21 G  PRE-CAM  BRIAN    GEOLOGY    OP    NORTH    AMERICA. 
is  as  painful  to  make  as  it  is  necessary  in  the  interests  of  sound  geol- 
ogy. The  anorthosite  is  provisionally  correlated  with  the  Norian  of 
the  Province  of  Quebec,  but  as  this  correlation  is  merely  a  hypothesis 
the  name  Carltonian  is  suggested  for  this  formation. 
Grant,179  in  1893,  describes  and  maps  the  geology  of  Kekequabic 
Lake  in  northeastern  Minnesota.  By  far  the  larger  proportion  of 
the  rocks  represented  are  elastics,  which  are  divided  for  convenience 
into  four  groups.  The  first  and  most  extensive  group  is  the  slate 
formation,  consisting  largely  of  argillites  with  smaller  amounts  of 
fine  and  coarse  graywackes  and  grits,  the  coarser  phases  becoming 
distinctly  conglomeratic  in  places.  The  second  group  consists  of 
coarse  conglomeratic  rocks,  which  are  a  part  of  the  Ogishke  con- 
glomerate. The  third  group  is  made  up  of  certain  fissile  green 
schists,  which  are  believed  to  be  water  deposited,  and  probably 
originally  formed  from  fine  volcanic  ash.  The  fourth  group  con- 
sists of  volcanic  fragmental  material,  in  part  deposited  in  water. 
All  of  these  clastic  rocks  iioav  stand  in  nearly  vertical  positions,  with 
a  strike  a  little  north  of  east. 
Sharply  marked  off  from  the  clastic  rocks  are  four  types  of 
igneous  rocks — hornblende  porphyrite,  granite,  diabase,  and  gabbro. 
The  granite  is  divisible  into  two  types,  ordinary  granite  and  granite 
porphyry,  in  both  of  which  the  ferromagnesian  constituent  is  almost 
exclusively  pyroxene  and  the  predominating  feldspar  anorthoclase. 
The  origin  of  the  granite  is  truly  eruptive,  having  broken  through 
the  surrounding  elastics;  the  rock  is  not  formed,  as  held  by  N.  IT. 
and  A.  Winchell,  from  the  recrystallization  in  situ  of  the  sedi- 
mentaries  of  the  region. 
The  slate  formation,  the  green  schist,  and  the  volcanic  tuff  belong 
to  the  Keewatin,  the  Minnesota  equivalent  of  the  Lower  Huronian. 
The  conglomerate  contains  pebbles,  many  of  which  are  similar  to 
some  of  the  Keewatin  rocks,  and  it  seems  to  belong  to  a  newer  series, 
although  as  yet  no  unconformity  between  the  conglomerate  and 
other  rocks  has  been  discovered.  FolloAving  Lawson,  it  is  believed 
that  the  conglomerate  is  a  part  of  the  Keewatin,  probably  separated 
from  the  lower  part  of  that  series  by  an  unconformity,  and  that  it 
is  much  older  than  the  Animikie.  However,  the  question  whether 
the  elastics  belong  to  one  or  two  series  is  as  yet  open. 
The  porphyrite  and  the  granite  are  of  Keewatin  age.  The  por- 
phyrite is  regarded  as  contemporaneous  with  the  deposition  of  vol- 
canic tuff  and  green  schist,  and  the  granite  is  believed  to  date  from 
the  folding  of  the  Keewatin.  The  age  of  the  diabase  dikes  is  not 
known;  they  are  perhaps  contemporaneous  with  the  great  diabase 
intrusions  in  the  Animikie.  The  gabbro  is  of  early  Keweenawan 
age. 
