Yol.  49.] 
PEBBLES  PROM  THE  GLACIAL  DRIFT. 
319 
The  Thames  Gravel  near  the  Mar  Dyke  is  shown,  and  in  it  I 
have  found  Northern  Drift  pebbles  (100  feet  O.D.).  The  Grays 
brickfield  is  2  miles  east  of  the  point  where  the  Tilbury  and 
Southend  Railway  crosses  the  line  of  section.  The  hills  near 
Detcham  are  capped  by  very  pebbly  gravel  with  Lower  Greensand 
debris,  probably  Southern  Drift. 
South  of  Dartford  Heath  I  have  not,  at  present,  found  Northern 
Drift  pebbles  ;  indeed,  such  sections  as  I  have  seen  in  the  long¬ 
shaped  patches  of  gravel  which  run  up  the  Longfield  and  Orpington 
valleys  lead  me  to  believe  that  the  materials  are  almost  entirely 
derived  from  beds  of  Red  Clay-with -Hints  or  from  the  Chalk. 
It  seems  to  me  that  the  facts  enumerated  in  the  present  commu¬ 
nication  strongly  support  the  conclusions  expressed  in  the  last 
communication  on  this  subject  which  I  laid  before  the  Society ; 
and  also  prove  that,  if  (as  I  think  probable)  rivers  flowing  in  the 
direction  of  the  present  Thames  and  Ivennet  existed  at  the  time 
when  the  Glacial  Gravel  was  spread  out,  the  valleys  in  which  those 
rivers  now  flow  have  to  a  large  extent  been  excavated  since. 
