72 
ME.  HOENEE  ON  THE  ALJjJJYlKL  LAND  OF  EGYPT. 
memoir  of  M.  Gikaed  in  the  above  vrork,  afterwards  published  in  the  Memou-s  of  the 
Institute*.  I have  been  naturally  led  to  examine  very  carefully  the  data  upon  which 
the  result  arrived  at  by  M.  Gieaed  was  founded,  and  have  seen  reason  to  differ  from 
his  conclusion.  In  thus  differing  from  an  authority  sanctioned,  apparently,  by  the  assent 
of  subsequent  authors  for  so  long  a time,  although  that  assent  may  have  been  given 
without  special  examination,  I should  not  feel  justified  in  expressing  my  dissent,  with- 
out stating  the  grounds  of  it,  and  without  giving  such  extracts  from  M.  Gieaed’s 
memoir  as  will  show  the  data  on  which  his  conclusion  is  founded.  This  must  be  my 
apology  for  a somewhat  lengthened  statement  which  I have  given  in  the  Appendix, 
Note  C,  p.  80. 
The  statement  of  a secular  increase  of  the  soil  over  a valley  extending  580  miles, 
even  if  it  were  determined  by  the  most  accru’ate  data,  gives  us  no  information  of  real 
value ; for  that  mean  may  be  inapplicable  to  any  one  particular  part  of  the  valley ; and 
it  is  especially  inapplicable  in  relation  to  the  geological  history  of  the  allmial  land,  when 
a depth  of  one  foot  of  soil  in  one  place  and  of  three  feet  in  another,  might,  according 
to  circumstances,  have  been  deposited  during  the  same  amount  of  time,  the  object  of 
inquiry  being  to  ascertain  the  probable  age  of  the  lowest  layer  of  the  Nile  sediment.  In 
every  situation  where  the  experiment  is  made,  we  must  have  a fixed  point  in  time  to  start 
from,  viz.  the  known  age  of  a monument  whose  foundation  rests  upon  Nile  sediment,  and 
upon  whose  sides  it  has  accumulated  by  subsequent  inundations.  If  there  have  been  no 
local  causes  to  disturb  the  probability  that  the  sediment  above  and  below  the  foundation 
has  accumulated  at  the  same  rate,  we  divide  the  amount  above  the  foundation  by  the 
number  of  Centuries  known  to  have  elapsed  from  the  erection  of  the  monument  to  the 
present  time,  and  then  apply  the  same  chronometric  scale  to  the  greatest  ascertained 
depth  of  sediment  below  the  foundation.  If  many  experiments  were  thus  made  in  dif- 
ferent localities  throughout  the  valley,  where  monuments  of  a known  age  exist,  upon 
whose  sides  the  sediment  had  accumulated  and  whose  foundations  were  ascertained  to 
rest  upon  the  same  kind  of  sediment,  we  should  obtain  reliable  results,  applicable  to  the 
valley  generally.  But  in  no  one  locality,  however  deep  the  sediment,  could  we  affh’m 
that  we  had  reached  the  oldest  layer,  unless  it  rested  on  limestone  (the  rock  foundation 
of  the  valley),  because  of  the  more  than  probable  inequality  of  the  surface  of  its  bottom. 
We  might  arrive  at  a very  high  antiquity,  but  we  could  not  say  that  we  had  arrived  at 
the  highest.  To  divide  a depth  of  sediment  above  the  foundation  of  a monument  by 
an  assumed  mean  secular  increase  of  it  over  the  whole  valley,  and  thence  to  determine,  as 
has  been  done,  the  age  of  the  monument,  is  obviously,  for  the  reasons  stated,  a falla- 
cious inference. 
* Sur  la  Vallee  d’Egypte,  et  sur  Texhaussement  seculaire  du  sol  qui  la  recouvre ; Memoii’es  de  I’Academie 
Eoyale  des  Sciences  de  ITnstitut,  annee  1817. 
