﻿HORNBLENDE-SCHISTS 
  ETC. 
  OF 
  THE 
  LIZARD. 
  519 
  

  

  33. 
  Notes 
  on 
  the 
  Hornblende-schists 
  and 
  Banded 
  Crystalline 
  

   Rocks 
  of 
  the 
  Lizard. 
  By 
  Major-General 
  C. 
  A. 
  M 
  c 
  Mahon, 
  

   F.G.S. 
  (Read 
  May 
  22, 
  1889.) 
  

  

  I 
  haye 
  visited 
  the 
  Lizard 
  on 
  three 
  occasions, 
  namely, 
  in 
  1887 
  in 
  

   company 
  with 
  several 
  Members 
  of 
  the 
  Geologists' 
  Association 
  under 
  

   the 
  able 
  guidance 
  of 
  Messrs. 
  Howard 
  Fox, 
  F.G.S., 
  and 
  E. 
  A. 
  Wiinsch, 
  

   F.G.S. 
  ; 
  in 
  1888 
  by 
  myself; 
  and 
  in 
  .February 
  of 
  the 
  present 
  year 
  

   in 
  company 
  with 
  Mr. 
  Howard 
  Fox, 
  to 
  whom 
  I 
  feel 
  under 
  the 
  

   greatest 
  obligation, 
  his 
  intimate 
  acquaintance 
  with 
  the 
  Lizard 
  

   rocks 
  and 
  his 
  local 
  knowledge 
  rendering 
  his 
  aid 
  in 
  the 
  field 
  ex- 
  

   tremely 
  valuable. 
  

  

  On 
  each 
  occasion 
  I 
  collected 
  numerous 
  specimens 
  of 
  the 
  rocks, 
  and 
  

   studied 
  thin 
  slices 
  of 
  them 
  under 
  the 
  microscope 
  during 
  the 
  intervals 
  

   between 
  my 
  visits, 
  so 
  that 
  I 
  have 
  had 
  an 
  opportunity 
  of 
  correcting 
  

   the 
  impressions 
  made 
  in 
  the 
  field 
  by 
  microscopic 
  work 
  in 
  the 
  study, 
  

   and 
  of 
  again 
  testing 
  ideas 
  formed 
  in 
  the 
  study 
  by 
  fresh 
  observations 
  

   in 
  the 
  field. 
  I 
  have 
  examined 
  under 
  the 
  microscope 
  over 
  ninety 
  

   thin 
  slices 
  of 
  my 
  own 
  specimens, 
  nearly 
  fifty 
  have 
  been 
  lent 
  me 
  by 
  

   Mr. 
  Howard 
  Pox, 
  and 
  Mr. 
  J. 
  J. 
  Harris 
  Teali 
  kindly 
  placed 
  those 
  of 
  

   his 
  own 
  collection 
  at 
  my 
  disposal. 
  

  

  Before 
  stating 
  the 
  results 
  of 
  my 
  own 
  investigations 
  I 
  think 
  it 
  desir- 
  

   able 
  to 
  refer 
  briefly 
  to 
  the 
  views 
  expressed 
  by 
  previous 
  observers. 
  

   Sir 
  Henry 
  De 
  la 
  Beche's 
  opinion 
  may 
  be 
  gathered 
  from 
  the 
  following 
  

   extract 
  from 
  his 
  ' 
  Report 
  on 
  the 
  Geology 
  of 
  Cornwall 
  and 
  Devon 
  ' 
  * 
  : 
  — 
  

   "If 
  it 
  were 
  not 
  for 
  the 
  occurrence 
  of 
  the 
  hornblende 
  slate 
  in 
  the 
  

   conglomerate 
  of 
  the 
  Nare 
  Point, 
  we 
  might 
  suppose 
  that 
  it 
  [the 
  horn- 
  

   blende 
  slate] 
  was 
  a 
  mass 
  of 
  that 
  trappean 
  or 
  ancient 
  volcanic 
  ash 
  

   which 
  is 
  detected 
  so 
  abundantly 
  amid 
  the 
  grauwacke 
  of 
  Devon 
  and 
  

   Cornwall, 
  upon 
  which 
  the 
  mass 
  of 
  serpentine 
  and 
  diallage 
  rock, 
  now 
  

   nearly 
  covering 
  it 
  up, 
  has 
  been 
  poured 
  in 
  a 
  melted 
  state 
  ; 
  and 
  that 
  

   being 
  thus 
  retained 
  long 
  beneath 
  it 
  in 
  a 
  heated 
  condition, 
  the 
  water 
  

   amid 
  its 
  loose 
  laminse 
  prevented 
  from 
  escape 
  upwards 
  by 
  the 
  hot 
  

   rock 
  above, 
  the 
  hornblendic 
  and 
  felspathic 
  particles 
  ol 
  which 
  it 
  was 
  

   composed 
  arranged 
  themselves 
  into 
  crystalline 
  forms, 
  the 
  mass 
  retain- 
  

   ing 
  its 
  original 
  laminated 
  structure. 
  ... 
  It 
  must, 
  however, 
  be 
  

   confessed 
  that, 
  unless 
  we 
  suppose 
  the 
  hornblende 
  slate, 
  in 
  the 
  

   conglomerate 
  of 
  the 
  Nare 
  Point, 
  to 
  be 
  derived 
  from 
  some 
  other 
  source, 
  

   this 
  explanation 
  is 
  not 
  so 
  good 
  as 
  could 
  be 
  desired." 
  

  

  The 
  blocks 
  in 
  the 
  conglomerate 
  alluded 
  to 
  are 
  said 
  by 
  De 
  la 
  Beche, 
  

   in 
  another 
  place 
  (Report, 
  p. 
  94), 
  to 
  be 
  " 
  generally 
  decomposed," 
  and 
  

   his 
  correlation 
  needs 
  confirmation, 
  especially 
  by 
  the 
  modern 
  methods 
  

   of 
  microscopic 
  investigation 
  ; 
  but 
  even 
  if 
  the 
  rocks 
  which 
  supplied 
  

   the 
  boulders 
  were 
  satisfactorily 
  shown 
  to 
  be 
  the 
  hornblende- 
  

  

  * 
  Report, 
  p. 
  34. 
  

  

  