Following  the  time  when  sedges  or  other  advancing  plants  have  closed
in  over  the  surface  of  the  open  water  of  a  lake  or  pond,  however,  the
mat  slowly  thickens,  as  has  already  been  pointed  out,  both  by  the
building  up  of  the  surface,  through  the  accumulation  of  vegetable  debris
and  by  the  sinking  of  the  mat,  partly  through  the  replacement  of  air
and  other  gases  held  in  the  plant  tissues  by  the  water,  water-logging,
but  more  because  of  the  increased  load  upon  the  surface,  after  the
heavier  tree  associations  become  established.  This  sinking  is,  however,
a  slow  process,  and  only  goes  so  far  as  is  necessary  to  establish  equili-
brium, i.  e.,  until  the  mat  is  built  up  suflSciently  to  sustain  the  weight
upon  it.  The  thickness  of  this  mat,  even  at  its  maximum  development,
is  small  relative  to  the  entire  depth  of  the  deposit,  rarely  more  than  3
or  4  feet,  and  in  but  a  single  case  in  the  series  examined,  showing  as
much  as  6  feet  of  material,  which  could  be  considered  true  mat,  before
the  fluid,  or  semi-fluid,  structureless  peat  below  the  mat  was  reached.
