46  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull.  80. 
ing  at  one  place  at  an  angle  of  50°.  Also  the  foliation  of  the  granite- 
gneisses  abnts  almost  at  right  angles  against  the  line  of  contact  at  one 
place.  The  contact  here,  then,  has  all  the  characteristics  of  one  of  erosive 
unconformability.  Upon  the  second  island,  instead  of  a  clear  line  of 
contact  between  the  conglomerate  and  the  basement  complex  there  is 
an  apparent  gradation,  the  change  occurring  within  5  or  6  feet.  Here 
the  solid  granite-gneiss  is  first  broken;  then  in  passing  npward  the  an- 
gular fragments  have  moved  somewhat;  in  passing  still  farther  upward 
they  become  roundish  and  are  mingled  with  extraneous  material,  until 
a  bowlder  conglomerate  is  reached  which  is  in  every  respect  like  that 
before  described.  This  relation  is  not  uncommon  when  an  encroaching 
shore-line  overrides  a  rock  formation. 
It  is  concluded  that  between  the  lowest  members  of  the  Original 
Huronian  series  and  the  granite-gneiss-schist  basement  complex  which 
Logan  has  called  Laurentian,  there  is  the  clearest  evidence  of  a  very 
great  unconformity.  Also,  that  the  Laurentian  series,  instead  of  being 
a  simple  one,  consists  of  rocks  of  many  different  kinds  and  has  a  most 
complex  history. 
SUMMARY  OF  RESULTS. 
Bigsby's  Transition  formation  is  that  to  which  the  term  Huronian  was 
later  applied  by  Logan  and  Murray,  and  his  description  of  this  series 
is  hence  the  earliest.  From  the  first  it  is  plain  that  Murray  does  not 
consider  the  series  as  metainorphic,  since  it  is  excluded  from  the  rocks 
to  which  that  tennis  applied.  It  is  also  plain  that  the  true  nature  of 
the  interbedded  greenstones  was  appreciated.  Logan  was  distinctly  a 
stratigraphical  geologist,  who  believed  in  extreme  metamorphism  of 
sedimentary  beds,  yet  he  also  clearly  saw  that  the  greenstones  associ- 
ated with  the  fragmental  rocks  could  not  be  regarded  as  other  than 
of  igneous  origin. 
As  to  the  relations  of  the  Huronian  and  Laurentian  north  of  lake 
Huron,  Murray  made  the  distinction  in  1857  to  rest  upon  age  and  upon 
lithological  characteristics,  the  Laurentian  being  older  and  more  com- 
pletely crystalline  than  the  Huronian.  While  the  Huronian  and  Lau- 
rentian by  Logan  and  Murray  are  not  described  at  any  definite  locality 
as  having  unconformable  relations,  the  former  states  that  the  Huro- 
nian is  a  stratified  series  and  reposes  discordantly  upon  the  Lauren- 
tian system,  and  in  1858  he  again  clearly  indicates  the  same  thing  by 
the  statement  that  in  the  slate  conglomerates  are  bowlders  and  peb- 
bles derived  from  the  subjacent  gneiss  and  that  the  lower  formation 
was  consequently  converted  into  gneiss  and  probably  greatly  disturbed 
before  the  upper  series  was  laid  down.  In  1865,  Logan  further  says 
that  the  Huronian  unconformably  overlies  the  lower  Laurentian,  and 
is  believed,  although  not  found  in  contact  with  it,  to  be  more  recent 
than  the  upper  Laurentian.  These  statements  are  emphasized  by  his 
sections  published  in  1863,  which  represent  the  Huronian  as  resting 
unconformably  upon  the  Laurentian.  The  first  to  describe  an  actual 
contact  between  the  underlying  gneissic  series  and  the  overlying  Huro- 
