Development  of  present  land  surfaces. — The  layers  of
the  earth  which  are  now  exposed  have  been  modified
not  only  by  the  action  of  heat  and  cold,  wind  and  water,
but  also  by  animals  and  plants.  During  recent
geological  times  great  changes  in  temperature  have
caused  immense  collections  of  ice,  called  glaciers,  to  lie
upon  and  flow  or  glide  very  slowly  over  large  areas  of
land  near  the  north  and  south  poles  of  the  globe.  These
glaciers,  which  once  pushed  much  farther  out  from  the
poles  toward  the  equator  than  now,  have  done  much  to
transport  the  particles  of  soil  from  place  to  place,  mix-


GEOLOGICAL    HISTORY   OF   THE   EARTH  35

ing  the  stony  substances  together,  or  depositing  them  in
layers,  as  the  case  might  be.  Water  and  winds  have  also
done  much  in  this  mixing  of  soils.  In  moist,  low  places
vegetation  has  grown  and  been  preserved  by  water  cov-
ering it,  and  thus  collected  into  beds  of  peat.  Peat  beds
thus  made  in  former  times,  and  afterward  covered  with
clay,  in  some  cases  have  been  gradually  transformed
into  coal.
