As  regards  the  higher  districts,  however,  his  data  are
perhaps  not  far  wrong,  and  if  we  apply  them  to  the
valley  of  the  Somme,  where  the  excavation  is  about
200  feet  in  depth,  they  would  indicate  an  antiquity  for
the  Palaeolithic  Epoch  of  from  100,000  to  240,000  years,
which,  though  arrived  at  from  perfectly  different  data,
agrees  with  the  periods  A  and  B  in  the  calculation  made
by  Messrs  Croll  and  Stone.
In  addition  to  the  causes  already  alluded  to,  there  is  at
least  one  other  astronomical  phenomenon — namely,  the
change  in  the  obliquity  of  the  ecliptic,  which  must  be
taken  into  account  in  considering  the  effects  which
cosmical  causes  may,  or  must,  have  exercised  on  climate.
The  whole  question  then  is  one,  not  only  of  extreme
interest,  but  also  of  very  great  difficulty,  and  we  are  not,
I  think,  at  present  in  a  position  to  estimate  with  con-
fidence the  effects  on  climate  which  may  have  been
produced  by  these  various  causes.
Several  other  points  connected  with  the  glacial  period
receive  a  natural  explanation  from  the  suggestions  of
M,  Adhemar  and  Mr  Croll.  Thus  M.  M^orlot  ^  some
years    ago    pointed    out    that    there    are    in    Switzerland

'  Bui/,   de   la  Soc.    Vaudoise  des  Sciences   Naturclles^   March    1854  ;
Bibl.  Universelle  de  Geneve^  May  1858.
412  PREHISTORIC   TIMES

evidences  of  several  periods  of  cold,  during  what  is
called  the  glacial  epoch,  separated  by  an  interval  of
mildness.
Of  this  the  most  striking  instance  is  afforded  by  the
Dilrnten  beds,  where  a  layer  of  coal  or  lignite  no  less
than  twelve  feet  thick,  lies  between  two  glacial  deposits.
Again,  Mr  Croll  gives ^  particulars  of  250  borings  through
the  surface  deposits  of  the  mining  districts  in  Scotland.
