166  PRE-CAMBRIAN    GEOLOGY    OP    NORTH    AMERICA. 
The  Mansfield  formation  was  a  mudstone,  which  has  subsequently 
been  transformed  into  a  slate  or  schist.  The  Hemlock  formation  is 
mainly  a  great  volcanic  mass,  including  both  basic  and  acidic  rocks, 
lavas,  and  tuffs,  but  it  contains  also  subordinate  interbedded  sedi- 
mentary rocks.  This  formation  occupies  a  larger  area  than  any 
other  of  the  Lower  Huronian  formations,  and  is  perhaps  the  most 
characteristic  feature  of  the  Crystal  Falls  district.  The  Groveland 
is  the  iron-bearing  formation.  It  includes  sideritic  rocks,  cherts, 
jaspilites,  iron  ores,  and  other  varieties  characteristic  of  the  iron- 
bearing  formations  of  the  Lake  Superior  region.  In  all  important 
respects  these  rocks  are  similar  to  those  of  the  Negaunee  formation 
of  the  Marquette  district,  with  the  exception  that  in  the  southeastern 
part  of  the  Crystal  Falls  district,  associated  with  the  nonclastic 
material  there  is  a  considerable  proportion  of  clastic  deposits.  The 
Groveland  formation  contains  iron  carbonate  and  possibly  glauconite, 
from  which  its  other  characteristic  rocks  were  derived. 
The  variability  in  the  character  of  the  deposits  overlying  the  Rand- 
ville  formation  is  probably  caused  by  the  great  volcanic  outbreaks  in 
the  western  part  of  the  district.  In  the  southern  and  southeastern 
parts  the  deposit  overlying  the  Randville  formation  is  the  Mansfield 
slate  and  schist.  North  of  Michigamme  Mountain  and  of  the  Mans- 
field area  the  Mansfield  formation  is  replaced  along  the  strike  by  the 
Hemlock  volcanic  formation,  which  directly  overlies  the  limestone 
for  most  of  the  way  about  the  western  Archean  oval.  The  effect  of 
the  volcanic  outbreak  apparently  did  not  reach  so  far  as  the  north- 
eastern part  of  the  district. 
Overlying  the  Mansfield  formation  in  the  southeastern  part  of  the 
district  and  the  Randville  formation  in  the  central  part  is  the  Grove- 
land iron-bearing  formation.  In  the  Mansfield  slate  area  the  iron- 
bearing  rocks  appear  near  the  top  of  the  Mansfield  formation  inter- 
calated with  the  slates.  The  Groveland  formation  can  not  be  cer- 
tainly traced  farther  north  than  the  northeastern  portion  of  the  west- 
ern Archean  oval.  It  is  apparently  replaced  along  the  strike  by  the 
Hemlock  volcanics. 
In  the  northeastern  part  of  the  district  the  Groveland  formation, 
equivalent  to  the  Negaunee  formation  of  the  Marquette  district  of 
Michigan,  is  found  above  the  Ajibik  formation.  The  occupation,  in 
the  western  part  of  the  district,  by  the  Hemlock  volcanics  of  the  same 
part  of  the  geological  column  that  is  occupied  by  the  Hemlock  vol- 
canics east  of  the  western  Archean  oval,  the  Mansfield  slate,  and  the 
Groveland  formation  is  explained  by  the  fact  that  in  the  western 
part  of  the  district  the  volcanoes  broke  out  and  there  continued  their 
activity  longest.     While  the  volcanic  rocks  were  being  laid  down 
