Sir  C.  Lyell  attempted  ■'■  to  form  an  estimate  of  the
duration  of  the  glacial  epoch,  on  the  assumption  that  the
different  movements  of  elevation  and  depression  pro-
ceeded at  an  average  rate  of  2|  feet  in  a  century.  As
the  simplest  "  series  of  changes  in  physical  geography
which  can  possibly  account  for  the  phenomena  of  the
glacial  period,"  he  gave  the  following  : —

"  First,  a  continental  period,  towards  the  close  of  which
the  forest  of  Cromer  flourished  ;  when  the  land  was  at
least  500  feet  above  its  present  level."

"  Secondly,  a  period  of  submergence,  by  which  the  land
north  of  the  Thames  and  Bristol  Channel,  and  that  of
Ireland  was  gradually  reduced  to  an  archipelago."

"  Thirdly,  a  second  continental  period,  when  the  bed
of  the  glacial  sea,  with  its  marine  shells  and  erratic  blocks,
was  laid  dry,  and  when  the  quantity  of  land  equalled  that
of  the  first  period."

It  is  evident  that  such  changes  as  these  would  require
a  great  lapse  of  time.  Sir  Charles  Lyell  admits  that  the
average  change  of  2^  feet  in  a  century  is  a  purely
arbitrary  and  conjectural  rate,  and  that  there  are  cases  in
which  a  change  of  as  much  as  six  feet  in  a  century  appears
to  have  taken  place  :  still,  it  is  in  his  opinion  probable
that  the  rate  assumed  in  a  century  is,  if  anything,  above
the  average,  and  in  this  I   believe  most  geologists  would

^  Antiquity  of  Mufi,  pp.  282,  285.
GEOLOGICAL   EVIDENCE  417

be  disposed  to  agree  with  him.  On  this  hypothesis  he
estimates  that  the  whole  series  of  changes  would  have
required  some  220,000  years.
