Yol.  49.] 
PEEBLES  FROM  THE  GLACIAL  ERIFT. 
309 
village  of  Tilehurst  stands.  The  highest  part  of  that  plateau  i& 
343  feet  O.D.,1  but  the  best  section  is  in  a  gravel-pit  near  Norcot 
Kiln  at  290  feet  O.D.2  The  gravel  is  roughly  stratified,  and  is 
composed  of  the  following  classes  of  material : — 
(a)  Derived  from  the  Chalk  and  Tertiary  beds,  mostly  local. 
1.  Flints  internally  black  and  often  very  little  rolled  or  waterworn. 
2.  Flint-pebbles  abundant,  some  probably  from  older  gravels,  but  mostly 
from  Tertiary  pebble -beds. 
3.  Sarsens,  many  rolled  blocks. 
(b)  Derived  from  older  gravels. 
4.  Brown,’  subangular  flints. 
5.  Pebbles  of  quartz. 
(c)  Origin  doubtful,  probably  erratics  of  Glacial  Drift. 
6.  Black  grit-pebbles,  possibly  from  Bunter  pebble-beds. 
(d)  Erratics  of  Glacial  Drift. 
7.  Quartzites.  Eeddish  quartzite-boulders,  more  than  3  inches  long,  are 
common  ;  white  quartzite-pebbles,  reddish  vitreous  quartzite,  etc. 
8.  Large  blocks  of  quartz. 
9.  Igneous-rock  boulders. 
10.  Ironstone — small  pebbles  abundant,  very  characteristic  of  the  Glacial 
Gravel  on  the  north  side  of  the  Thames  in  this  neighbourhood. 
I  submitted  a  section  cut  from  a  block  of  igneous  rock  found  here 
and  given  to  me  by  Mr.  0.  A.  Shrubsole,  F.G.S.,  to  Prof.  Bonney,  and 
he  pronounced  it  to  be  a  true  igneous  rock,  probably  once  a  glass,  now 
devitrified,  with  indications  of  fiuidal  structure,  and  a  few  micro- 
porphyritic  crystals  of  felspar,  two  or  three  of  which  show  Carlsbad 
twinning,  probably  sanidine.  He  considered  that  the  rock'  had  been 
a  glassy  rhyolite,  now  devitrified,  and  perhaps  also  a  little  silicified, 
as  in  the  case  of  the  Arvonian  at  St.  David’s. 
The  diagrammatic  section  (fig.  1,  p.  311)  shows  the  relation  of  the 
gravel  at  Tilehurst  to  the  other  gravels  of  this  part  of  the  country. 
At  the  northern  end  are  Chalk  hills,  having  an  altitude  of  over  500 
feet.  The  patches  of  gravel  at  Greenmoor  Hill  and  Checkendon 
(Woodcote  Common),  described  as  Westleton  Shingle  by  Prof. 
Prestwich,3  being  a  little  west  of  the  line  of  section,  are  indicated 
in  outline.  Between  them  and  the  Thames  at  Tilehurst  are  exten¬ 
sive  patches  *  of  Glacial  Gravel,  and  on  the  south  of  the  river  is  the 
Tilehurst  Plateau.  Between  this  plateau  and  the  Thames  there  is 
a  patch  of  gravel  about  275  feet  O.D.,  shown  in  the  figure.  It  is  a 
reddish  gravel,  with  many  pebbles  of  red  and  brown  quartzite,  and 
very  like  the  Glacial  Gravel  in  composition,  but  its  position  and  level 
lead  me  to  believe  that  it  is  really  a  high-level  river-gravel  newer 
than,  and  mainly  derived  from,  Glacial  Gravel.  South  of  the  valley 
1  The  altitudes  in  this  paper  are  taken  from  the  Ordnance  Survey  maps  as 
far  as  possible,  and  given  in  figures  above  the  Ordnance  datum,  written  shortly 
O.D. 
2  See  J.  H.  Blake,  Proc.  Geol.  Assoc,  vol.  x.  (1888)  p.  495. 
3  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  p.  140,  &  pi.  vii.  fig.  1. 
