van-  hise.j  DISCUSSIONS    OF    PRINCIPLES.  4 13  7 
small  as  to  be  useless  tor  the  purposes  of  correlation.  The  structure  of 
individual  districts  and  regions  could  be  worked  out  and  the  formations 
correlated,  but  the  attempt  to  equate  the  Cambrian,  Silurian,  Devonian, 
or  Carboniferous  of  one  region  with  rocks  of  the  same  age  in  a  far  dis- 
tant one  would  be  an  almost  hopeless  undertaking.  In  the  Carbonif- 
erous the  beds  of  coal  would  serve  as  an  important  guide,  but  if  im- 
plicitly followed  and  no  fossils  were  available  the  Triassic  of  Virginia, 
the  Carboniferous  of  the  central  United  States,  and  the  Cretaceous  of 
the  west  would  be  placed  together.  If  the  iron  carbonate  formations 
of  the  Algonkian  in  the  lake  Superior  region,  which  appear  to  be  the 
most  characteristic  of  any  one  kind  of  rock,  were  followed  as  a  guide, 
the  results  would  probably  be  as  far  from  the  truth. 
We  may,  perhaps,  go  so  far  in  some  cases  as  to  correlate  series  which 
occur  in  different  districts  of  the  same  region  whe'n  a  set  of  character- 
istic formations  forming  the  series  occur  in  like  order  and  the  series  as 
a  whole  is  in  the  same  relative  position  to  overlying  and  subjacent 
series,  one  or  both  of  which  are  known  to  be  identical  in  both  districts. 
It  is  probable,  when  several  pre-Cambrian  series  occur  of  the  same 
general  character,  with  like  relations  to  each  other  and  to  the  Archean 
and  Cambrian,  and  not  so  far  apart  as  to  be  outside  of  the  same  geo- 
logical basin,  that  a  provisional  correlation  is  warranted.  While,  then, 
it  is  not  practicable  to  subdivide  the  Algonkian  into  general  systems 
which  shall  cover  the  whole  of  North  America,  it  is  often  possible  so  to 
do  in  a  single  geological  basin,  or  in  adjacent  basins  in  which  the  rela- 
tions  of  the  separate  formations  and  series  can  be  worked  out. 
Before  considering  the  principles  applicable  to  the  subdivision  and  cor- 
relation of  the  Algonkian  series,  it  will  perhaps  be  well  to  review  briefly 
the  regions  in  which  pre-Cambrian  rocks  occur,  and  indicate  their  char- 
acter and  relations,  as  well  as  their  relations  to  the  Archean.  The  order 
followed  is  that  of  the  review  of  literature.  No  attempt  is  made  to  give 
detailed  evidence  for  the  conclusions  stated.  For  this  it  will  be  neces- 
sary to  refer  to  the  fuller  accounts  of  the  several  regions  in  the  previous 
chapters. 
THE   ORIGINAL  LAUEENTIAN   AND   ASSOCIATED    AREAS. 
Iii  this  region  are  Algonkian  rocks  at  the  following  localities :  Hast- 
ings district,  lake  Mpissing,  Ottawa  river,  and  Upper  St.  Lawrence 
river.  The  Grenville  area  of  the  Ottawa  is  the  original  Laurentian 
type  district  and  the  one  mapped  in  most  detail.  While  the  maps  do 
not  connect  these  areas,  the  similarity  of  their  clastic  rocks  is  such  as  to 
indicate  a  present  or  former  continuity,  with  the  exception,  perhaps,  of 
those  of  lake  Mpissing.  The  elastics  consist  of  interstratified  lime- 
stones, quartzites,  conglomerates,  green  slates  and  schists,  mica- 
schists,  hornblende-schists,  and  regularly  bedded  gneisses,  together 
estimated  to  be  thousands  of  feet  thick.  Associated  with  these  are 
diabasic  and  chloritic  rocks,  both  massive  and  schistose. 
Bull.  86 32 
