Attempts  have  also  been  made  to  calculate  the  time
required  to  excavate  the  ravine  between  the  present
Niagara  Falls  and  Queenstown,  which  has  been  estimated
at  about  35,000  years  ;  ^  others,  however,  make  it
much  less.
Again,  whatever  cause  or  causes  may  have  produced
the  great  change  which  has  taken  place  in  the  climate
of  Western  Europe,  there  can  be  little  doubt  that  this
change  indicates  a  very  considerable  lapse  of  time.  We
are  indebted  to  Mr  Hopkins  for  a  very  interesting
memoir  on  this  subject.  Among  the  possible  causes  of
change  he  discusses —

Firstly.    A  variation  in  the  intensity  of  solar  radiation.
To  this  theory  Mr  Hopkins  sees  no  a  priori  objection  ;
but  he  does  not  feel  disposed  to  attach   much  weight  to

1  Appendix  to  third  edition,  p.  16.     See  also  Geological  Journal,  1869,

vol.  XXV.  p.    I  I.
-  Geikie,  Fhys.  Geog.,  p.  280.
CHANGE    OF    CLIMATE  397

it,  because  it  is  "  a  mere  hypothesis  framed  to  account
for  a  single  and  limited  class  of  facts,  and  unsupported
by  the  testimony  of  any  other  class  of  allied  but
independent  phenomena."

It  is,  moreover,  open  to  the  objections  stated  with  great
force  by  Professor  Tyndall,^  who  argues  that  the  ancient
glaciers  indicate  the  action  of  heat  as  much  as  of  cold.
"  Cold,"  he  says,  "  will  not  produce  glaciers.  You  may
have  the  bitterest  north-east  winds  here  in  London
throughout  the  winter,  without  a  single  flake  of  snow.
