890  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
A  second  belt  of  metamorphic  rocks  forms  the  major  part  of  the 
Alaska  Range,  and  Ordovician0  fossils  have  been  found  at  a  horizon 
which  probably  occurs  near  the  base  of  this  series,  but  it  is  not  im- 
possible that  pre-Cambrian  sediments  are  also  included. 
A  broad  belt  of  metamorphosed  sediments  is  shown  on  the  map 
between  Yukon  and  Tanana  rivers,  and  in  this  are  known  to  be 
Devonian h  and  Carboniferous,  and  probably  Silurian,  horizons. 
Here,  too,  pre-Cambrian  beds  may  be  included. 
Schrader0  has  described  a  great  complex  of  metamorphic  sedi- 
ments, in  which  Carboniferous  fossils  have  been  found  near  the  top 
and  some  doubtful  Silurian  forms  in  a  limestone  occurring  near  the 
bottom.  Beneath  this  limestone  are  mica  schists  or  phyllites,  which 
Schrader  has  named  the  Totsen  "  series," d  and  which  have  been 
described. 
These  facts  indicate  that  pre-Cambrian  horizons  will  probably  not 
be  found  among  the  terranes  here  classed  as  metamorphosed  rocks, 
chiefly  Paleozoic  and  Mesozoic,  though  definite  proof  of  their  absence 
must  await  detailed  surveys. 
SUMMARY. 
Archean  rocks  are  probably  not  present  in  Alaska,  though  some 
gneissic  areas  lying  in  the  Yukon-Tanana  region  have  been  pro- 
visionally assigned  to  this  period.  The  proof  of  the  presence  of  pre- 
Cambrian  sediments  is  also  lacking.  Though  highly  metamorphosed 
sediments  have  been  found  in  many  different  localities,  the  evidence 
a])] tears  to  point  to  the  conclusion  that  they  are  for  the  most  part 
Paleozoic,  with  some  Mesozoic  terranes.  In  the  Yukon-Tanana 
region  there  are  extensive  areas  of  mica  schists,  termed  the  Birch 
Creek  schist,  which  are  probably  Cambrian  or  pre-Cambrian.  North 
of  the  Yukon  the  same  horizon  may  possibly  be  recognized  in 
Schrader's  Totsen  "series."  In  Seward  Peninsula  there  is  an  exten- 
sive development  of  metamorphic  rocks  known  to  be  pre- Silurian. 
The  oldest  of  these,  the  Kigluaik  group,  including  a  great  thickness 
of  mica  schists,  with  some  crystalline  limestones,  appear  to  be  the 
oldest  sediments  which  have  been  found  in  Alaska.  This  group  out- 
crops along  what  seems  to  be  the  eroded  axis  of  the  anticlinal  uplift 
in  the  heart  of  the  Kigluaik  Mountains. 
a  Brooks,  Alfred  H.,  An  exploration  in  the  Mount  McKinley  region,  Alaska  :  Prof.  Paper 
U.  S.  Geol.  Survey.      (In  manuscript.) 
6  Prindle,  L.  M.,  and  Hess,  F.  L.,  unpublished  notes. 
c  Schrader,  F.  C,  A  reconnaissance  in  northern  Alaska,  etc.  :  Prof.  Paper  U.  S.  Geol. 
Survey  No.  20,  1904,  pp.  55-72. 
d  The  writer  has  elsewhere  discussed  the  evidence  by  which  the  Totsen  group  is  consid- 
ered older  than  the  limestone  carrying  the  doubtful  Silurian  fossils.  Compare  The  geog- 
raphy and  geology  of  Alaska  :  Prof.  Paper  U.  S.  Geol.  Survey  No.  45,  1906. 
