18  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
studied  continuously  since  geological  study  began  in  America,  the 
extreme  difficulty  and  complexity  of  New  England  geology  may  be 
realized. 
In  New  York,  Pennsylvania,  New  Jersey,  and  Maryland  and  adja- 
cent parts  of  Virginia  a  greater  proportional  advance  has  been  made. 
For  most  of  this  region  the  pre-Cambrian  has  been  separated  from 
the  later  rocks  and  the  lithological  relations  have  been  determined. 
There  is  also  partial  agreement  as  to  the  succession  and  correlation  of 
these  units,  but  there  remain  important  differences  of  opinion  in  these 
respects. 
In  the  southern  Piedmont  region  detailed  work  has  been  done  in 
several  areas  aggregating  7,000  square  miles,  resulting  in  the  separa- 
tion of  the  pre-Cambrian  units.  Outside  of  these  areas  the  great 
problem  is  still  the  separation  of  the  pre-Cambrian  from  the  later 
rocks.  The  extreme  weathering  is  the  principal  obstacle  in  the  way 
of  geological  mapping. 
The  isolated  areas  of  the  Mississippi  Valley  are  all  well  known 
and  for  the  most  part  adequately  outlined. 
The  distribution  of  the  pre-Cambrian  rocks  in  the  Rocky  Mountain 
region  is  fairly  well  known,  though  there  are  areas  now  assigned  to 
the  pre-Cambrian  which  may  be  found  to  be  of  later  age.  Detailed 
studies  of  the  pre-Cambrian  of  this  region  have  been  made  in  but  few 
localities. 
In  the  Great  Basin  region  the  crystalline  axes  of  the  mountain 
ranges  were  called  pre-Cambrian  to  a  large  extent  before  their  re- 
lations to  the  Cambrian  were  determined.  To  what  extent  these  rocks 
are  really  pre-Cambrian  and  to  what  extent  post-Cambrian  is  only 
partially  known.  There  has  been  almost  no  detailed  mapping  of 
the  pre-Cambrian  rocks  in  this  region. 
In  the  Sierra  Nevada,  in  the  Coast  Range,  and  in  the  Klamath 
Mountains  the  crystalline  core  rocks  have  been  thought  to  be  partly 
pre-Cambrian,  but  the  next  overlying  rocks  are  so  late  in  age  and  the 
region  has  been  so  deformed  by  late  earth  movements  that  a  dis- 
crimination of  the  pre-Cambrian  from  later  rocks  is  yet  unsatisfac- 
tory. Almost  nothing  has  been  done  in  the  way  of  the  subdivision 
of  the  pre-Cambrian. 
DISTINGUISHING  FEATURES  OF  PRE-CAMBRIAN  ROCKS. 
There  is  difficulty  in  defining  general  distinguishing  features  of 
the  pre-Cambrian  rocks  because  they  differ  greatly  among  them- 
selves. It  will  be  seen  later  (pp.  19-25)  that  they  may  be  naturally 
divided  into  two  systems.  The  physical  contrasts,  from  almost  every 
point  of  view,  between  these  two  systems  are  in  places  greater  than 
between  the  later  system  and  the  lower  Paleozoic.  Practically  the 
only  uniform  distinction  between  the  later  system  and  the  Paleozoic  is 
