AREA   NOETH   AND    NORTHEAST   OF   LAKE    HURON.  409 
and  the  overlying  Huronian.  Here  a  basal  conglomerate,  containing 
partly  rounded  and  angular  fragments  up  to  2  feet  in  diameter, 
largely  derived  from  the  immediately  underlying  gneiss,  rests  directly 
upon  the  upturned  edges  of  the  gneissic  series.  Such  a  contact  indi- 
cates a  great  structural  break,  whether  the  underlying  gneissic  series 
is  of  eruptive  or  of  sedimentary  origin.  If  sedimentary,  it  must  have 
been  metamorphosed  to  its  present  crystalline  condition  and  upturned 
before  the  fragmentals  were  deposited  upon  it;  if  eruptive,  its 
coarsely  crystalline  character  shows  that  it  belongs  to  the  deep-seated 
rocks  which  must  have  crystallized  at  great  depth,  and  therefore  that 
it  must  have  been  subjected  to  profound  erosion  in  order  to  appear  at 
the  surface.  Such  a  contact  is  also  found  at  two  or  three  other  points 
on  the  Algoma  branch  of  the  Canadian  Pacific  Railway,  and  particu- 
larly near  the  mouth  of  Serpent  River.  Logan's  a  green  chloritic  slate 
is  composed  of  diabase  sheets  and  a  little  interleaved  fragmental  ma- 
terial, perhaps  partly  volcanic  ash. 
Winchell  (N.  H.),21  in  1888,  describes  many  localities  within  the 
Original  Huronian.  Logan's  chloritic  slates,  as  well  as  the  green- 
stones, are  regarded  as  accidental  features,  the  former  being  a  part  of 
the  basic  eruptive  rocks  of  the  region.  Vast  outflows  of  greenstone 
cover  many  square  miles  in  the  Thessalon  Valley  and  constitute  hill 
ranges  as  conspicuous  as  those  of  any  hill  rock  in  the  region.  This 
series  is  classified  and  parallelized  with  the  Minnesota  rocks,  as 
follows : 
Original  Huronian.  Minnesota  equivalents. 
_,,  .  (Pewabic  quartzite   (?) 
Otter   Tail   qnartzite : LT        TT1        T>  ,  ,    w     „ 
m.         .  ...  New   Ulm,    Pokegama    and    AYaus- 
Thessalon   quartzite ,   ., 
[     waugonmg  quartzites. 
Black    slate Animikie  black  slate. 
"Lithographic  stone"  and  fine  grayquartzite__Not  known. 
Red  fclsite Felsites  at  Duluth  and  probably  the 
Great  Palisades. 
Mississagui  quartzite Not  known. 
Slate  conglomerate Ogishki   conglomerate. 
Chert  and  quartzite  pebbles  in  the  Thessalon  quartzite  lead  to  the 
inference  that  this  is  unconformably  upon  the  black  slate.  The  exist- 
ence of  granite  bowlders  in  the  slate  conglomerate  indicates  another 
unconformity  between  it  and  the  granites  of  the  region.  In  this 
latter  case  the  evidence  is  conclusive,  and  in  the  former  it  is  incon- 
siderable. 
Winchell  (Alexander),-2  in  1888,  makes  further  observations 
upon  the  Original  Huronian.  In  the  Huronian  system  is  a  large 
volume  of  eruptive  rock,  with  a  great  thickness  of  rocks  of  undoubted 
sedimentary  origin,  and  an  equal   volume  of  an  obscure  slaty  char- 
°See  note  2,  Chapter  V,  p.  482. 
