LAKE    SUPERIOR   REGION.  213 
the  Saganaga  granite  as  the  Ogishki  conglomerate,  the  same  thing  is 
true  of  the  Animikie  and  Keewatin.  The  Ogishki  conglomerate  is 
younger  than  the  most  of  the  Keewatin,  but  is  considered  as  a  part 
of  it. 
Hall  and  Sardeson,175  in  1892,  describe  the  upper  Cambrian  rocks 
of  southeastern  Minnesota  as  resting  unconformably  upon  a  pre-Paleo- 
zoic  floor.  The  base  of  the  Potsdam  is  usually  conglomeratic.  At 
Minneopa  a  well  800  feet  deep  passed  through  a  conglomerate  for 
a  distance  of  225  feet,  the  pebbles  of  which  are  vitreous  quartzite 
like  those  occurring  in  Cortland,  Watonwan,  and  Cottonwood  coun- 
ties. A  conglomerate  containing  granite  debris  is  found  on  Snake 
River  about  2  miles  above  Mora,  and  3  miles  distant  from  the  Ann 
River  knobs  of  hornblende-biotite  granite,  the  clastic  appearing  to  be 
derived  from  the  granite.  At  Taylors  Falls  a  conglomerate  made 
iij)  of  pebbles  of  diabase  rests  upon  the  diabase  of  St.  Croix  River. 
These  underlying  formations  are  Archean  and  Algonkian  rocks. 
Bayley,176  in  1893,  1894,  and  1895,  gives  a  detailed  petrographic 
study  of  the  basic  massive  rocks  of  the  Lake  Superior  region,  and 
especially  of  the  great  gabbro  of  northeastern  Minnesota.  The  nor- 
mal phase  of  the  gabbro  is  found  to  have  a  typical  granitic  structure 
and  to  differ  essentially  from  all  of  the  basic  intrusive  rocks  of  the 
Animikie  series  and  from  all  other  Keweenawan  basic  rocks,  none  of 
which  have  a  distinctly  granitic  structure.  Upon  the  border  of  the 
main  mass  of  gabbro  are  peculiar  rocks  which  are  interlaminated 
with  quartzose  bands.  These  are  shown  to  be  but  peripheral  phases 
of  the  gabbro.  It  is  concluded  that  further  field  work  will  probably 
show  that  the  gabbro  is  either  a  batholith,  well  toward  the  base  of  the 
Keweenawan  series,  or  that  it  is  an  eroded  mass,  upon  top  of  w7hich 
the  later  Keweenawan  beds  have  been  deposited. 
Winchell  (N.  H.),177  in  1893,  gives  the  following  as  the  general 
consensus  of  opinion  of  several  geologists  as  to  the  descending  suc- 
cession of  the  rocks  of  northeastern  Minnesota. 
1.  KewTeenawan  or  Xipigon  series  unconformably  beneath  rocks 
bearing  the  Dikelocephalus  fauna,  and  consisting  of  fragmental  and 
eruptive  beds,  the  upper  portions  being  almost  entirely  red  sand- 
stones. 
2.  Alternating  beds  of  eruptive  sheets  and  fragmental  rocks.  The 
fragmentals  are  thin-bedded  slates,  actinolite  schists,  magnetitic  jas- 
pers, cherts,  and  quartzites.  The  sheets  are  ordinary  eruptives  or 
pyroclastics. 
3.  Immense  quantities  of  true  gabbro,  much  of  it  bearing  titanifer- 
ous  magnetite,  arc  associated  with  contemporaneous  fclsites,  quartz 
porphyries,  and  red  granites.  This  gabbro  includes  several  masses 
of  the  next  older  strata,  particularly  the  Pewabio  quartzite. 
