﻿HATJGHTON 
  — 
  GRANITES 
  OF 
  IRELAND. 
  

  

  279 
  

  

  (a) 
  Oligoclase 
  * 
  from 
  Ytterby 
  t, 
  Sweden, 
  

  

  

  Per-centage. 
  

  

  

  Atoms 
  

  

  Silica 
  

  

  . 
  . 
  . 
  62-66 
  

  

  

  1415 
  

  

  Alumina 
  

  

  23-45 
  

  

  

  451 
  

  

  

  3-53 
  

  

  . 
  . 
  . 
  126 
  1 
  

  

  

  

  005 
  

  

  2 
  

  

  417 
  

  

  Soda 
  

  

  7-91 
  

  

  255 
  f 
  

  

  

  Potash 
  

  

  1-59 
  

  

  34 
  J 
  

  

  

  100-19 
  

  

  (b) 
  Black 
  Mica 
  of 
  Swedish 
  Granite 
  and 
  Gneiss. 
  

  

  

  No. 
  1. 
  

  

  No. 
  2. 
  

  

  

  39-70 
  

   12-25 
  

   23-55 
  

  

  0-96 
  

   1-00 
  

   4-48 
  

   7-25 
  

   0-47 
  

   7-30 
  

   1-00 
  

  

  32-60 
  

   15-50 
  • 
  

   2794 
  

   7-45 
  

  

  0-80 
  

   115 
  

   4-79 
  

  

  0-82 
  

   4-30 
  

   6-80 
  

  

  

  Iron, 
  peroxide 
  

  

  

  

  

  

  Soda 
  

  

  Potash 
  

  

  Loss 
  bv 
  ienition 
  

  

  t/ 
  D 
  

  

  Totals 
  

  

  97-96 
  

  

  102-21 
  

  

  

  No. 
  1. 
  Jonesed. 
  — 
  From 
  the 
  black 
  micaceous 
  gneiss 
  of 
  Jonesecl, 
  near 
  the 
  Troll- 
  

   hatta 
  Falls 
  : 
  this 
  gneiss 
  is 
  composed 
  of 
  red 
  felspar 
  and 
  black 
  mica, 
  

   and 
  is 
  much 
  contorted 
  even 
  in 
  hand-specimens 
  ; 
  the 
  direction 
  of 
  its 
  

   bedding 
  coincides 
  at 
  the 
  Falls 
  with 
  that 
  of 
  the 
  river, 
  W. 
  30° 
  S. 
  (mag.). 
  

  

  No. 
  2. 
  Ytterby. 
  — 
  Occurs 
  in 
  the 
  black 
  micaceous 
  partings 
  already 
  described 
  ; 
  

   in 
  large 
  fiat 
  crystalline 
  sheets. 
  

  

  (c) 
  White 
  Mica 
  of 
  Swedish 
  Granites. 
  

  

  The 
  specimen 
  of 
  white 
  mica 
  that 
  I 
  analyzed 
  was 
  taken 
  from 
  

   a 
  large 
  mass 
  found 
  by 
  me 
  in 
  the 
  quarry 
  at 
  Ytterby, 
  within 
  a 
  foot 
  

   of 
  the 
  black 
  sheets 
  of 
  mica. 
  It 
  had 
  the 
  following 
  composition 
  : 
  — 
  

  

  * 
  Throughout 
  the 
  entire 
  mass 
  of 
  the 
  large 
  crystals 
  of 
  oligoclase 
  examined, 
  

   minute 
  specks 
  of 
  quartz 
  were 
  occasionally 
  visible 
  — 
  a 
  circumstance 
  which 
  seems 
  

   to 
  me 
  irreconcileable 
  with 
  the 
  supposition 
  of 
  the 
  formation 
  of 
  this 
  oligoclase 
  by 
  

   fusion, 
  in 
  the 
  dry 
  way. 
  

  

  t 
  The 
  celebrated 
  quarry 
  on 
  the 
  Island 
  of 
  Ytterby, 
  near 
  Waxholm, 
  has 
  been 
  

   worked 
  for 
  upwards 
  of 
  half 
  a 
  century 
  for 
  cbina-felspar. 
  It 
  consists 
  of 
  a 
  dyke 
  

   12 
  feet 
  wide, 
  vertical, 
  and 
  bearing 
  20° 
  east 
  of 
  north 
  (mag.). 
  This 
  dyke 
  is 
  filled 
  

   with 
  the 
  constituent 
  minerals 
  of 
  the 
  granite 
  developed 
  on 
  a 
  grand 
  scale, 
  the 
  milky 
  

   quartz 
  and 
  white 
  oligoclase 
  preponderating, 
  sometimes 
  in 
  masses 
  upwards 
  of 
  a 
  

   foot 
  in 
  length 
  ; 
  and 
  pink 
  orthoclase, 
  in 
  large 
  tabular 
  crystals, 
  is 
  intimately 
  mixed 
  

   up 
  with 
  them. 
  The 
  black 
  and 
  white 
  micas 
  are 
  developed 
  in 
  parallel 
  sheets 
  tra- 
  

   versing 
  the 
  dyke 
  obliquely 
  ; 
  the 
  sheets 
  of 
  black 
  mica 
  are 
  from 
  ^ 
  inch 
  to 
  2 
  inches 
  

   asunder, 
  the 
  intervals 
  being 
  filled 
  with 
  orthoclase 
  and 
  oligoclase. 
  So 
  far 
  as 
  I 
  

   could 
  judge, 
  the 
  rare 
  minerals 
  of 
  the 
  quarry, 
  yttrite. 
  yttrotantalite, 
  phosphate 
  

   of 
  yttria, 
  and 
  others, 
  were 
  only 
  developed 
  in 
  the 
  felspar 
  between 
  the 
  sheets 
  of 
  

   black 
  mica. 
  

  

  VOL. 
  xx. 
  

  

  -PART 
  I. 
  

  

  I' 
  

  

  