198  PRE-CAMBRTAN   ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
to  some  extent  true.  That  the  rocks  in  the  different  districts  referred 
to  these  periods  have  the  same  absolute  duration  would  hardly  be  ex- 
pected. Sedimentation  in  the  series  correlated  probably  continued 
longer  in  certain  parts  of  the  region  than  in  others.  A  break  even  if 
as  widespread  as  believed  probably  did  not  begin  nor  end  everywhere 
at  the  same  time;  and  certainly  it  would  be  true,  after  a  certain  move- 
ment and  erosion  had  ended,  as  the  sea  began  to  encroach,  that  sedi- 
mentation would  begin  in  one  district  before  it  reached  another.  So 
that  if  these  correlations  are  correct  and  the  breaks  really  general,  as 
is  believed,  it  does  not  follow  that  the  periods  opened  or  closed  simul- 
taneously, but  that  they  stand  in  a  general  way  as  equivalent.  The 
equivalency  advocated  may  be  much  more  strongly  asserted  of  the  dis- 
tricts immediately  adjacent  to  lake  Superior  than  of  the  more  remote 
districts. 
The  further  question  arises,  whether  as  work  continues  new  breaks  of 
considerable  magnitude,  not  now  recognized,  will  be  found.  This  is  not 
improbable.  In  fact,  there  is  already  some  indication  of  such  a  break, 
although  not  now  capable  of  being  proved  at  any  point.  Does  the  series 
of  great  conglomerates  which  are  placed  at  the  base  of  the  Upper  Hu- 
ronian,  the  debris  being  derived  mostly  from  the  Lower  Huronian,  grade 
conformably  upward  into  the  Animikie,  or  is  there  here  a  considerable 
additional  break?  It  is  by  no  means  certain  that  the  truth  is  not  with 
the  latter  alternative,  for  the  conglomerates  certainly  seem  to  have  suf- 
fered more  intense  dynamic  action  than  the  adjacent  Animikie.  Their 
thickness  is  great,  and  it  may  well  be  that  in  the  Thunder  bay  district 
will  be  found  a  considerable  break  which  in  a  part  at  least  of  the  lake 
Superior  region  will  subdivide  the  Upper  Huronian. 
Also  as  a  problem  for  the  future  is  the  real  nature  of  the  Archean 
schists.  Are  they  clastic  or  igneous  in  origin?  Are  they,  as  has  been 
supposed,  a  real  fundamental  complex,  or  will  this  be  subdivided  upon 
a  structural  basis?  A  few  years  ago  all  below  the  pre-Cambrian  was  a 
fundamental  complex.  Will  not  the  future  find  our  present  funda. 
mental  complex  further  divisible  and  the  real  fundamental  complex  at 
a  still  lower  horizon? 
Accepting  the  general  stratigraphy  as  given  above,  how  far  will  it  be 
possible  to  correlate  the  individual  formations  of  the  series?  How  far 
are  the  Quartz- Slate  member,  the  Iron-bearing  member,  and  the  Up- 
per Slate  member  of  the  Penokee  series  equivalent  to  those  of  the  Ani- 
mikie? When,  as  in  this  case,  three  like  formations  of  great  thickness 
are  found  in  the  same  order,  and  the  two  series  as  wholes  bear  identical 
relations  to  underlying  and  overlying  series,  the  correlation  may  per- 
haps be  made  with  a  considerable  degree  of  probability,  and  later  closer 
work  rather  leads  to  the  conclusion  that  much  will  be  accomplished  in 
the  direction  of  correlating  formations ;  that  is,  several  of  the  series  may 
be  divided  into  two  or  three  or  more  members,  which  may  with  a  con- 
siderable degree  of  probability  be  correlated  with  equivalent  members 
