THE    CORDILLERAS.  885 
The  information  now  at  hand  points  to  the  absence  of  a  pre- 
Cambrian  complex  in  Alaska  that  may  be  assigned  to  the  Archean. 
The  rocks  formerly  assigned  to  the  Archean  have  been  found  for  the 
most  part  to  be  altered  igneous  rocks  intruded  into  what  are  chiefly 
Paleozoic  sediments. 
METAMORPHIC  SEDIMENTS  OF  UNDETERMINED  AGE. 
The  oldest  sediments  of  the  province  form  a  part  of  a  complex 
metamorphic  series  of  which  little  is  known  except  their  general 
distribution.  These  form  a  broad  belt,  in  many  places  broken  by 
areas  of  younger  rocks,  which  stretches  from  British  Columbia  across 
central  Alaska  and  northwestward  toward  Bering  Strait.  In  this 
are  included  rocks  of  diverse  lithological  types  and  ranging  in  age 
from  Ordovician,  probably  the  Cambrian,  or  pre-Cambrian,  through 
the  Paleozoic  column,  but  are  chiefly  pre-Devonian.  It  is  by  no 
means  impossible  that  even  some  Mesozoic  terranes  may  occur  in  this 
great  complex,  but  little  has  been  accomplished  toward  its  differen- 
tiation and  mapping. 
In  the  Yukon-Tanana  region  a  series  of  closely  folded  siliceous 
schists,  phyllites,  and  quartzites,  associated  with  some  crystalline 
limestones,  appears  to  include  the  oldest  sediments.  According  to 
SpuiT,"  this  series,  called  the  Birch  Creek  schist,  typified  by  highly 
arenaceous  sediments,  often  carrying  graphite,  occurs  above  the 
arkose  conglomerate.  Spurr's  estimate  of  25,000  feet  thickness 
for  these  rocks  is  probably  much  too  great,  for  Prindle's  h  more  de- 
tailed studies  in  the  same  field  have  shown  that  the  folding  is  so 
complex  as  almost  to  defy  accurate  measurements.  Field  studies 
made  in  1906  go  to  show  that  the  Birch  Creek  schist  is  probably  un- 
coil formably  overlain  by  Ordovician  limestones. 
The  arenaceous  sediments  pass  gradually  upward  into  calcareous 
rocks  and  these  are  succeeded  by  crystalline  limestones  interbedded 
with  quartzites.  The  latter,  called  the  Fortymile  "  series"  by  Spurr,c 
are  terranes  typically  made  up  of  marbles  and  quartzites,  with  horn- 
blende, garnetiferous,  and  sometimes  graphitic  schists.  Igneous  in- 
trusives  are  abundant  in  both  the  Birch  Creek  schist  and  the  Forty- 
mile  "  series,"  as  are  also  quartz  veins,  the  latter  being  the  source  of 
the  placer  gold.  The  Rampart  "series,"  probably  of  Devonian  age, 
appears  to  overlie  these  two  terranes  unconformable. 
a  Spurr,  J.  E.,  The  geology  <>f  the  Yukon  gold  district  :  Eighteenth  Ann.  Rept.  D".  S. 
Geol.  Survey,  pt.   3,   1898,  pp.   140-145. 
b  Prindle,  L.  M.,  The  gold  placers  <>f  the  Fortymile,  Birch  Creek,  and  Fairbanks  regions  : 
Bull.  U.  S.  Geol.  Survey  No.   251,  1905. 
cOp.  cit.,  pp.    145-154. 
