882  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
SECTION  12.     ALASKA. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
The  following  summary  has  been  prepared  by  Alfred  H.  Brooks : 
INTRODUCTION. 
As  the  systematic  investigations  of  the  geology  of  Alaska  have  been 
carried  on  for  less  than  a  decade  and  there  remain  many  large  almost 
unexplored  areas  in  the  Territory,  the  conclusions  herein  presented  a 
must  be  regarded  as  tentative.  The  work  having  been  directed  along 
economic  lines,  considerable  attention  has  been  paid  to  the  metamor- 
phic  rocks  which  are  the  source  of  the  gold,  but  the  problems  they 
present  are  complex.  Few  definite  statements  regarding  their  succes- 
sion and  structure  can  be  made. 
No  positive  evidence  has  yet  been  found  of  the  presence  of  pre- 
Cambrian  rocks  in  Alaska,  but  there  are  many  facts  which  have  been 
interpreted  to  indicate  such  a  conclusion.  A  series  of  gneisses  and 
mica  schists  in  the  Yukon  basin,  formerly  accepted  as  the  base  of  the 
stratigraphic  succession  and  provisionally  referred  to  the  Archean 
as  this  term  is  used  by  the  United  States  Geological  Survey,  are  now 
known  to  be  chiefly  altered  intrusives.  These  gneisses  are  associated 
with  a  great  complex  of  metamorphic  sediments,  intruded  by  many 
igneous  rocks,  whose  age  and  stratigraphic  relations  are  still  largely 
in  doubt.  Fossils  from  some  of  these  metamorphic  rocks  range  in  age 
from  the  Ordovician  to  the  Carboniferous  and  possibly  to  the 
Triassic.  Eliminating,  however,  all  of  the  rocks  wThich,  on  direct  or 
indirect  evidence,  are  assignable  to  the  Paleozoic,  there  still  remain 
vast  thicknesses  of  metamorphic  sediments  whose  age  is  indetermin- 
able and  may  be  pre-Cambrian. 
Therefore,  though  little  is  known  of  the  oldest  horizons,  the  areas 
to  which  they  are  probably  limited  are  fairly  well  defined.  As  part  of 
the  Paleozoic  and  even  the  Mesozoic  rocks  have  been  highly  met  amor- 
phosed  and  may  include  pre-Cambrian  horizons,  it  becomes  necessary 
here  to  consider  briefly  all  of  the  areas  of  metamorphic  rocks,  but 
detailed  discussion  will  be  limited  to  those  terranes  which  seem  the 
most  likely  to  include  pre-Cambrian  rocks. 
Of  Alaska's  586,400  square  miles,  less  than  150,000  have  been  geo- 
logically mapped,  but  there  are  random  notes  on  much  of  the  remain- 
der, and  these,  together  with  the  inferences  drawn  from  known 
areas,  form  the  basis  for  four  subdivisions.  The  youngest  embraces 
all  those  areas  believed  to  be  occupied  by  rocks  which  include  no  pre- 
Cambrian  and  no  metamorphic  horizons.     In  the  next  younger  divi- 
« This  paper  is  in  part  abstracted  from  The  geography  and  geology  of  Alaska,  by 
Alfred  H.  Brooks  :  Prof.  Paper  U.  S.  Geol.  Survey  No.  45,  1906. 
