34  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull.  86. 
underlie  the  elastics.  This  is  explicable  by  regarding  the  anorthosite 
as  an  intrusive  which  has  pushed  up  the  bedded  elastics,  causing  the 
latter  to  dip  away  from  it  in  every  direction.  In  the  Ontario  area  it 
appears  that  the  "  anorthosite  ,?  has  actually  burst  through  and  over- 
flowed the  elastics. 
Comparing  the  bedded  clastic  division  of  the  Laurentian  with  the 
Original  Huronian,  the  description  of  the  latter  by  Bell  at  Mpissing 
approaches  closely  in  lithological  character  that  of  the  Huronian,  to 
which  division  the  series  was  referred  by  Selwyn.  The  clastic  series 
in  the  vicinity  of  Madoc,  before  Yennor  realized  that  it  is  probably 
a  continuation  of  that  to  which  the  term  Laurentian  was  applied  at 
Ottawa  by  Logan,  was  considered  as  Huronian,  and  by  Logan  himself 
was  supposed  to  be  higher  than  the  Grenville  series. 
These  two  districts  are  intermediate  in  position  as  in  character  between 
the  type  areas  of  upper  Laurentian  and  the  Huronian. 
The  elastics  of  the  Laurentian  nowhere  come  in  contact  with  those  oi 
the  Original  Huronian  area,  so  that  we  have  no  evidence  whatever  as 
to  their  relative  age.  The  former  are  underlain  by  a  banded  and  con- 
torted gneissic  and  granitic  series.  The  same  is  true  of  the  Huronian 
of  lake  Huron.  In  this  latter  area  the  clastic  series  rests  uncon- 
formable' upon  a  lower  gneissic  one,  but  in  eastern  Ontario  we  have  no 
positive  evidence  that  similar  relations  obtain ;  but  it  is  not  improb- 
able, as  maintained  by  Yennor,  that  there  is  at  the  base  of  the  clastic 
series  a  true  unconformity.  On  the  other  haud,  it  is  possible  that  the 
relations  between  the  clastic  series  and  the  underlying  completely  crys- 
talline series  are  those  of  sedimentary  rocks  and  later  intrusives. 
Bearing  in  favor  of  an  unconformity  between  the  lower  granite- gneiss 
and  at  least  a  part  of  the  clastic  series  are  several  lines  of  evidence : 
(1)  In  Leeds,  Frontenac,  and  Lanark  counties  the  granites  which  cut  the 
Laurentian  gneisses  in  a  network  of  veius  never  penetrate  the  series  of 
quartzites,  conglomerates,  limestones,  etc.,  although  sometimes  found 
in  Yennor's  middle  or  dioritic  division.  This  indicates  that  the  gran- 
ite-gneiss not  only  existed  but  had  been  intruded  by  granitic  eruptions 
before  the  elastics  were  deposited.  (2)  The  abundant  large  pebbles 
and  bowlders  of  granite  and  syenite  found  at  many  localities  in  the 
Laurentian  region,  and  particularly  about  lake  Mpissing  and  near 
Shannon  ville,  show  that  earlier  than  the  elastics  existed  gneisses 
and  granites  identical  in  character  with  those  now  designated  as  Lau- 
rentian. The  probability  is  that  these  pebbles  are,  as  supposed  by 
Logan  and  Wilkins,  derived  directly  from  the  Laurentian.  If  this  is 
the  case,  between  the  two  must  have  existed  a  great  time  gap,  for  these 
fragments  when  deposited  show  that  the  rocks  from  which  they  are 
derived  were  then  at  the  surface  in  their  present  completely  crystal- 
line condition.  (3)  The  rocks  of  plainly  clastic  origin  are  associated  and 
occur  in  trough  like  areas,  although  having  interstratified  -with  them 
comparatively  thin  belts  of  gneiss,   which,  however,  may  be  meta- 
