In  studying  the  work  of  the  surface  agents,  the  logical  order  of
treatment  requires  that  the  destructive  operations  be  considered
first.  The  agencies  to  be  examined  are:  (i)  the  atmosphere,
(2)  running  water,  (3)  ice,  (4)  lakes,  (5)  the  sea,  (6)  animals  and
plants.  Of  these  various  agents  the  work  is  principally  mechanical,
but  water,  in  its  various  forms,  is  a  slow  but  extremely  efl&cient
agent  of  chemical  changes.
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CHAPTER  IV
DESTRUCTIVE  PROCESSES. —THE  ATMOSPHERE

The  atmospheric  agencies  are  by  far  the  most  important  of  the
destructive  or  denuding  agents,  because  no  part  of  the  land  surface
is  altogether  exempt  from  their  activity.  Their  work  is  described
by  the  general  term  weatherings  and  is  shown  at  once  by  the  dif-
ferent appearance  of  freshly  quarried  stone  from  that  which  has
been  long  exposed  in  the  face  of  a  cliff,  or  even  in  ancient  build-
ings. While  such  agents  as  rivers  and  the  sea  do  work  that  is
much  more  apparent  and  striking  than  that  of  the  atmosphere,
yet  they  are  more  locally  confined,  and  even  in  their  opera-
tions the  atmosphere  renders  important  aid.  Though  no  part
of  the  land  surface  is  entirely  free  from  the  destructive  activity  of
the  atmosphere,  the  rapidity  and  intensity  of  this  activity  vary
much  in  different  places.  There  are,  in  the  first  place,  the  great
differences  of  climate  to  be  considered,  differences  in  the  amount
and  distribution  of  the  rainfall,  of  temperature,  and  of  the  winds.
