96 
PROCEEDINGS  OE  THE  GEOLOGICAL  SOCIETY. 
[May  1893, 
Taking  as  the  known  quantity  the  results  supplied  by  the  obser¬ 
vations  in  Greenland,  the  equation,  he  says,  will  be  as  follows  : — 
A  surplus-ice  overflow  equal  to  one  mile  advance  in  eight  or  twelve 
years,  minus  (1)  the  retardation  due  to  friction  and  irregularities 
of  the  surface,  and  (2)  to  seasonal  changes  of  temperature  (the  so- 
called  interglacial  periods)  :  plus  (l)the  increase  of  discharge  due  to 
progressive  secular  refrigeration  and  (2)  further  to  increased  preci¬ 
pitation.  The  known  quantity,  he  says,  gives  us  from  4000  to  6000 
years.  He  allows  that  of  the  unknown  quantities  we  can  at  present 
form  but  a  distant  idea,  although  he  thinks  it  will  be  found  that 
the  time  required  for  the  formation  and  spread  of  the  great  ice- 
sheets  in  Europe  and  America  need  not  be  extended  beyond  25,000 
years  at  the  utmost,  including  in  this  estimate  the  time  during 
which  the  cold  was  increasing,  or  pre-glacial  time,  and  that  during 
which  the  cold  was  diminishing,  or  post-glacial  time.  The  close  of 
the  Glacial  Period,  i.  e.  the  final  melting  of  the  ice-sheet,  might  have 
taken  place  from  8000  to  10,000  years  ago. 
We  must  all  admit  the  interest  which  attaches  to  calculations 
based  on  the  rate  of  motion  in  the  Greenland  ice-sheet,  yet  the 
unknown  elements,  and  especially  the  uncertainty  as  to  the  duration 
of  the  period  of  refrigeration,  seem  to  cast  a  shadow  upon  this 
attempt  to  estimate  the  duration  of  the  Glacial  Period.  However, 
the  Author  himself  explains  that  he  does  not  attempt  to  fix 
actual  terms  of  years,  but  only  wishes  to  show  that  we  must  not 
unhesitatingly  accept  such  large  measures  of  time  as  have  in  some 
instances  been  required. 
It  is  curious  to  note  how  different  a  view  Mr.  Mellard  Beade 
takes  of  the  duration  of  what  is  usually  termed  post-glacial  time. 
While  Prof.  Prestwich  thinks  that  the  final  melting  of  the  Great 
Ice  Sheet  is  not  likely  to  have  occurred  more  than  10,000  years 
ago,  Mr.  Beade  claims  for  events  succeeding  the  deposition  of  the 
low-level  Boulder-clay  of  Lancashire  a  period  of  57,500  years, 
which  is  made  up  as  follows : — He  assigns  a  period  of  40,000 
years  to  the  elevation  and  erosion  of  the  Boulder-clay,  one  of  15,000 
years  for  the  deposition  of  the  post-glacial  beds,  such  as  estuarine 
silt  and  Scrobicularia- clay  succeeded  by  peat  and  forest-beds,  and 
a  further  2500  years  for  the  blown  sand  which  covers  these  in  many 
places. 
It  seems  tolerably  certain  that,  when  an  author  is  bold  enough  to 
attempt  a  chronological  scheme  of  this  nature,  the  arguments  of  most 
of  the  speakers  aim  at  demonstrating  the  impossibility  of  reducing 
