314 
ME.  H.  W.  MONCKTON  ON  BOHLDEKS  AND  [Aug.  I^93? 
The  igneous-rock  boulders  are  very  much  decayed.  One  of  them, 
from  which  I  brought  some  fragments,  measured  12^  x  6  x  4  inches. 
It  is  a  greenish-grey  rock,  not  unlike  that  already  described  from 
the  Tilehurst  Plateau  ;  under  the  microscope,  too,  the  rocks  bear  a 
considerable  resemblance  to  one  another.  In  both  are  porphyritic 
crystals  of  orthoclase ;  but  while  in  the  Tilehurst  rock  they  are  fairly 
free  from  enclosures,  in  that  from  Bisham,  as  Prof.  Bonney  has 
pointed  out  to  me,  the  felspars  contain  enclosures  rather  con¬ 
spicuously.  There  is  also  an  absence  of  the  fluidal  structure  in 
this  latter  rock,  while  it  is  very  conspicuous  in  that  from  Tilehurst. 
On  the  whole,  Prof.  Bonne}^  is  of  opinion  that  the  two  rocks,  though 
not  from  the  same  rock-mass,  may  well  have  come  from  the  same 
district.  Where  that  district  may  be  is  a  question  upon  which  it 
would  not  be  wise  to  express  an  opinion,  until  a  considerably  larger 
series  of  the  igneous  rocks  found  in  the  Glacial  Drift  near  London 
have  been  examined. 
Patches  of  gravel  with  red  quartzites  occur  at  Cookham  Dean  at 
300  feet  O.D. ; 1  and  at  250  feet  O.D.  there  is  a  patch  of  gravel 
capping  Winter  Hill,  mapped  as  ‘  River  Gravel.’  Red  quartzite- 
pebbles  abound  in  it,  and  I  saw  one  of  white  quartz  measuring 
3-|  x  2  inches.  Clearly  this  is  a  very  early  Thames  Gravel,  mainly 
derived  from  Glacial  Gravel. 
Hear  Cookham  Station  are  patches  of  gravel  100  to  146  feet  above 
O.D.,  mainly  composed  of  black  and  white  flints  from  the  Chalk  ; 
but  red  quartzites  are  fairly  common.  The  level  of  the  Thames 
here  is  about  80  feet  O.D.  These  gravel-patches  thus  afford  good 
evidence  of  the  gradual  excavation  of  this  part  of  the  Thames  Yalley 
after  the  Glacial  Gravel  had  been  laid  down,  for  all  contain  fragments 
or  pebbles  which  may  be  reasonably  supposed  to  be  derived  from  it. 
YI.  Ashley  and  Bowsey  Hills. 
On  Ashley  Hill  and  Bowsey  Hill  are  patches  of  gravel  which  have 
been  assigned  to  the  Westleton  Shingle  by  Prof.  Prestwich  ; 2  and 
the  great  abundance  of  white  quartz-pebbles,  together  with  the 
scarcity  of  subangular  flints  and  Lower  Greensand  fragments,- tells  in 
favour  of  the  view  that  this  gravel  is,  at  least  very  largely,  composed 
6f  debris  from  the  Westleton  Shingle.  The  presence  of  other 
material,  which  may  most  probably  have  come  from  the  north, 
leads  one  to  suspect  that  this  gravel  has  been  rearranged  in  more 
recent  times,  and  has  received  a  mixture  of  material  from  the 
Glacial  Gravel.  A  large  collection  of  pebbles  from  these  hills 
has  been  made  by  Mr.  H.  J.  Osborne  White,3  and  he  has  kindly 
lent  me  a  series  of  specimens  which  I  have  compared  with  those 
collected  by  Mr.  R.  S.  Herries  and  myself.  There  are  amongst 
them  a  number  of  black  pebbles,  with  veins  of  white  quartz,  up  to 
l\  inch  long.  I  have  had  a  microscope-section  cut  from  one  of 
1  See  Whitaker,  ‘  Geology  of  London,’  vol.  i.  p.  301,  1889. 
2  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  p.  141. 
3  See  his  paper,  Proc.  Geol.  Assoc,  vol.  xii.  (1892)  p.  379. 
