438  PRE-CAMBRIAN   GEOLOGY   OF    NORTH    AMERICA. 
ding,  both  within  and  without  the  original  Huronian  district,  that 
there  is  an  unconformity  between  the  upper  slate  conglomerate  and 
the  lower  slate  conglomerate,  but  he  placed  the  limestone  with  the 
upper  group.  In  1892  Pumpelly  and  Van  Hise  visited  the  area  and 
found  contacts  which  proved  the  existence  of  an  unconformity  be- 
tween the  two  conglomerates.  But  the  upper  slate  conglomerate 
rests  upon  and  carries  fragments  of  the  limestone,  where  this  is  not 
entirely  removed  by  erosion,  thus  showing  that  this  formation  be- 
longs to  the  lower  rather  than  the  upper  group.  In  1902  the  district 
was  again  visited  by  Van  Hise  and  Leith  and  some  six  weeks  spent 
in  attempting  to  ascertain  the  importance  of  this  unconformity. 
The  results  of  this  work,  summarized  on  a  preceding  page,  indicate 
that  the  unconformity  is  one  of  considerable  magnitude,  representing 
a  time  interval  long  enough  to  allow  of  the  truncation  of  the  under- 
lying group.  This  unconformity  was  for  the  most  part  not  recog- 
nized by  the  Canadian  geologists  until  the  time  of  the  meeting  of 
the  international  geological  committee  in  this  district  in  1904. 
Thickness  of  the  Huronian  sediments. — The  structural  work  of 
Van  Hise  and  Leith  in  1902  showed  that  the  thickness  of  the  Huro- 
nian has  probably  been  greatly  overestimated. 
Relations  of  Huronian  sediments  to  underlying  rocA'S.-^-Smce 
Logan  and  Murray's  time  the  "  Laurentian  "  granite  has  been  until 
recently  regarded  by  Canadian  geologists  not  only  as  intrusive  into 
what  they  call  the  "Lower  Huronian,"  but  by  Barlow  as  intrusive 
even  into  the  "  Upper  Huronian."  The  first  to  describe  an  actual 
contact  between  the  underlying  gneissic  series  and  the  overlying 
Huronian  sediments  was  Irving,  who,  in  1887,  clearly  showed  that 
such  an  unconformity  4occurs.  The  observations  of  Pumpelly  and 
Van  Hise  reinforced  this  conclusion,  and  show  that  between  the 
lowest  clastic  member  of  the  Huronian  and  the  Laurentian  complex 
there  is  a  very  great  discordance.  This  was  further  confirmed  in  1902 
by  Van  Hise  and  Leith  and  in  1904  by  the  joint  committee.  The  com- 
mittee also  observed  that  the  same  formation  which  rests  unconform- 
able upon  granite  also  rests  unconformably  upon  the  Thessalon  group 
of  Keewatin  rocks  (Logan's  chloritic  slates),  and  that  the  Thessalon 
Keewatin  is  intruded  by  the  Laurentian  granite.  It  being  admitted 
that  the  Thessalon  contact  of  Huronian  with  Laurentian  and  Kee- 
watin  rocks  represents  a  true  erosional  unconformity,  the  question 
may  still  be  asked  whether  this  conglomerate  is  at  the  base  of  the 
lower  Huronian.  It  is  difficult  to  show  this  beyond  doubt,  but  the 
evidence  that  such  is  the  fact  is  very  strong. 
In  the  Sudbury  district  to  the  northeast  granites  abundantly 
intrude  the  Huronian  series. 
