive 
  

   rocks, 
  although 
  plenty 
  of 
  decomposition-products 
  were 
  present. 
  

   The 
  crushed 
  material 
  in 
  the 
  so-called 
  shear-zone 
  at 
  West 
  Malvern 
  

   he 
  regarded 
  as 
  possibly 
  a 
  friction-breccia. 
  Some 
  of 
  these 
  zones 
  

   are, 
  very 
  likely, 
  mere 
  dislocations. 
  There 
  was 
  no 
  correspondence 
  

   between 
  the 
  direction 
  of 
  the 
  faults 
  and 
  the 
  strike 
  of 
  the 
  bedding 
  

   and 
  schistosity. 
  He 
  still 
  adhered 
  to 
  his 
  previous 
  conclusions. 
  

  

  Mr. 
  Teall 
  confessed 
  that 
  his 
  ideas 
  on 
  the 
  general 
  question 
  were 
  

   in 
  an 
  inchoate 
  stage. 
  He 
  was 
  much 
  interested 
  in 
  the 
  paper, 
  and 
  

   agreed 
  with 
  many 
  of 
  the 
  points, 
  but 
  should 
  require 
  more 
  detailed 
  

   knowledge 
  of 
  the 
  locality. 
  With 
  regard 
  to 
  shear-zones, 
  he 
  was 
  

   somewhat 
  of 
  Mr. 
  Rutley's 
  opinion 
  ; 
  he 
  was 
  not 
  quite 
  certain 
  that 
  

   he 
  understood 
  the 
  Author. 
  Foliation 
  as 
  evidence 
  of 
  shearing 
  was 
  

   the 
  point 
  under 
  discussion 
  ; 
  but 
  the 
  Author 
  appeared 
  to 
  take 
  it 
  for 
  

   granted. 
  As 
  evidence 
  that 
  the 
  shearing 
  motion 
  has 
  taken 
  place, 
  

   the 
  Author 
  describes 
  the 
  mechanical 
  deformation 
  of 
  minerals 
  ; 
  are 
  

   they 
  invariably 
  found 
  along 
  the 
  zones 
  where 
  foliation 
  occurs 
  ? 
  Many 
  

   of 
  the 
  rocks 
  do 
  not 
  show 
  a 
  trace 
  of 
  shearing 
  of 
  this 
  kind. 
  There 
  

   was 
  no 
  evidence 
  of 
  this 
  tearing 
  asunder 
  of 
  the 
  minerals. 
  The 
  main 
  

   point 
  is, 
  How 
  can 
  we 
  tell, 
  when 
  dealing 
  with 
  holocrystalline 
  rock, 
  

   whether 
  the 
  structure 
  is 
  original 
  or 
  superinduced, 
  so 
  far 
  as 
  micro- 
  

   scopic 
  evidence 
  is 
  concerned, 
  and 
  without 
  reference 
  to 
  the 
  results 
  of 
  

   field-work? 
  As 
  subsidiary 
  points, 
  there 
  was 
  the 
  conversion 
  of 
  

   hornblende 
  into 
  black 
  mica, 
  but 
  he 
  was 
  surprised 
  to 
  hear 
  that 
  this 
  

   had 
  been 
  effected 
  through 
  the 
  intervention 
  of 
  chlorite. 
  He 
  agreed 
  

   that 
  white 
  micas 
  had 
  been 
  derived 
  from 
  the 
  felspars. 
  He 
  referred 
  

   also 
  to 
  the 
  development 
  of 
  granulitic 
  aggregates 
  of 
  quartz 
  in 
  the 
  

   place 
  of 
  felspar, 
  and 
  asked 
  if 
  the 
  Author 
  was 
  sure 
  that 
  the 
  grains 
  

   were 
  quartz 
  and 
  not 
  felspar. 
  

  

  Dr. 
  Hicks 
  agreed 
  with 
  the 
  Author 
  to 
  a 
  certain 
  extent, 
  but 
  he 
  

   failed 
  to 
  see 
  why 
  the 
  original 
  minerals 
  should 
  remain 
  unaltered 
  

   or 
  merely 
  deformed 
  so 
  near 
  the 
  shear-zones, 
  if 
  the 
  general 
  meta- 
  

   morphism 
  was 
  produced 
  by 
  shearing. 
  As 
  to 
  the 
  granite-veins, 
  are 
  

   they 
  really 
  intrusions 
  or, 
  as 
  is 
  more 
  probable, 
  segregation-veins 
  

   only 
  ? 
  If 
  so, 
  this 
  would 
  show 
  that 
  the 
  greatest 
  amount 
  of 
  change 
  

   in 
  these 
  rocks 
  is 
  due 
  to 
  secondary 
  deposition 
  along 
  lines 
  of 
  weak- 
  

   ness. 
  

  

  Prof. 
  Blake 
  was 
  glad 
  to 
  hear 
  that 
  Mr. 
  Teall 
  admitted 
  the 
  existence 
  

   of 
  schists 
  in 
  which 
  there 
  was 
  no 
  evidence 
  of 
  shearing. 
  What 
  

   might 
  be 
  the 
  origin 
  of 
  these 
  parallel 
  structures 
  was 
  quite 
  a 
  different 
  

   question 
  from 
  that 
  of 
  the 
  production 
  of 
  sheared 
  rocks. 
  In 
  the 
  case 
  

   of 
  the 
  latter 
  the 
  amount 
  and 
  direction 
  of 
  the 
  motion 
  that 
  had 
  taken 
  

   place 
  was 
  directly 
  ascertainable 
  by 
  an 
  examination 
  of 
  the 
  position 
  

   of 
  the 
  fragments 
  of 
  the 
  deformed 
  crystals. 
  

  

  