GENERAL   ACCOUNT    OF    THE    PRE-CAMBRIAN    ROCKS.  37 
are  entirely  satisfactory  as  a  basis  of  classification  where  they  conflict. 
The  broad  problem  of  harmonizing  these  criteria  in  a  satisfactory 
classification  remains  open.  In  the  meantime  the  paleontological 
basis  for  classification  will  be  adhered  to  because  for  the  most  part  it 
is  in  accord  with  structural  evidence ;  it  has  been  the  accepted  stand- 
ard in  the  past, 
The  difficulty  of  determining  the  plane  of  demarcation  between 
known  Cambrian  and  pre-Cambrian  series  arises  for  only  a  small  part 
of  North  America.  For  much  the  larger  portion  of  the  continent  dis- 
tinctly recognizable  Cambrian  rocks  rest  with  profound  unconformity 
upon  distinctly  recognizable  pre-Cambrian  rocks.  Most  of  the 
Archean  and  Algonkian  continent  was  practically  base-leveled  before 
and  during  the  encroachment  of  the  Cambrian  sea.  Occasional 
monadnocks  stood  out  upon  this  plain.  About  these  the  base  of  the 
Cambrian  is  marked  by  coarse  conglomerates,  but  for  the  most  part 
the  disintegrated  and  decomposed  surface  of  the  peneplain  yielded, 
to  the  encroaching  Cambrian  sea,  above  conglomerate  beds  of  only 
moderate  thickness,  somewhat  fine-grained  and  well-assorted  frag- 
mental  sediments,  such  as  quartz  sands  and  muds. 
Difficulty  in  discriminating  the  Cambrian  from  the  pre-Cambrian 
rocks  sometimes  arises  from  the  fact  that  for  extensive  areas  the 
lowest  recognizable  Cambrian  rocks  are  of  upper  Cambrian  age, 
unconformably  below  which  are  nonfossiliferous  rocks  referred  to 
the  Algonkian.  Because  the  overlying  rocks  are  upper  Cambrian,  in 
some  cases  it  has  been  argued  that  the  lower  rocks  are  Cambrian. 
This  position  has  been  taken  in  reference  to  the  Keweenawan  series 
of  the  Lake  Superior  region,  its  nonfossiliferous  character  being  ex- 
plained by  volcanism  during  its  deposition.  In  cases  of  this  sort  no 
hard  and  fast  rule  can  be  laid  down  for  their  classification,  but  each 
case  should  be  decided  independently  on  its  own  merits.  In  the  Lake 
Superior  region  it  is  believed  that  the  magnitude  of  the  unconformity 
separating  the  Cambrian  from  the  Keweenawan,  with  the  great 
thickness  of  the  Keweenawan  and  the  lack  of  fossiliferous  content, 
combined  with  the  fact  that  this  unconformity  is  a  part  of  the 
great  slanting  unconformity  already  mentioned  as  extending  over  a 
large  part  of  the  continent,  the  deposition  of  which  extended  from 
Algonkian  to  Ordovician  time,  throws  the  probability  strongly 
toward  the  pre-Cambrian  age  of  the  Keweenawan  rocks.  (See 
pp.  349-351.) 
UNCONFORMITIES  WITHIN  THE  ALGONKIAN. 
Three  marked  unconformities  are  known  within  the  Algonkian  of 
the  Lake  Superior  and  Lake  Huron  regions.  These  are  evidenced  by 
basal  conglomerates,  by  erosion  of  the  underlying  series,  by  difference 
