When  fired  in  a  kiln,  a  blue  day  will  yield  red  bricks,  by  the
conversion  of  FeCOj  into  FcaOg.  In  nature,  rain-water  effects  a
similar  change,  and  the  contrast  between  the  superficial  and  deep-
seated  parts  of  the  same  rock  is  often  as  great  as  between  blue  clay
and  red  brick.  Weathered  blocks  stained  red  on  the  outside  are
often  blue,  grey,  or  nearly  black  on  the  inside,  because  the  change
has  not  affected  the  whole  mass.
An  especially  important  and  wide-spread  change  is  carbonation,
due  to  the  carbon  dioxide  which  all  natural  waters  contain  in

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RAIN  103

greater  or  less  quantity.  The  silicates  are  attacked  and  decom-
posed in  a  manner  that  will  be  explained  below,  and,  under
certain  circumstances,  the  insoluble  ferric  hydrates  are  converted
into  soluble  ferrous  carbonate.
Finally,  solution  plays  a  highly  unportant  r61e  in  the  destructive
work  of  the  rain.  All  rocks  contaiit  some  soluble  material,  and
when  this  soluble  material  is  removed,  the  rock  crumbles  into  a
friable  mass,  which,  on  complete  disintegration,  forms  soil.  This
may  be  illustrated  by  a  block  of  frozen  earth,  which  is  as  hard
as  many  rocks,  being  cemented  by  the  ice  crystals,  which  bind
the  particles  of  soil  together.  When  the  ice  is  melted,  the  mass
immediately  becomes  incoherent.  So,  in  the  rocks,  the  re-
moval of  even  a  small  quantity  of  soluble  material  often  causes
the  whole  to  crumble.  Except  vegetable  moulds,  all  soils  are
derived  from  the  decay  and  disintegration  of  rocks.
The  chemical  composition  of  the  rock-forming  minerals  varies
so  much,  that  the  processes  which  destroy  them  must  vary  corre-
spondingly, differing  in  the  case  of  the  igneous  rocks,  on  the  one
hand,  from  that  of  the  sedimentary  rocks,  on  the  other.
