﻿626 
  MESSRS. 
  C. 
  I. 
  GARDINER 
  AND 
  S. 
  H. 
  KEYNOLDS 
  ON 
  [Dec. 
  1 
  897, 
  

  

  similar 
  in 
  character 
  to 
  the 
  igneous 
  rocks 
  described 
  above, 
  though 
  

   they 
  are 
  much 
  altered, 
  and, 
  especially 
  to 
  the 
  south-west 
  of 
  the 
  tower, 
  

   are 
  cleaved 
  and 
  much 
  veined 
  with 
  quartz. 
  

  

  The 
  rocks 
  south 
  of 
  the 
  tower 
  were 
  mapped 
  by 
  the 
  officers 
  of 
  the 
  

   Geological 
  Survey 
  as 
  ash 
  ; 
  but, 
  though 
  some 
  ash 
  perhaps 
  occurs, 
  they 
  

   consist 
  mainly 
  of 
  altered 
  lavas. 
  North 
  of 
  the 
  tower 
  they 
  are 
  certainly 
  

   all 
  lavas. 
  In 
  the 
  field 
  they 
  are 
  seen 
  to 
  be 
  green 
  and 
  purple 
  rocks, 
  

   the 
  most 
  marked 
  type 
  being 
  a 
  coarse 
  porphyrite 
  with 
  a 
  purple 
  ground- 
  

   mass 
  and 
  large. 
  green 
  porphyritic 
  felspars. 
  This 
  is 
  described 
  by 
  

   the 
  Geological 
  Survey 
  as 
  being 
  faulted 
  against 
  the 
  rocks 
  to 
  the 
  

   south, 
  the 
  latter 
  being 
  also 
  described 
  as 
  altered 
  when 
  they 
  come 
  in 
  

   contact 
  with 
  the 
  coarse 
  rock, 
  but 
  we 
  found 
  no 
  evidence 
  of 
  either 
  

   the 
  fault 
  or 
  the 
  alteration. 
  

  

  Sections 
  of 
  these 
  rocks 
  show 
  a 
  prominent 
  groundmass, 
  as 
  a 
  

   rule 
  much 
  altered, 
  but 
  showing 
  small 
  acicular 
  felspars 
  and 
  generally 
  

   much 
  iron 
  ore 
  ; 
  ilmenite 
  and 
  magnetite 
  are 
  most 
  plentiful, 
  but 
  

   haematite 
  and 
  pyrites 
  are 
  sometimes 
  abundant, 
  while 
  chalybite 
  

   occurs 
  in 
  one 
  slide. 
  The 
  porphyritic 
  constituents 
  are(l) 
  felspars, 
  

   probably 
  labradorite, 
  generally 
  replaced 
  partly 
  or 
  wholly 
  by 
  epidote, 
  

   calcite, 
  or 
  chlorite 
  — 
  the 
  alteration 
  having 
  gone 
  so 
  far 
  that 
  some- 
  

   times 
  it 
  is 
  barely 
  possible 
  in 
  polarized 
  light 
  to 
  distinguish 
  between 
  

   the 
  porphyritic 
  felspars 
  and 
  the 
  groundmass 
  ; 
  and 
  (2) 
  augites, 
  also 
  

   much 
  altered. 
  

  

  In 
  many 
  of 
  the 
  rocks 
  calcite 
  has 
  come 
  in 
  largely, 
  and 
  often 
  per- 
  

   meates 
  the 
  groundmass 
  as 
  well 
  as 
  fills 
  the 
  vesicles. 
  Other 
  vesicles 
  

   are 
  filled 
  with 
  chlorite 
  or 
  with 
  chlorite 
  and 
  quartz, 
  while 
  some 
  

   contain 
  a 
  mineral 
  which 
  is 
  probably 
  serpentine. 
  

  

  The 
  lavas, 
  then, 
  are, 
  like 
  those 
  at 
  the 
  northern 
  end 
  of 
  the 
  section, 
  

   mainly 
  augite-andesites. 
  

  

  The 
  junction 
  between 
  the 
  igneous 
  and 
  sedimentary 
  rocks 
  is 
  seen 
  

   in 
  the 
  sandy 
  bay 
  south 
  of 
  St. 
  Kenny's 
  Well 
  ; 
  its 
  character 
  is 
  not 
  

   clear, 
  but 
  it 
  is 
  probably 
  a 
  faulted 
  junction, 
  like 
  that 
  between 
  

   the 
  igneous 
  and 
  sedimentary 
  rocks 
  at 
  the 
  northern 
  end 
  of 
  the 
  

   section. 
  It 
  will 
  be 
  described 
  more 
  fully 
  in 
  dealing 
  with 
  the 
  sedi- 
  

   mentary 
  rocks. 
  

  

  (b) 
  Those 
  exposed 
  Inland. 
  

  

  The 
  igneous 
  rocks 
  at 
  the 
  southern 
  end 
  of 
  the 
  section 
  exposed 
  

   inland 
  occur 
  in 
  the 
  neighbourhood 
  of 
  Balcarrick 
  House. 
  They 
  

   consist 
  of 
  augite-andesites, 
  sometimes 
  amygdaloidal, 
  and 
  similar 
  in 
  

   the 
  main 
  to 
  those 
  forming 
  the 
  foreshore, 
  but 
  often 
  in 
  a 
  much 
  fresher 
  

   condition, 
  the 
  porphyritic 
  augites 
  being 
  in 
  some 
  cases 
  specially 
  well 
  

   preserved. 
  In 
  the 
  groundmass, 
  too, 
  fresh 
  augite-granules 
  are 
  well 
  

   seen, 
  and 
  some 
  of 
  the 
  rocks 
  are 
  more 
  nearly 
  holocrystailine 
  than 
  are 
  

   any 
  of 
  those 
  on 
  the 
  foreshore. 
  One 
  rock, 
  grey- 
  green 
  in 
  colour, 
  with 
  

   numerous 
  large 
  augites 
  visible 
  in 
  a 
  hand- 
  specimen, 
  occurs 
  in 
  the 
  

   farmyard 
  of 
  Balcarrick 
  House 
  : 
  this 
  rock, 
  under 
  the 
  microscope, 
  is 
  

   seen 
  to 
  have 
  a 
  not 
  very 
  prominent 
  groundmass, 
  somewhat 
  decom- 
  

   posed, 
  but 
  still 
  showing 
  small 
  felspar-needles 
  and 
  augite-granules. 
  

   Scattered 
  about 
  in 
  this 
  groundmass 
  are 
  numerous 
  porphyritic 
  

  

  