40  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
In  general  wherever  an  unconformity  between  the  Algonkian  and 
the  Archean  is  known  there  is  likely  to  be  evidence  of  long-continued 
erosion  of  the  Archean  rocks  prior  to  the  deposition  of  the  Algonkian. 
The  basal  Algonkian  sediments  are  so  frequently  of  a  cleanly  sorted 
quartz-sand  type  as  to  imply  a  preceding  period  of  weathering  of 
long  duration. 
Theoretically  there  should  be  some  place  where  sedimentation  was 
continuous  between  the  Archean  and  the  Algonkian,  for  sedimentary 
rocks  are  known  in  both.  Such  places  may  be  found,  but  positive 
proof  of  such  continuity  is  not  yet  known.  A  number  of  geologists 
have  cited  cases  of  "  gradation  "  between  the  rocks  here  designated 
Archean  and  Algonkian,  but  wherever  the  localities  have  been  ex- 
amined it  has  been  found  that  progressive  metamorphism  has  prob- 
ably obliterated  the  evidence  of  unconformity. 
CORRELATION   OF  PRE-CAMBRIAN  SERIES. 
In  relative  amount  of  deformation  and  metamorphism,  in  lithology, 
in  relative  abundance  of  igneous  and  sedimentary  rocks,  in  stratig- 
raphy, in  the  number  of  unconformities,  and  above  all  in  our  knowl- 
edge concerning  these  rocks,  there  is  wide  variation  among  the  pre- 
Cambrian  areas  of  North  America.  Notwithstanding  these  difficul- 
ties, we  are  able,  for  many  regions,  to  apply  the  dual  classification 
of  the  pre-Cambrian — Archean  and  Algonkian. 
It  has  been  intimated  that  the  Archean  and  Algonkian  are  equiva- 
lent in  magnitude  to  the  Paleozoic  and  Mesozoic.  The  Paleozoic  is 
divided  into  systems  upon  the  basis  of  fossil  evidence.  In  the  Lake 
Superior  region  is  a  fauna  called  Silurian,  similar  to  the  fauna  in  the 
Silurian  rocks  of  New  York  and  Wales,  but  no  way  has  been  found  to 
tell  whether  one  of  the  Algonkian  series  of  the  Lake  Superior  region 
is  equivalent  to  one  which  occurs  in  the  Grand  Canyon  district,  in 
Scotland,  or  in  China.  The  science  of  geology  has  not  sufficiently 
advanced  to  correlate  the  pre-Cambrian  rocks  upon  a  physical  basis 
from  province  to  province.  When  the  physical  history  of  the  con- 
tinents has  been  worked  out,  when  we  know  more  about  paleogeog- 
raphy,  and  when  we  learn  how  extensive  are  the  great  unconformities, 
we  may  be  able  to  correlate  the  pre-Cambrian  series  from  province  to 
province  and  possibly  even  from  continent  to  continent.  Because  of 
these  facts,  no  attempt  is  made  to  correlate  the  several  series  of  the 
Algonkian  in  different  geological  provinces  with  one  another.  How- 
ever, since  the  classification  of  the  Archean  rocks  is  upon  the  basis  of 
lithology  rather  than  upon  stratigraphy,  we  do  not  hesitate  to  carry 
over  to  certain  provinces  the  series  terms  Keewatin  and  Laurentian, 
using  them  with  the  same  significance  given  to  them  in  their  type 
locality,  the  Lake  Superior  region. 
