﻿372 
  MISS 
  C. 
  A. 
  KAISI2* 
  ON 
  THE 
  PETROLOGY 
  [Aug. 
  1 
  89 
  7 
  

  

  include 
  grains 
  of 
  felspar, 
  sometimes 
  probably 
  a 
  lime-pyroxene 
  like 
  

   the 
  picrite 
  of 
  St. 
  David's. 
  1 
  Apparently 
  an 
  added 
  growth 
  occurs 
  in 
  

   one 
  or 
  two 
  cases. 
  2 
  In 
  several 
  specimens 
  an 
  actinolitic 
  aggregate 
  is 
  

   the 
  only 
  representative 
  of 
  the 
  pyroxenic 
  constituent, 
  especially 
  in 
  

   what 
  seem 
  to 
  have 
  been 
  originally 
  gabbros, 
  although 
  the 
  diallage 
  

   structure 
  is 
  rarely 
  recognizable 
  in 
  them. 
  This 
  actinolitic 
  develop- 
  

   ment 
  is 
  not 
  necessarily 
  connected 
  with 
  any 
  dynamo-metamorphism, 
  

   and 
  may 
  be 
  considered 
  to 
  be 
  probably 
  an 
  example 
  of 
  the 
  second 
  

   class 
  of 
  alteration 
  suggested 
  by 
  Prof. 
  Bonney. 
  3 
  The 
  rocks 
  ex- 
  

   hibit 
  other 
  secondary 
  minerals, 
  the 
  most 
  interesting 
  of 
  which, 
  

   perhaps, 
  is 
  rutile, 
  richly 
  developed 
  in 
  one 
  case 
  within 
  crystals 
  

   idiomorphic 
  in 
  form, 
  but 
  now 
  consisting 
  of 
  an 
  actinolitic 
  aggregate, 
  

   like 
  the 
  development 
  within 
  serpentine 
  in 
  certain 
  altered 
  peridotites 
  

   where 
  the 
  outline 
  of 
  the 
  original 
  olivine-grain 
  is 
  retained. 
  4 
  In 
  

   another 
  rock, 
  rutile 
  apparently 
  is 
  formed 
  from 
  ilmemte, 
  now 
  replaced 
  

   by 
  leucoxene. 
  In 
  more 
  than 
  one 
  case, 
  mica 
  has 
  developed 
  apparently 
  

   across 
  structure-planes, 
  as 
  in 
  the 
  rocks 
  described 
  by 
  Prof. 
  Bonney. 
  5 
  

  

  3. 
  Pressure-effects. 
  

  

  The 
  area 
  described 
  by 
  Capt. 
  Lyons 
  as 
  consisting 
  of 
  ' 
  schistose 
  ' 
  

   rocks, 
  and 
  so 
  marked 
  on 
  the 
  accompanying 
  map 
  (PL 
  XXVI), 
  is 
  

   illustrated 
  in 
  this 
  collection 
  by 
  only 
  five 
  specimens. 
  Judging 
  from 
  

   these, 
  the 
  rocks 
  all 
  show 
  effects 
  of 
  pressure, 
  which 
  in 
  the 
  probable 
  

   phyllites 
  are 
  very 
  strongly 
  marked. 
  

  

  A 
  crushed 
  and 
  brecciated 
  vein-quartz 
  is 
  interesting 
  as 
  exhibiting 
  

   a 
  separation 
  of 
  the 
  quartz, 
  and 
  of 
  the 
  minute 
  disseminated 
  iron-oxide. 
  

   In 
  this, 
  however, 
  as 
  in 
  so 
  many 
  other 
  examples, 
  doubtless 
  the 
  crushed 
  

   material 
  facilitated 
  the 
  percolation 
  of 
  water, 
  through 
  the 
  action 
  of 
  

   which 
  separation 
  and 
  recrystallization 
  probably 
  were 
  caused. 
  The 
  

   fragmental 
  rocks 
  have 
  undergone 
  pressure- 
  modification, 
  as 
  in 
  one 
  

   gritty 
  limestone, 
  and 
  doubtless 
  several 
  of 
  the 
  basic 
  rocks 
  (altered 
  

   gabbros 
  and 
  diabase), 
  but 
  these 
  would 
  offer 
  much 
  resistance 
  and 
  

   would 
  not 
  easily 
  exhibit 
  effects 
  from 
  this 
  cause. 
  

  

  4. 
  Meteorological 
  Action. 
  

  

  One 
  phenomenon 
  of 
  which 
  there 
  is 
  clear 
  evidence 
  is 
  the 
  erosion 
  

   by 
  wind 
  and 
  desert-sand, 
  and 
  the 
  formation 
  of 
  a 
  weathered 
  coating. 
  

   The 
  worn 
  faces 
  of 
  the 
  rocks 
  usually 
  are 
  lumpy 
  or 
  irregular, 
  but 
  

   with 
  projections 
  and 
  depressions, 
  smoothed 
  or 
  polished. 
  In 
  many 
  of 
  

   the 
  crystalline 
  rocks 
  the 
  pyroxenic 
  constituent 
  is 
  slightly 
  hollowed 
  

   out 
  on 
  the 
  weathered 
  surface, 
  and 
  apparently 
  is 
  the 
  part 
  first 
  to 
  

   acquire 
  the 
  brown 
  coating 
  described 
  by 
  Walther 
  and 
  other 
  observers. 
  6 
  

   Whatever 
  be 
  the 
  origin 
  of 
  this 
  layer, 
  the 
  peculiar 
  polish 
  which 
  

   accompanies 
  it 
  is 
  almost 
  certainly 
  due 
  to 
  the 
  wind-action 
  noticed 
  

   above. 
  

  

  1 
  T. 
  G-. 
  Bonney, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xli. 
  (1885) 
  p. 
  519. 
  

  

  2 
  See 
  also 
  Whitman 
  Cross, 
  Am. 
  Journ. 
  Sci. 
  ser. 
  3, 
  vol. 
  xxxix. 
  (1890) 
  p. 
  360. 
  

  

  3 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xlix. 
  (1893) 
  p. 
  102. 
  

  

  4 
  Ibid. 
  vol. 
  liii. 
  (1897) 
  pp. 
  253, 
  262. 
  

  

  5 
  Ibid. 
  vol. 
  xlix.. 
  (1893) 
  p. 
  104. 
  

  

  6 
  ' 
  Die 
  Denudation 
  in 
  der 
  Wiiste,' 
  J. 
  Walther, 
  1891, 
  pp. 
  109-117. 
  

  

  