36  PRE-CAMBR1AN    GEOLOGY    OF    NORTH    AMERICA. 
ozoic  are  not  sufficient  to  serve  any  useful  purpose  in  correlation  from 
province  to  province.  Thus  the  Algonkian  of  the  Lake  Superior 
region  is  much  less  metamorphosed  than  the  Paleozoic  rocks  of  New 
England.  At  most  these  criteria  can  be  used  only  for  discrimina- 
tions in  the  same  region.  Thus  in  the  Lake  Superior  region  the 
Algonkian  rocks  are,  on  the  average,  more  metamorphosed  than  the 
Cambrian,  but  the  upper  Keweenawan  sandstones  differ  little  from 
the  Cambrian  in  degree  of  metamorphism. 
UNCONFORMITY  BENEATH  THE  CAMBRIAN. 
In  late  Algonkian  time  a  large  part  of  the  Algonkian  continent  was 
above  water  and  undergoing  erosion.  With  the  ensuing  encroach- 
ment of  the  sea,  perhaps  from  the  southeast,  south,  and  southwest, 
there  were  deposited,  onto  the  older  basement  over  Avhich  the  sea  was 
transgressing,  lower  Cambrian,  middle  Cambrian,  upper  Cambrian, 
and  Ordovician  sediments,  each  successive  series  overlapping  the 
earlier  ones.  Thus  the  unconformity  at  the  base  of  the  Cambrian 
slants  across  the  base  of  each  of  the  Cambrian  series  into  the  Ordo- 
vician. Also  there  is  much  evidence  to  show  that  this  transgression 
began  in  late  Algonkian  time.  Near  the  margins  of  the  old  Al- 
gonkian land  areas  are  found  Algonkian  sediments  which  appear  to 
be  in  continuous  downward  conformable  succession  below  the  Cam- 
brian rocks.  Thus  in  the  southern  Appalachian  area  about  half  of 
the  so-called  Ocoee  sediments  extends  downward  conformably  several 
thousand  feet  below  the  known  Olenellus  beds,  and  in  the  Wasatch 
and  Uinta  mountains  a  great  quartzite  series  extends  for  12,000  feet 
conformably  below  the  Olenellus  horizon.  The  Bow  River  group  of 
British  Columbia  is  thought  by  Walcott  possibly  to  represent  con- 
tinuous deposition  between  the  Belt  Algonkian  and  the  Cambrian. 
The  question  naturally  arises  whether  these  nonfossiliferous  down- 
ward extensions  of  the  Olenellus  beds  shall  be  called  Cambrian  or  Al- 
gonkian, especially  in  view  of  the  fact  that  they  are  separated  from 
Algonkian  rocks  below  by  great  unconformities.  Since  the  base  of 
the  Cambrian  has  been  defined  as  marked  by  the  Olenellus  horizon, 
the  great  conformable  downward  extensions  have  been  thrown  tenta- 
tively into  the  pre-Cambrian,  and  at  the  present  time  there  seems  to 
be  no  adequate  reason  for  changing  this  usage,  although  in  the  future 
evidence  sufficient  for  including  some  of  these  formations  with  the 
Paleozoic  may  appear.  If  the  unconformity  at  the  base  rather  than 
the  Olenellus  horizon  be  taken  as  the  dividing  plane,  it  would  be 
necessary,  at  places  of  continuous  sedimentation  between  Algonkian 
and  Cambrian,  to  include  considerable  parts  of  the  so-called  Al- 
gonkian in  the  Cambrian,  unless  discriminative  fossil  evidence  should 
be  developed.     Neither  the  paleontological  nor  the  structural  criteria 
