218  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
syenite,  mica  schist,  graywacke,  etc.,  greenstone,  and  quartz  porphyry. 
The  gabbro  is  the  most  recent,  and  covers  part  of  the  older 
rocks.  The  syenite  is  older  than  the  gabbro,  and  is  younger  than 
the  greenstone  and  mica  schist,  both  of  which  it  cuts.  The  mica 
schists,  graywackes,  etc.,  are  vertical,  and  have  a  general  east-north- 
east strike.  These  have  been  formerly  mapped  as  belonging  to  the 
Vermilion  series,  but  there  seems  to  be  good  reason  for  putting  all 
of  this  type  of  rock  in  the  area  mapped  into  the  Keewatin.  The 
greenstone  is  presumably  of  Keewatin  age,  and  is  probably  younger 
than  the  mica  schists,  graywackes,  etc.  Quartz  porphyry  dikes  are 
found  cutting  the  greenstones  in  several  places,  but  they  have  not 
been  seen  in  the  other  rocks  in  the  immediate  vicinity. 
Lawson,182  in  1893,  gives  a  resume  of  the  geology  of  northeastern 
Minnesota  adjacent  to  Lake  Superior.  Surrounding  the  lake  are 
four  geological  provinces,  viz  (from  the  top  downward),  the  Pots- 
dam, Keweenian,  Animikie,  and  Archean. 
The  rocks  of  the  Potsdam  are  flat-lying  shaly  sandstones,  generally 
of  a  red  color. 
The  Keweenian  occupies  the  entire  Minnesota  coast  from  Duluth 
to  Grand  Portage.  The  series  consists  in  this  area  of  a  well-stratified 
series  of  volcanic  flows,  having  a  gentle  lakeward  dip,  which  does  not 
generally  exceed  10°.  The  sedimentary  formations  are  represented  in 
the  series,  but  occupy  less  than  one-half  per  cent  of  the  coast  line. 
The  lavas  are  largely  vesicular  or  amygdaloidal  in  character,  and  in 
those  of  acidic  composition,  in  which  the  vesicular  structure  is  not  so 
well  developed,  are  numerous  irregular  joints.  The  series  has  been 
invaded  by  many  later  intrusive  masses,  which  occur  as  nearly  ver- 
tical dikes,  or  more  commonly  as  injected  sills  which  coincide  with 
the  planes  of  stratification  of  the  bedded  flows.  Since  the  time  of  the 
outflow  of  the  Keweenian  rocks  the  strata  have  suffered  comparatively 
little  disturbance,  the  prevalent  lakeward  dip  being  probably  due  to 
the  attitude  of  the  slopes  upon  which  the  lavas  flowed,  rather  than 
entirely  to  a  differential  movement  of  once  horizontal  strata.  The 
pre-Keweenian  labraclorite  rocks  exposed  at  a  number  of  points  were 
profoundly  eroded  before  the  Keweenian  was  deposited  upon  them, 
and  they  were  presumably  Archean. 
The  Animikie  rocks  occupy  the  shore  of  the  lake  from  Grand  Port- 
age to  Port  Arthur.  The  series  is  composed  altogether  of  sedimentary 
strata,  and  consists  mainly  of  fine-grained  sandstones,  which  are 
locally  quartzites,  and  carbonaceous  shales  or  slates,  and  in  small 
part  of  cherts  and  jaspers,  beds  of  carbonate  of  iron,  hematite,  and 
magnetite,  conglomerate,  and  occasional  lenses  of  nonferruginous 
carbonate  in  the  slates.  Except  in  local  instances  the  rocks  have  been 
disturbed  very  little  from  the  horizontal,  the  average  dip  of  the 
strata  being  in  a  southeasterly  direction  at  an  angle  probably  not  ex- 
