GENERAL   ACCOUNT    OP    THE    PRE-CAMBRIAN    ROCKS.  25 
fication  is  based  on  the  largest  number  of  essential  factors  common  to 
many  districts,  it  is  of  value  as  an  aid  in  grasping  the  essential  points 
in  similarity  of  geology  and  serves  as  a  standard  for  the  measure- 
ment and  expression  of  local  variations.  In  some  regions  it  is  diffi- 
cult, indeed  impossible,  to  place  the  dividing  plane  between  the  Cam- 
brian and  the  Ordovician.  For  instance,  in  the  eastern  United 
States  it  has  been  found  so  impracticable  to  do  this  that  a'  great  lime- 
stone formation  is  known  as  the  Cambro-Ordovician  limestone.  In 
various  regions  it  is  difficult  to  place  the  plane  between  the  Paleozoic 
and  the  Mesozoic,  and  elsewhere  between  the  Mesozoic  and  the  Ceno- 
zoic.  The  same  is  true  also  of  the  Cambrian  and  the  pre-Cambrian. 
For  instance,  although  it  is  easily  possible  to  place  the  boundary 
between  the  Paleozoic  and  the  pre-Cambrian  in  Montana,  yet  in  the 
area  along  the  international  boundary  and  to  the  north  in  Canada 
it  is  extremely  difficult  to  make  this  division,  and  the  discrimination 
has  not  yet  been  made.  These  difficulties  in  making  discrimina- 
tions locally  between  geological  divisions  which  are  generally 
recognized  are  especially  likely  to  be  found  where  there  is  continuity 
in  the  sediments  of  different  periods,  where  fossils  are  absent  or  have 
been  obliterated,  where  there  has  been  strong  metamorphism,  and 
where  great  igneous  intrusions  have  taken  place. 
The  fact  of  the  matter  is  that  geological  classification  must  be 
made  up  from  regions  where  the  relations  are  known  rather  than 
from  regions  where  the  relations  are  not  known.  To  follow  any 
other  method  than  this  would  hopelessly  bar  geological  progress.  As 
has  been  pointed  out,  in  the  great  area  of  the  pre-Cambrian  shield 
of  Canada,  which  should  give  the  basis  for  the  division  of  the  pre- 
Cambrian  rocks  of  North  America,  it  has  been  practicable,  wherever 
close  studies  have  been  made,  to  separate  the  Archean  and  the  Algon- 
kian.  The  same  has  been  equally  true  in  the  great  pre-Cambrian 
area  of  northern  Europe,  including  Scotland,  Scandinavia,  and 
Finland,  and  also  in  China. 
It  is  therefore  certain  that  the  discrimination  between  the  Archean 
and  the  Algonkian  is  one  of  fundamental  importance  in  the  vast 
areas  of  pre-Cambrian  which  should  give  the  basis  for  classification. 
In  some  outlying  areas,  where  it  is  not  practicable  or  advisable  in 
the  present  state  of  knowledge  to  make  the  division  between  Archean 
and  Algonkian,  the  rocks  may  be  called  pre-Cambrian.  In  this  mat- 
ter we  follow  well-known  procedure.  For  instance,  in  various 
regions  all  the  rocks  antedating  the  Mesozoic  are  so  complexly  inter- 
mingled and  so  difficult  to  discriminate  that  they  arc  called  simply 
pre-Mesozoic,  leaving  to  the  future  the  separation  of  the  different 
units  of  the  basement  complex,  which  may  include  all  rocks  ante- 
dating the  Mesozoic.  Many  other  illustrations  of  the  same  principle 
could  be  given. 
