﻿522 
  MESSRS. 
  C. 
  I. 
  GARDINER 
  AND 
  S. 
  H. 
  REYNOLDS 
  ON 
  [Dec. 
  1 
  897, 
  

  

  the 
  farm 
  at 
  the 
  north-eastern 
  corner 
  of 
  the 
  area. 
  These 
  rocks 
  

   in 
  a 
  hand-specimen 
  are 
  seen 
  to 
  have 
  a 
  compact 
  groundmass, 
  and 
  

   frequently 
  show 
  porphyritic 
  felspars 
  of 
  no 
  great 
  size. 
  They 
  are 
  

   often 
  slightly 
  and 
  sometimes 
  extremely 
  amygdaloidal, 
  the 
  amygdules 
  

   being 
  formed 
  of 
  pink 
  calcite. 
  Veins 
  of 
  epidote 
  are 
  of 
  frequent 
  

   occurrence, 
  and 
  jaspers 
  are 
  numerous 
  at 
  certain 
  spots. 
  

  

  Some 
  twenty 
  sections 
  of 
  rocks 
  from 
  this 
  portion 
  of 
  the 
  coast 
  have 
  

   been 
  examined 
  microscopically. 
  They 
  are 
  all 
  seen 
  to 
  be 
  from 
  

   andesites 
  which 
  have 
  undergone 
  considerable 
  alteration, 
  accompauied 
  

   by 
  the 
  formation 
  of 
  various 
  secondary 
  minerals. 
  

  

  The 
  groundmass 
  is 
  always 
  fairly 
  prominent, 
  and 
  in 
  some 
  cases 
  

   very 
  prominent 
  indeed 
  ; 
  it 
  is 
  invariably 
  much 
  altered, 
  its 
  original 
  

   constituents 
  being 
  replaced 
  by 
  chlorite 
  and 
  epidote. 
  In 
  many 
  

   sections 
  the 
  groundmass 
  contains 
  numerous 
  needle-shaped 
  crystals 
  

   of 
  felspar 
  which 
  extinguish 
  approximately 
  parallel 
  to 
  their 
  lengths, 
  

   and 
  occasionally 
  show 
  well-marked 
  flow-structure. 
  Iron-ores 
  are 
  

   abundant, 
  some 
  slides 
  showing 
  magnetite, 
  others 
  leucoxene 
  or 
  

   haematite, 
  while 
  in 
  some 
  iron 
  pyrites 
  occurs 
  as 
  well. 
  

  

  The 
  porphyritic 
  constituents 
  are 
  (1) 
  felspars, 
  probably 
  labra- 
  

   dorites, 
  occurring 
  as 
  fair-sized 
  crystals, 
  which, 
  though 
  much 
  altered, 
  

   worn, 
  and 
  replaced, 
  still 
  at 
  times 
  show 
  twinning; 
  (2) 
  augites, 
  

   now 
  entirely 
  replaced 
  by 
  calcite 
  or 
  by 
  a 
  brown 
  mineral 
  traversed 
  

   by 
  veins 
  of 
  chlorite; 
  and 
  (3) 
  apatite, 
  which 
  has 
  been 
  observed 
  

   in 
  only 
  one 
  section 
  of 
  a 
  rock 
  from 
  this 
  area, 
  though 
  it 
  has 
  been 
  

   frequently 
  found 
  in 
  similar 
  andesitic 
  rocks 
  occurring 
  on 
  Lambay 
  

   Island. 
  

  

  The 
  amygdules, 
  as 
  previously 
  mentioned, 
  are 
  formed 
  for 
  the 
  

   most 
  part 
  of 
  calcite, 
  but 
  some 
  contain 
  chlorite 
  enclosing 
  little 
  

   patches 
  of 
  silica. 
  

  

  Immediately 
  north 
  of 
  the 
  farm, 
  at 
  the 
  point 
  where 
  the 
  coast-line 
  

   turns 
  to 
  the 
  south, 
  two 
  somewhat 
  different 
  types 
  of 
  rock 
  occur, 
  one 
  

   of 
  which 
  probably 
  forms 
  a 
  dyke 
  in 
  the 
  andesites, 
  as 
  its 
  strike 
  does 
  

   not 
  run 
  in 
  the 
  same 
  direction 
  as 
  do 
  those 
  of 
  the 
  andesites. 
  

  

  The 
  dyke, 
  a 
  foot 
  or 
  so 
  in 
  breadth, 
  runs 
  in 
  an 
  east-and-west 
  

   direction, 
  is 
  of 
  a 
  light 
  green 
  colour, 
  and 
  shows 
  large, 
  greenish-white, 
  

   platy 
  felspars. 
  It 
  is 
  identical 
  in 
  character 
  with 
  some 
  of 
  the 
  

   Lambay 
  porphyrites, 
  which 
  are 
  sometimes 
  intrusive 
  and 
  sometimes 
  

   interbedded. 
  Under 
  the 
  microscope 
  it 
  shows 
  an 
  extensive 
  ground- 
  

   mass, 
  composed 
  of 
  a 
  yellow 
  decomposition-product 
  and 
  felspar- 
  

   microlites, 
  and 
  porphyritic 
  felspars, 
  fairly 
  fresh 
  but 
  much 
  worn 
  at 
  

   their 
  edges. 
  

  

  The 
  second 
  rock, 
  whose 
  strike, 
  like 
  those 
  of 
  the 
  other 
  andesites, 
  is 
  

   south-west 
  and 
  north-east, 
  is 
  noticeable 
  from 
  the 
  very 
  large 
  

   amount 
  of 
  red 
  iron-oxide 
  which 
  is 
  aggregated 
  round 
  the 
  felspars 
  

   and 
  vesicles. 
  Small 
  felspars 
  are 
  to 
  be 
  seen 
  in 
  the 
  groundmass, 
  

   most 
  of 
  them 
  much 
  altered. 
  Porphyriiic 
  felspars 
  are 
  scarce, 
  while 
  

   patches 
  of 
  a 
  light 
  orange-brown 
  material 
  probably 
  represent 
  por- 
  

   phyritic 
  augites. 
  

  

  A 
  little 
  farther 
  on, 
  just 
  at 
  the 
  east 
  side 
  of 
  the 
  corner, 
  is 
  a 
  second 
  

  

  