﻿534 
  

  

  ilAJOK-GEXERAL 
  C. 
  A. 
  M'MAHON 
  OX 
  THE 
  HORNBLENDE- 
  SCHISTS 
  

  

  The 
  eruptive 
  character 
  of 
  this 
  rook 
  is 
  undoubted. 
  At 
  low 
  water, 
  

   during 
  spring-tides, 
  the 
  reefs 
  outside 
  the 
  island 
  at 
  Polpeor 
  are 
  exposed 
  

   for 
  about 
  a 
  mile 
  from 
  the 
  shore. 
  The 
  porphyritic 
  diorite 
  may 
  be 
  

   seen 
  in 
  these 
  reefs 
  cutting 
  right 
  across 
  the 
  strike 
  of 
  the 
  gneiss. 
  

   Nearer 
  land 
  it 
  is 
  an 
  intruder 
  in 
  the 
  green 
  schists, 
  generally 
  following 
  

   their 
  bedding, 
  but 
  constantly 
  shifting 
  from 
  one 
  horizon 
  to 
  another 
  ; 
  

   whilst 
  in 
  the 
  cliffs 
  of 
  the 
  mainland 
  it 
  frequently 
  shows 
  itself 
  at 
  a 
  

   still 
  higher 
  horizon 
  among 
  the 
  micaceous 
  and 
  hornblendic 
  schists. 
  

  

  This 
  rock 
  is 
  a 
  very 
  frequent 
  intruder 
  in 
  the 
  " 
  granulitic" 
  group, 
  

   and 
  as 
  it 
  appeared 
  prior 
  to 
  the 
  last 
  intrusion 
  of 
  granite, 
  and 
  has 
  the 
  

   habit, 
  in 
  this 
  group, 
  of 
  following 
  the 
  lines 
  of 
  bedding, 
  its 
  true 
  cha- 
  

   racter 
  might 
  easily 
  be 
  overlooked 
  by 
  an 
  observer 
  who 
  had 
  not 
  made 
  

   himself 
  familiar 
  with 
  it 
  in 
  localities 
  (like 
  the 
  reef 
  outside 
  Polpeor) 
  

   where 
  its 
  igneous 
  nature 
  is 
  evident. 
  

  

  This 
  diorite 
  is 
  often 
  profusely 
  porphyritic, 
  but 
  sometimes 
  the 
  

   porphyritic 
  crystals 
  of 
  felspar 
  are 
  small 
  or 
  sparse, 
  and 
  occasionally 
  

   they 
  are 
  absent, 
  when 
  it 
  becomes 
  impossible 
  to 
  distinguish 
  it 
  from 
  

   an 
  ordinary 
  diorite. 
  Now 
  and 
  then 
  (as 
  in 
  one 
  or 
  two 
  places 
  in 
  the 
  

   Polpeor 
  cliffs) 
  it 
  becomes 
  foliated. 
  

  

  The 
  following 
  table 
  gives 
  a 
  summary 
  of 
  the 
  microscopic 
  examina- 
  

   tion 
  of 
  several 
  samples 
  of 
  the 
  " 
  porphyritic 
  " 
  and 
  other 
  diorites. 
  

  

  Intrusive 
  Diorites. 
  

  

  11. 
  

   12. 
  

   13. 
  

   14. 
  

  

  Locality 
  from 
  which 
  the 
  

   specimens 
  were 
  taken. 
  

  

  © 
  

  

  

  

  

  

  

  — 
  

  

  

  

  

  

  

  - 
  

  

  

  

  

  

  

  -5 
  

  

  — 
  

  

  si 
  

  

  - 
  

  

  

  +3 
  

  

  © 
  

  

  z 
  

  

  - 
  

  

  ed 
  

  

  o 
  

  

  

  -= 
  

  

  — 
  '. 
  

  

  ~ 
  

  

  Z' 
  

  

  s 
  

  

  <4 
  

  

  m 
  

  

  Diorites. 
  

  

  Polbarrow 
  HF 
  Q 
  i 
  Mi 
  

  

  Between 
  Chough 
  Ogo 
  andi 
  

   Polbarrow* 
  HF 
  

  

  Between 
  Polbarrow 
  and 
  

   Cadgwith 
  H 
  F 
  

  

  Between 
  Caerleon 
  and 
  Ken- 
  

   nack 
  Coves 
  HF 
  

  

  Ap 
  Sph 
  Ma 
  

  

  Ap 
  Sph 
  

  

  Typical 
  'Porphyritic 
  Diorites. 
  

  

  South 
  of 
  KVnance 
  Cove 
  H 
  F 
  

  

  Polpeor 
  Cove 
  H 
  F 
  

  

  ditto 
  H 
  F 
  

  

  Caerleon 
  Cove 
  H 
  F 
  

  

  ditto 
  H 
  F 
  

  

  ditto 
  HF 
  

  

  ; 
  Ap 
  Sph 
  

   Ap 
  

  

  Sph 
  

   Sph 
  

   Sph 
  

  

  Diorites 
  of 
  Doubtful 
  Class. 
  

  

  Kennack 
  Cove 
  

  

  ditto 
  

  

  ditto 
  

  

  ditto* 
  ... 
  

  

  HF 
  

   HF 
  

   HF 
  

   HF 
  

  

  14 
  

  

  14 
  

  

  Mi 
  

  

  Ap 
  

   Ap 
  

  

  Sph 
  

   Sph 
  

   Sph 
  

   Sph 
  

  

  10 
  

  

  Ma 
  

  

  Ma 
  

   Ma 
  

   Ma 
  

   Ma 
  

  

  Remarks. 
  

  

  Chi 
  

  

  In 
  serpentine, 
  

   in 
  apparent 
  

   line 
  of 
  strike 
  

   with 
  No. 
  1. 
  

  

  In 
  serpentine 
  . 
  

  

  An 
  

  

  Chi 
  

  

  Chi 
  

  

  * 
  Parallelism 
  of 
  structure 
  is 
  developed 
  in 
  No. 
  2 
  and 
  No. 
  14. 
  

  

  