The  belt  ranges  from  about  10  miles  to  25  miles  in
width.  North  of  Point  Arena  it  is  bounded  on  the  west
by  the  Pacific  Ocean.  Southwestward  from  Point  Arena
it  is  bounded  on  the  west  by  the  San  Andreas  fault  to
a  point  a  few  miles  north  of  the  Russian  River.  The
coastal  belt  is  bounded  to  the  east  by  the  central  belt  of
Franciscan  rocks  throughout  most  of  Sonoma  and  Men-
docino Counties,  and  in  southern  Humboldt  County  by
the  Yager  formation  of  Ogle  (1953).  Rocks  similar  to
those  of  the  coastal  belt  underlie  relatively  small  areas
within  the  central  belt  of  the  Franciscan  formation.
The  sedimentary  rocks  of  th;  coastal  belt  consist
chiefly  of  graywacke,  shale,  and  minor  conglomerate
(photo  9).  Stain  tests  show  (Bailey  and  Irwin,  1959)  that
most  of  the  graywackes  of  the  coastal  belt  contain  ap-
preciable quantities  of  potassium  feldspar.  In  this  respect
the  graywackes  of  the  coastal  belt  are  similar  to  the  rocks
of  the  Sacramento  Valley  sequence,  but  differ  from  tlv:


graywackes  of  the  Franciscan  of  the  central  belt  and  the
metamorphosed  Franciscan  of  the  eastern  belt.
Volcanic  rocks  interbedded  with  graywacke  and  shale
are  found  at  a  few  places  in  the  coastal  belt  and  appar-
ently are  most  abundant  in  the  southern  third  of  the  belt.
