80  PBE-CAMBKIAN    GEOLOGY   OF    NOBTH    AMEBICA. 
It  is  denied  that  Eozoon,  beds  of  limestone,  the  presence  of  graphite 
in  crystalline  limestone,  or  any  other  discovered  material  in  the  pre- 
Potsdam  rocks  is  sufficient  evidence  for  the  presence  of  life. 
It  is  considered  that  we  are  fully  justified  in  saying  that  the  results 
of  geological  investigation  during  the  last  thirty-five  years  have  given 
no  encouragement  to  the  idea  that  below  the  well-known  Primordial 
zone — the  Potsdam  sandstone  of  American  geologists — there  is  an- 
other series  of  fossiliferous  rocks. 
If  the  Azoic  rocks  are  really  azoic,  as  is  believed,  then  it  follows,  as 
a  matter  of  course,  that  the  series  thus  designated  can  be  separated 
into  subsystems  only  on  purely  lithological  grounds;  if  they  are  fos- 
siliferous, as  held  by  the  Canada  Survey,  it  is  equally  clear  that  any 
subdivisions  proposed  for  them  should  have  a  paleontological  basis. 
It  is  denied  that  Aspidella  and  Arenicolites  spiralis  are  of  organic 
origin. 
If  we  examine  the  often  repeated  statement  that  the  Huronian  un- 
conformably  reposes  on  the  worn  edges  of  the  Laurentian  and  con- 
tains the  debris  of  the  latter,  it  will  be  found  that  in  the  seven  cases 
in  which  the  rocks  referred  to  these  two  formations  were  found  in 
contact  in  the  Canadian  district  the  Huronian,  with  but  two  excep- 
tions, is  said  to  be  conformable  with  and  to  generally  pass  imper- 
ceptibly into  the  Laurentian.  In  one  of  these  two  exceptions  the 
rocks  show  mutually  intrusive  relations,  and  in  the  other  the  Hu- 
ronian abuts  against  and  runs  under  the  Laurentian. 
In  all  cases  in  which  pebbles  and  fragments  of  the  Laurentian  have 
been  found  in  the  Huronian  they  were  seen  occurring  high  up  in  the 
latter  series,  and  not  forming  basement  conglomerates.  All  the  other 
so-called  proof  of  unconformity  has  been  made  out  of  the  fact  that 
the  strike  of  the  foliation  in  the  two  formations,  when  not  in  con- 
tact, has  been  found  to  be  discordant — worthless  evidence  unless  the 
rocks  observed  in  both  formations  be  proved  to  be  sedimentary  and 
the  foliation  be  shown  to  be  coincident  with  the  stratification.  Now, 
if  the  Laurentian  was  an  old  metamorphosed  sedimentary  formation 
which  had  been  upheaved  and  contorted,  and  on  whose  worn  edges 
the  Huronian  was  laid  down,  the  evidence  of  the  fact  ought  to  be 
overwhelming  in  amount  after  the  country  has  been  studied  for  so 
many  years. 
It  is  well  known  that  any  eruptive  rock  so  soon  as  it  comes  in  con- 
tact with  erosive  agencies  will  yield  fragmental  material,  even  before 
it  is  cold,  and  that  much  eruptive  matter  is  ejected  in  a  fragmental 
state,  so  that  in  a  mixed  series  of  eruptive  and  detrital  rocks  nothing 
is  more  common  than  to  have  the  debris  of  one  inclosed  in  another, 
without  that  inclosure  proving  that  the  rocks  differ  in  geological  age. 
This  is  well  known  to  be  the  case  with  the  copper-bearing  rocks  of 
Keweenaw  Point,  and  it  has  been  shown  that  the  iron  ores  of  the 
