310  BOULDERS  AND  PEBBLES  FROM  THE  GLACIAL  DRIFT.  [Aug.  1 893. 
of  the  Kennet  is  the  extensive  plateau  of  Mortimer  and  Burghfield 
Commons  (the  Silchester  Plateau),  which  is  1|  mile  west  of  the  line 
of  section  of  fig.  1.  It  is  capped  by  Southern  Drift.1 
III.  Beading. 
Around  Beading  are  several  terraces  of  gravel,  all  of  which  gravel 
strongly  resembles  that  of  Tilehurst  and  contains  pebbles  of  red 
quartzite,  etc.  These  terraces  are  therefore  obviously  of  newer  age 
than  the  Glacial  Gravel,  and  give  evidence  of  the  gradual  erosion  of 
the  valley  by  the  Bivers  Thames  and  Kennet 2  after  that  gravel  was 
spread  out  in  this  area. 
South  of  Beading  we  find  the  gravel-capped  plateau  which  extends 
from  that  town  to  Shinfield,  and  the  highest  part  of  which  is 
265  feet  O.D.  Sections  are  scarce,  but  there  is  sufficient  evidence 
to  show  that  the  gravel  is  that  of  a  river  flowing  into  the  Thames 
from  the  south.  Thus,  at  the  southern  or  Shinfield  end,  the  fields 
are  covered  with  stones,  among  which  Lower  Greensand  fragments 
are  common,  while  Northern  Drift  pebbles  are  apparently  absent ; 
whereas  at  the  northern  end  (Southern  Hill,  Beading),  Northern 
Drift  pebbles  abound  and  Lower  Greensand  fragments  become 
scarce.  As  the  valleys  were  cut  down  to  lower  levels  similar  con¬ 
ditions  prevailed,  and  the  gravel  of  the  terrace  on  the  southern  side 
of  the  plateau  up  to  Earley,  205  feet  O.D.,  contains  Lower  Green¬ 
sand  fragments,  but,  so  far  as  I  can  discover,  no  debris  from  the 
Glacial  Gravel.  I  therefore  believe  that  Southern  Hill  (Beading)  is 
on  the  boundary  of  the  area  over  which  debris  from  the  Glacial 
Gravel  are  distributed.  On  that  hill  there  is  a  gravel-pit  above 
Waterloo  Kiln,  locally  called  4  Bob’s  Mount,’  I  believe,  at  a  level  of 
210  feet  O.D.  I  found  there  an  abundance  of  red  quartzite-pebbles, 
and  also  some  black  pebbles,  which  appear  to  have  come  from  the 
Glacial  Drift.  Being  anxious  to  ascertain  the  origin  of  these  black 
pebbles,  I  had  a  section  cut  from  one,  a  very  hard,  close-grained,  sub- 
angular  fragment,  nearly  2|  inches  long  ;  a  quartz-vein  runs  through 
it.  It  turns  out  to  be  a  quartz-grit,  the  grains  of  quartz  being  in  places 
surrounded  by  a  minute  greenish-blue  material  which  Prof.  Bonney 
believes  to  be,  in  part  or  wholly,  tourmaline.  He  states  that  there 
are  in  his  collection  two  sections  from  a  somewhat  similar  rock — 
the  one  of  a  black  pebble  from  the  Bunter  Beds  of  Cannock  Chase, 
the  other  of  a  pebble  from  the  Neocomian  Beds,  near  the  ‘  coprolite  ’- 
bed  horizon  between  Sandy  and  Potton.  The  Beading  pebble  is 
quite  unlike  any  tourmaline-grit  known  to  Prof.  Bonney  from 
Cornwall.  The  evidence  seems,  therefore,  to  favour  the  derivation 
of  these  black  pebbles  from  the  north.  I  have  no  doubt  this  is  an 
old  river-gravel,  and,  as  it  is  south  of  both  the  Thames  and  Kennet, 
and  94  feet  above  the  level  of  the  present  junction  of  those  rivers, 
it  shows  to  how  small  an  extent  the  valleys  were  excavated  at  the 
time  of  its  deposition. 
1  See  my  former  paper  in  this  Journal,  vol.  xlviii.  (1892)  p.  39. 
2  See  Shrubsole,  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  p.  585. 
