Andesine  and  bytownite  are  closely  allied  varieties  of  similar
'habit,  the  first  being  a  trifle  more  acid,  and  the  second  more
basic  than  labradorite.  (4)  Anorthite,  or  lime  feldspar,  is
also  a  prominent  and  important  constituent  of  the  basic
eruptives,  and  has  been  found  in  meteorites  and  terrestrial
peridotites.
On  account  of  their  abundance  and  wide  distribution,  as  well
as  on  account  of  the  character  of  their  decomposition  products,
the  feldspars  are  to  be  considered  as  among  the  most  important
of  rock  constituents.  As  it  is  from  the  debris  of  the  older
feldspathic  rocks  that  have  been  made  up  a  large  proportion
of  all  the  sedimentaries  of  more  recent  date,  so  too  it  may  be
claimed  that  from  the  decomposition  of  this  feldspathic  con-
stituent has  been  derived  a  large  share  of  the  salts  of  potash,
lime,  and  soda,  as  well  as  aluminous  silicates  which  form  so
essential  a  portion  of  the  soils.  The  method  of  feldspathic
decomposition  as  commonly  understood  is  given  on  p.  237.
This  decomposition  usually  manifests  itself  by  a  whitening
of  the  mass,  accompanied  by  opacity  and  a  general  softening,
whereby  it  falls  away  to  loose  powder  unless  confined.  As  seen
in  thin  sections  under  the  microscope,  the  decomposition  goes
on  most  rapidly  along  lines  of  cleavage,  naturally  attacking  the
outer  portions  first,  so  that  the  crystals  show  fresh  unaltered
cores  surrounded  by  opaque  and  "  muddy  "  borders.  In  cases
where  the  feldspars  carry  iron  this  usually  makes  its  presence
known  by  a  reddening  or  browning  of  the  mass,  due  to  oxida-
tion. In  presence  of  abundant  carbonic  acid,  the  liberated  iron
may  enter  into  combination  as  a  carbonate  and  the  color  remain
unchanged.
