GENEEAL  ACCOUNT   OF   THE   PRE-CAMBKIAN    ROCKS.  17 
York  are  outliers  of  pre-Cambrian  rocks  surrounded  by  younger  sedi- 
ments, which,  while  closely  related  in  distribution  to  the  Appalachian 
system,  do  not  form  any  part  of  it.  In  western  United  States  pre- 
Cambrian  rocks  form  parts  of  some  of  the  Cordilleran  ranges,  and  in 
northern  Montana  and  Idaho  and  in  southern  British  Columbia  they 
occupy  very  large  areas. 
In  the  Mississippi  Valley  isolated  areas  of  pre-Cambrian  rocks 
appear  in  Wisconsin,  Minnesota,  Iowa,  Dakota,  Missouri;  Indian 
Territory,  and  Texas. 
The  general  map  (PL  I)  discloses  an  imperfect  concentric  group- 
ing of  the  pre-Cambrian  areas  of  North  America,  with  the  pre-Cam- 
brian shield  as  the  center.  Similarly  it  has  often  been  noted  that  the 
mountain  folds  of  North  America  are  roughly  concentric  with  the 
pre-Cambrian  shield.  Within  the  pre-Cambrian  shield  itself  there  is 
a  tendency  for  linear  distribution  of  the  pre-Cambrian  subdivisions 
parallel  to  its  borders,  corresponding  to  axes  of  folding. 
STATE  OF  KNOWLEDGE. 
Of  the  great  pre-Cambrian  shield  only  the  southern  margin,  border- 
ing on  the  St.  Lawrence  and  the  Great  Lakes,  has  been  studied  in  de- 
tail. Because  of  the  excellent  and  fresh  exposures  here  developed  by 
glacial  erosion,  because  of  the  fullness  of  the  pre-Cambrian  succession, 
and  finally  because  of  the  commercial  value  of  the  deposits  of  iron, 
copper,  nickel,  cobalt,  and  silver  in  the  pre-Cambrian  of  the  Lake 
Superior-Lake  Huron  region,  the  geology  of  the  southern  margin  of 
the  pre-Cambrian  shield  has  become  better  known  than  that  of  most 
other  pre-Cambrian  areas  of  North  America.  Knowledge  of  this 
region  is  of  special  importance  from  the  point  of  view  of  classification 
of  the  pre-Cambrian  formations,  since  this  region  is  the  southern  part 
of  the  great  pre-Cambrian  shield,  and  appears  to  exhibit  all  the  ele- 
ments of  that  shield.  The  area  of  the  pre-Cambrian  rocks  for  this 
immense  region  is  vastly  greater  than  for  all  of  the  remainder  of 
North  America,  and  the  fullness  of  the  record  there  exhibited,  so  far 
as  yet  known,  is  not  surpassed  elsewhere. 
In  Newfoundland,  Nova  Scotia,  New  Brunswick,  and  southern 
Quebec  the  mapping  of  the  pre-Cambrian  is  sufficiently  advanced  to 
make  possible  a  fairly  definite  statement  of  the  general  pre-Cambrian 
succession  of  the  regions.  For  much  of  New  England  the  pre-Cam- 
brian rocks  have  not  been  discriminated  from  crystalline  rocks  of  later 
age,  and  where  they  have  been  discriminated  the  details  of  the  pre- 
Cambrian  succession  for  large  areas  are  not  known.  In  a  few  area-. 
particularly  in  southern  New  England,  the  pre-Cambrian  has  been 
mapped  in  detail.  If  it  is  remembered  that  New  England  has  been 
the  home  of  many  of  America's  leading  geologists    and    has    been 
55721— Bull.  360—09 2 
