THE   CORDILLERAS.  873 
Abrams  mica  schist,  which  forms  the  South  Fork  Mountain,  and  is" 
prolonged  northwestward  to  and  probably  across  the  Klamath  River 
near  Weitchpec.  The  sandstones  of  the  Coast  Range  region  adjoin 
this  schist  belt  on  the  west.  According  to  Diller,  toward  the  north, 
approaching  Klamath  River,  long  narrow  belts  of  schist  alternate 
with  narrow  belts  of  sandstone,  the  latter  dipping  eastward  as  though 
going  under  the  schists.  This  apparent  anomaly  is  evidently  due  to 
a  series  of  faults.  It  is  further  evident  that  the  Coast  Range  forma- 
tions have  buried  the  western  portion  of  the  schist  belt,  which  may 
extend,  immediately  under  the  sandstone,  far  toward  the  coast. 
The  eastern  schist  belt  emerges  from  beneath  the  Cretaceous  sand- 
stones and  shales  in  the  Sacramento  Valley  west  of  Ono,  with  a  width 
of  8  miles,  which  gradually  increases  as  it  advances  northward  to  a 
maximum  of  about  12  miles  west  of  Scott  Valley.  Southward  from 
Trinity  River  the  pre- Paleozoic  area  is  occupied  chiefly  by  the 
Abrams  mica  schist,  the  hornblende  schist  being  confined  to  narrow 
strips,  but  northward  from  Trinity  River  the  hornblende  schist 
spreads  out  and  finally  nearly  excludes  the  mica  schist,  as  in  the  valley 
of  the  South  Fork  of  Salmon  River.  Still  farther  north,  in  the  moun- 
tains west  of  Scott  Valley,  the  mica  schist  has  again  asserted  its 
supremacy. 
Diller,177  in  1905,  summarizes  the  geology  of  the  crystalline  rocks 
of  northern  California.  The  oldest  rocks  exposed  in  the  northern  end 
of  the  Sierra  Nevada  contain  characteristic  Silurian,  possibly  Ordo- 
vician,  fossils.  There  is  a  very  small  area  of  them,  wTith  no  character- 
istic Devonian,  brought  up  by  an  overthrust  fault  to  the  east  of 
Grizzly  Ridge,  near  Taylorsville.a  Northwest,  beyond  the  Sierra,  in 
the  Klamath  Mountains,  the  oldest  fossiliferous  rocks  are  of  Devo- 
nian age.&  They  are  widely  distributed  and  where  not  bounded  by 
igneous  rocks  are  limited  on  the  one  hand  by  Carboniferous  sedi- 
ments and  on  the  other  by  crystalline  schists,  which  Hershey  c  referred 
in  a  general  way  to  about  the  horizon  of  the  Algonkian.  Although 
the  Devonian  is  well  established  in  the  Klamath  Mountains,  with 
older  rocks  beneath,  no  trace  of  Silurian  fossils  has  been  found  there. 
The  presence  of  Silurian  and  Devonian  sediments  in  the  northern 
part  of  the  State,  both  but  slightly  altered,  precludes  the  nietamorph- 
ism  of  Silurian  strata  in  the  Klamath  Mountains  before  the  deposi- 
tion of  the  Devonian  and  affords  a  good  reason  for  regarding  the 
schists  which  underlie  the  Devonian  as  pre-Silurian  in  age  and  pos- 
sibly older  than  the  Ordovician. 
The  pre-Silurian  rocks  of  the  Klamath  Mountains  are  chiefly  mica 
schists   and  occur  in  three   distinct  belts,   one   in   the   South   Fork 
a  Bull.  Geol.  Soc.  America,  vol.  3,  1892,  pp.  369-394. 
"Bull.  U.  S.  Geol.  Survey  No.  19G,  1902,  p.  63.  Also  Am.  Jour.  Sci.,  4th  ser.,  vol.  15, 
1903,  pp.  342-362. 
cAm.  Geologist,  vol.  27,  1901,  pp.  226-230,  239-245. 
