GENERAL  ACCOUNT   OF   THE   PRE-CAMBRIAN    ROCKS.  19 
the  relative  sparseness  of  fossils  in  the  pre-Cambrian.  As  a  whole, 
pre-Cambrian  rocks  are  characterized  by  a  relatively  high  degree  of 
crystallinity  and  metamorphism,  by  a  great  abundance  of  igneous 
rocks,  and  by  schists  and  gneisses  of  doubtful  origin,  in  all  of  these 
respects  differing  in  general  from  the  post-Cambrian  rocks.  Locally 
in  the  Appalachians,  where  the  post-Cambrian  rocks  have  been 
closely  folded  and  intruded  with  great  masses  of  igneous  rocks,  they 
have  been  so  metamorphosed  that  their  discrimination  from  the  pre- 
Cambrian  rocks  is  extremely  difficult.  On  the  other  hand,  in  the 
Lake  Superior  region  certain  of  the  later  pre-Cambrian  rocks  have 
been  so  little  metamorphosed  that  they  resemble  the  unmetamor- 
phosed  Cambrian  sediments  of  this  region,  and  geologists  have 
differed  as  to  whether  they  should  be  correlated  with  the  Cambrian 
or  the  pre-Cambrian.  In  various  parts  of  the  Cordilleras  igneous 
rocks  of  post-Cambrian  age  are  so  abundant  that  pre-Cambrian  age 
may  not  be  premised  for  a  series  predominantly  igneous.  The  re- 
verse of  this  appears  in  the  Lake  Superior  region.  Distinct  fossils, 
while  lacking  in  the  pre-Cambrian  of  most  areas,  are  found  in  at 
least  two  pre-Cambrian  areas  of  the  United  States.  The  pre-Cam- 
brian of  the  great  shield  is  separated  from  later  rocks  by  a  great  un- 
conformity. This  is  true  for  other  pre-Cambrian  areas,  but  not  for 
all.  In  the  Appalachian  and  Piedmont  regions  the  known  Cambrian 
grades  down  into  the  Ocoee  group,  here  assigned  in  part  to  the  pre- 
Cambrian.  In  northwestern  United  States  and  adjacent  parts  of 
British  Columbia  the  Belt  series  and  its  equivalents  are  separated 
from  the  Cambrian  by  an  inconspicuous  erosion  interval,  and  in 
places  seem  to  grade  directly  into  the  Cambrian. 
The  pre-Cambrian  rocks  illustrate  all  processes  of  metamorphism, 
including  katamorphism  and  anamorphism.  Indeed,  so  largely  are 
they  altered  that  the  study  of  pre-Cambrian  geology  involves  a  study 
of  the  principles  of  metamorphism.  Especially  well  shown  are  the 
changes  which  accompany  the  development  of  schistose  structures 
under  deep-seated  conditions.  The  exposures  of  the  pre-Cambrian 
of  northern  North  America  have  been  so  stripped  of  their  Aveathered 
products  by  glaciation  that  excellent  opportunities  are  afforded  for 
studying  deeper-seated  alterations. 
SUBDIVISION  OF  THE  PRE-CAMBRIAN. 
THE    DUAL   CLASSIFICATION. 
The  United  States  Geological  Survey  has  since  1889  recognized  a 
dual  division  of  the  pre-Cambrian  into  Archean   and  Algonkian.0 
a  Tenth  Ann.  Kept,  (for  1888-89^  U.  S.  Geol.  Survey,  1890,  p.  66;  also  Twenty-fourth 
Ann.  Rept.   (for  1902-3)   U.  S.  Geol.  Survey,  1903,  p.  26. 
