﻿Yol. 
  53.] 
  OF 
  THE 
  NUBIAN 
  DESERT. 
  365 
  

  

  felspar, 
  whitish, 
  and 
  fatty-looking, 
  and 
  grass-green 
  pyroxene 
  or 
  

   amphibole 
  with 
  one 
  cleavage 
  and 
  a 
  well-marked 
  form. 
  The 
  appear- 
  

   ance 
  recalls 
  that 
  of 
  some 
  Alpine 
  gabbros. 
  — 
  Microsc. 
  Diallagoid 
  and 
  

   hornblendic 
  cleavages, 
  and 
  sometimes 
  twinning, 
  are 
  seen 
  in 
  the 
  

   greenish 
  mineral 
  in 
  the 
  midst 
  of 
  a 
  felted 
  hornblendic 
  mass. 
  The 
  

   rock 
  bears 
  no 
  definite 
  signs 
  of 
  crushing. 
  In 
  15 
  the 
  green 
  mineral 
  

   (less 
  in 
  amount) 
  is 
  partly 
  now 
  a 
  chloritic 
  aggregate, 
  partly 
  a 
  

   yellowish-green, 
  non-dichroic 
  product 
  which 
  seems 
  flaky 
  as 
  if 
  

   derived 
  perhaps 
  from 
  diallage. 
  The 
  edges 
  are 
  corroded 
  by 
  the 
  

   surrounding 
  mass. 
  Much 
  of 
  the 
  felspar 
  is 
  clear 
  and 
  is 
  plagioclase 
  

   (labradorite), 
  but 
  secondary 
  minerals 
  are 
  developed. 
  

  

  jSTos. 
  10 
  and 
  19 
  are 
  mottled 
  greenish 
  and 
  grey, 
  with 
  small 
  

   patches 
  which 
  are 
  jade-like 
  in 
  appearance. 
  The 
  former 
  specimen 
  

   is 
  bounded 
  by 
  joint-planes, 
  the 
  second 
  has 
  a 
  sand-worn 
  surface. 
  — 
  

   Microsc. 
  In 
  both, 
  the 
  mineral 
  constituents 
  are 
  replaced, 
  but 
  in 
  19, 
  

   where 
  the 
  grains 
  are 
  larger 
  and 
  clearer, 
  actinolite 
  is 
  grouped 
  in 
  

   curving 
  sheaves, 
  suggesting 
  the 
  possibility 
  of 
  pressure-effects. 
  In 
  

   10 
  two 
  or 
  three 
  isolated, 
  fairly 
  large 
  crystals 
  of 
  a 
  brownish 
  dichroic 
  

   mineral 
  (pale 
  to 
  cinnamon-brown) 
  with 
  bluish 
  dusty 
  centre, 
  probably 
  

   are 
  tourmaline. 
  They 
  terminate 
  irregularly, 
  enclose 
  foreign 
  sub- 
  

   stances, 
  and 
  appear 
  to 
  be 
  a 
  late 
  formation, 
  and 
  to 
  have 
  grown 
  across 
  

   the 
  hornblende-zoisite 
  aggregate, 
  almost 
  eating 
  it 
  up, 
  as 
  the 
  mica 
  

   described 
  by 
  Prof. 
  Bonney 
  across 
  a 
  schist. 
  1 
  The 
  form, 
  however, 
  

   would 
  be 
  unusual 
  for 
  tourmaline 
  in 
  a 
  rock 
  of 
  this 
  kind, 
  and 
  it 
  may 
  

   be 
  not 
  only 
  secondary 
  but 
  also 
  pseudomorphic. 
  

  

  (b) 
  Diabasic 
  Hocks 
  with 
  Lustre-mottled 
  Hornblende. 
  — 
  16 
  (Murrat) 
  

   is 
  from 
  the 
  Southern, 
  and 
  1 
  (Tilat 
  Abda) 
  & 
  33 
  are 
  from 
  the 
  

   Eastern 
  region. 
  In 
  all, 
  well-cleaved 
  blackish 
  or 
  dark-green 
  lustre- 
  

   mottled 
  hornblende 
  is 
  associated 
  with 
  felspar, 
  which 
  is 
  greenish- 
  

   white 
  saussuritic-looking 
  in 
  1 
  or 
  an 
  opaque 
  white 
  in 
  16. 
  — 
  

   Microsc. 
  The 
  felspar 
  is 
  replaced 
  by 
  a 
  micaceous 
  or 
  kaolin 
  aggregate 
  

   with 
  flaky 
  yellowish 
  viridite, 
  or 
  pseudophite. 
  and 
  granular 
  epidote, 
  

   some 
  oligoclase 
  remaining 
  in 
  1. 
  The 
  hornblende 
  is 
  dichroic, 
  2 
  has 
  

   colourless 
  and 
  other 
  patches, 
  and 
  where 
  the 
  differences 
  are 
  most 
  

   marked, 
  as 
  in 
  33, 
  the 
  varieties 
  apparently 
  pass 
  into 
  each 
  other 
  by 
  

   secondary 
  change. 
  It 
  encloses 
  grains 
  of 
  felspar 
  often 
  altered 
  ; 
  also 
  

   in 
  1 
  some 
  pyroxenic 
  grains 
  and 
  some 
  of 
  granular 
  calcite, 
  but 
  there 
  

   appear 
  to 
  be 
  gradations 
  between 
  these, 
  as 
  if 
  the 
  pyroxene 
  might 
  be 
  

   a 
  lime-pyroxene 
  — 
  a 
  malacolite 
  — 
  such 
  as 
  occurred 
  in 
  the 
  picrite 
  of 
  

   St. 
  David's 
  described 
  by 
  Prof. 
  Bonney. 
  3 
  Clustered 
  epidote 
  and 
  

   scattered 
  actinolite 
  are 
  abundant. 
  In 
  33 
  iron-oxide, 
  a 
  few 
  zircons, 
  

   apatite, 
  and 
  epidote 
  occur, 
  and 
  a 
  quartz-vein. 
  Much 
  of 
  this 
  slide 
  

   consists 
  of 
  dusty 
  felspars, 
  broadly 
  oblong, 
  closely 
  set 
  in 
  a 
  ground 
  of 
  

   coarse 
  quartz. 
  4 
  

  

  1 
  'On 
  a 
  Secondary 
  Development 
  of 
  Biotite 
  and 
  of 
  Hornblende 
  ,' 
  Quart. 
  

  

  Journ. 
  Geol. 
  Soe. 
  vol. 
  xlix. 
  (1893) 
  p. 
  104. 
  

  

  2 
  In 
  1 
  c, 
  pale 
  brown 
  ; 
  b, 
  dull 
  olive-green 
  ; 
  a, 
  reddish 
  brown 
  to 
  green. 
  

  

  3 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xli. 
  (1885) 
  p. 
  519. 
  

  

  4 
  I 
  incline 
  to 
  the 
  view 
  that 
  the 
  rock 
  is 
  akin 
  to 
  the 
  diorites, 
  notwithstanding 
  

   the 
  form 
  of 
  the 
  felspars 
  : 
  and 
  from 
  the 
  character 
  of 
  the 
  hornblende, 
  it 
  seems 
  

   best 
  described 
  here. 
  Compare 
  also 
  Nos. 
  4, 
  30, 
  27, 
  infra, 
  pp. 
  366, 
  367. 
  

  

  