Moreover  we  saw  that  over  all  these  surfaces,  in  ad-
dition to  the  vast  amount  of  erosion  which  must  have
been  effected  in  Palaeozoic,  Secondary  and  older  Tertiary
times,  renewed  denudations  accompanied  by  great  cold
occurred  at  a  late  Tertiary  epoch.  The  result  of  the
last  abrasion  has  been  to  cover  the  surface  more  or  less
with  loose  superficial  detritus,  upon  which  part  of  the
fertility  of  portions  of  the  country,  and  the  peculiarity
of  some  of  its  soils  depend.  We  then  passed  on  to
notice  what  I  considered  to  be  a  very  remarkable  result
of  this  last  great  denudation  brought  about  under  the
influence  of  ice,  by  which  the  chief  part  (I  by  no  means
say  all) — but  by  which  the  chief  part  of  the  lakes  of  our
country  have  been  formed;  and  not  of  our  country
alone,  but  of  a  large  part  of  the  northern  hemisphere.
It  is  a  remarkable  thing,  indeed,  to  consider,  if  true —
and  I  firmly  believe  it  to  be  true, — that  most  of  those
great  hollows  in  which  our  lakes  lie,  have  been  scooped
out  by  the  slow  and  long-continued  passage  of  great
sheets  of  glacier  ice,  quite  comparable  to  those  vast
masses  that  cover  the  extreme  northern  and  southern
regions  of  the  world  at  this  day.  The  water  drainage
of  the  country  is  likewise  seen  to  be  dependent  on
geological  structure.  Our  large  rivers  chiefly  drain  to


Summary.  197

the  east,  the  smaller  ones  to  the  west,— because  the
great  axes  of  disturbance  happened  to  lie  nearer  our
western  than  our  eastern  coasts.  Again,  the  quality  of
water  in  these  rivers  depends,  as  we  have  seen,  on  the
nature  of  the  rocks  through  which  they  flow,  and  of  the
springs  by  which  they  are  supplied.
