LAKE    SUPERIOR   REGION.  245 
into  ferruginous  cherts  of  the  iron  formation  rather  abruptly,  though 
usually  at  the  contact  the  chert  and  quartzite  are  interleaved  for  a 
few  feet.  The  conglomerate  of  the  iron  formation  rests  upon  Poke- 
gama  quartzite,  indicating  a  slight  erosion  interval  between  the 
Biwabik  and  Pokegama  formations,  although  the  interval  is  not 
shown  by  discordance  in  bedding,  which  is  parallel  in  both.  Hereto- 
fore the  quartzite  and  conglomerate  in  the  iron  formation  have  not 
been  discriminated  from  the  rocks  of  the  Pokegama  formation.  In 
the  eastern  portion  of  the  range  the  iron  formation  is  in  contact  with 
the  Keweenawan  gabbro  and  granite,  and  near  this  contact  has  suf- 
fered profound  metamorphism.  The  characteristic  rocks  of  this  area 
are  amphibole-magnetite  cherts.  The  thickness  of  the  formation  may 
vary  from  200  to  2,000  feet.     The  average  may  be  1,000  feet. 
The  Virginia  slate  is  essentially  a  soft  slate  or  shale  formation,  but 
it  contains  graywacke  phases,  near  its  base  a  little  limestone,  and 
near  its  contact  with  the  gabbro  is  metamorphosed  into  a  cordierite 
horn f els.  The  normal  slate  phases  of  the  formation  may  be  distin- 
guished with  difficulty  in  isolated  occurrences  from  the  slate  layers 
in  the  Biwabik  formation.  The  separation  of  the  twTo  is  of  im- 
portance to  the  explorer,  and  hence  an  attempt  is  made  to  determine 
criteria  for  their  discrimination.  The  thickness  of  the  Virginia  for- 
mation can  not  be  measured  wTithin  the  district,  but  from  analogy 
with  the  Penokee-Gogebic  district  and  the  extent  of  the  low,  flat- 
lying  area  south  of  the  Mesabi  range  supposed  to  be  occupied  by  the 
slate,  the  formation  is  believed  to  have  a  very  considerable  thickness. 
The  slate  grades,  both  vertically  and  laterally,  into  the  Biwabik 
formation. 
The  entire  Upper  Huronian  series  is  well  bedded,  conformable  in 
structure  (although  having  a  thin  conglomerate  between  the  Biwabik 
and  Pokegama  formations),  and  dips  in  southerly  directions  at 
angles  varying  from  5°  to  20°  and  exceptionally  at  higher  or  lower 
angles.  The  series  is  gently  cross  folded  and  the  axes  of  the  cross 
folds  pitch  in  southerly  directions.  Accompanying  the  folding  is 
considerable  jointing,  especially  in  the  brittle  Pokegama  and  Biwabik 
formations.  Indeed,  in  these  two  formations  the  folding  is  brought 
about  mainly  through  relatively  minute  displacements  along  joints, 
while  in  the  Virginia  formation  the  folding  has  taken  place  mainly 
by  the  actual  bending  of  the  strata. 
The  thickness  of  the  Upper  Huronian  series  within  the  limits  of  the 
district  mapped  may  average  about  1,500  feet;  but  if  the  total  thick- 
ness of  the  slate  formation  outside  the  limits  of  the  district  be  taken 
into  account,  the  total  thickness  of  the  Upper  Huronian  series  is 
probably  several  times  this  figure. 
The  relations  of  the  Upper  Huronian  series  to  the  subjacent  forma- 
tions are  those  of  unconformity,  as  evidenced  by  basal  conglomerate. 
