﻿Tol. 
  53.] 
  OF 
  TB.E 
  NUBIAN 
  DESERT. 
  373 
  

  

  5. 
  Relative 
  Age 
  of 
  the 
  flocks, 
  and 
  Comparison 
  with 
  other 
  

   Localities. 
  

  

  While 
  the 
  coarse 
  gneissie 
  and 
  granitoid 
  rocks 
  of 
  the 
  Xile 
  Valley 
  

   near 
  Assuan 
  are 
  poorly 
  represented, 
  some 
  likeness 
  to 
  those 
  of 
  a 
  

   more 
  basic 
  or 
  intermediate 
  type 
  from 
  Wadi 
  Haifa 
  is 
  exhibited. 
  1 
  

   The 
  rocks 
  which 
  appeared 
  to 
  belong 
  to 
  an 
  ancient 
  series 
  from 
  that 
  

   part 
  of 
  the 
  Mle 
  Valley 
  were, 
  however, 
  generally 
  dioritic. 
  The 
  more 
  

   modified 
  types 
  in 
  this 
  collection 
  from 
  east 
  of 
  the 
  Nile 
  are 
  usually 
  

   coarser, 
  and 
  are 
  altered 
  gabbro 
  rather 
  than 
  diorite. 
  Much 
  of 
  the 
  

   hornblende 
  is 
  similar 
  to 
  that 
  in 
  the 
  less-altered 
  diorites 
  of 
  the 
  valley, 
  

   although 
  in 
  the 
  latter 
  the 
  plagioclase 
  is 
  often 
  somewhat 
  fresher 
  in 
  

   appearance. 
  In 
  one 
  rock 
  from 
  the 
  Second 
  Cataract 
  unchanged 
  

   augite 
  occurred, 
  as 
  in 
  No. 
  26 
  in 
  this 
  collection 
  (p. 
  367). 
  

  

  The 
  altered 
  gabbros 
  with 
  remains 
  of 
  diallage, 
  occasionally 
  still 
  

   recognizable, 
  rather 
  resemble 
  the 
  well-known 
  euphotide 
  (Alpine 
  

   gabbro), 
  2 
  and 
  this 
  type 
  has 
  not 
  been 
  noticed, 
  so 
  far 
  as 
  I 
  am 
  aware, 
  

   from 
  Upper 
  Egypt. 
  The 
  almost 
  pure 
  hornblendite 
  seems 
  not 
  to 
  be 
  

   represented 
  nearer 
  than 
  Pangani 
  or 
  Southern 
  Abyssinia. 
  The 
  rarity 
  

   of 
  these 
  types 
  in 
  this 
  southerly 
  region 
  can 
  be 
  seen 
  by 
  referring 
  to 
  

   the 
  exhaustive 
  list 
  given 
  by 
  A. 
  Rosiwal. 
  3 
  Notwithstanding 
  specific 
  

   or 
  varietal 
  differences, 
  however, 
  the 
  conclusions 
  arrived 
  at 
  as 
  regards 
  

   this 
  area 
  are 
  similar 
  to 
  those 
  suggested 
  by 
  examination 
  of 
  specimens 
  

   from 
  parts 
  of 
  the 
  Nile 
  Valley 
  ; 
  and 
  even 
  similar 
  in 
  the 
  difficulty, 
  

   sometimes, 
  of 
  identifying 
  with 
  certainty 
  all 
  the 
  members 
  of 
  the 
  

   ancient 
  Archaean 
  series. 
  4 
  

  

  It 
  seems 
  clear 
  that 
  the 
  complex 
  in 
  this 
  district 
  east 
  of 
  the 
  Nile 
  

   exhibits 
  the 
  old 
  floor 
  of 
  crystalline 
  rocks 
  of 
  which 
  Capt. 
  Lyons 
  has 
  

   .already 
  noticed 
  indications 
  in 
  the 
  country 
  west 
  of 
  the 
  river 
  (for 
  

   example, 
  the 
  gneiss 
  of 
  Jebel 
  Abu 
  Bayan). 
  It 
  is 
  interesting 
  to 
  find 
  

   these 
  basement-rocks 
  exposed 
  along 
  what 
  Capt. 
  Lyons 
  had 
  suggested 
  

   may 
  be 
  an 
  anticlinal 
  axis. 
  5 
  The 
  resemblances 
  also 
  to 
  rocks 
  which 
  

   have 
  been 
  described 
  from 
  near 
  Wadi 
  Haifa 
  render 
  this 
  coincidence 
  

   the 
  more 
  striking, 
  since 
  it 
  is 
  by 
  way 
  of 
  this 
  locality 
  that 
  the 
  

   suggested 
  anticlinal 
  would 
  cross 
  the 
  Nile 
  Valley. 
  

  

  1 
  G-eol. 
  Mag. 
  1893, 
  p. 
  437. 
  I 
  have, 
  to 
  thank 
  Mr. 
  L. 
  Fletcher, 
  M.A., 
  F.K.S., 
  

   Keeper 
  of 
  Mineralogy 
  at 
  the 
  British 
  Museum 
  of 
  Natural 
  History, 
  for 
  kindly 
  

   allowing 
  me 
  to 
  re-examine 
  the 
  specimens 
  described, 
  as 
  the 
  rocks 
  and 
  slides 
  have 
  

   been 
  deposited 
  in 
  that 
  collection. 
  

  

  2 
  ' 
  Petrological 
  Notes 
  on 
  the 
  Euphotide 
  or 
  Saussurite-smaragdite 
  Gabbro 
  of 
  

   the 
  Saasthal,' 
  T. 
  G-. 
  Bonney, 
  Phil. 
  Mag. 
  ser. 
  5, 
  vol. 
  xxxiii. 
  (1892) 
  p. 
  237 
  ; 
  'On 
  

   some 
  Specimens 
  of 
  Gabbro 
  from 
  the 
  Pennine 
  Alps,' 
  T. 
  G. 
  Bonney, 
  Min. 
  Mag. 
  

   •vol. 
  ii. 
  (1878) 
  p. 
  5. 
  

  

  3 
  Denkschr. 
  der 
  Kais. 
  Akad. 
  der 
  "Wiss. 
  Wien, 
  vol. 
  lviii. 
  (1891): 
  'Beitr. 
  

   zur 
  geol. 
  Kenntniss 
  des 
  bstlicben 
  Afrika,' 
  pt. 
  ii. 
  pp. 
  531-548. 
  

  

  4 
  Geol. 
  Mag. 
  1893, 
  p. 
  43b\ 
  

  

  5 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  1. 
  (1894) 
  p. 
  539. 
  

  

  