vanhise.]  LAKE    SUPERIOR    REGION.  7l 
constitute  the  Steep  Kock  series.  Lying  across  theedgesof  fche  Steep 
Eock  series,  at  the  southeastern  part  of  the  lake,  is  a  later  series  of 
granites,  porphyries,  and  hornblende  rocks  which  pass  upward  into 
the  schists  of  the  Atieokan  river  and  are  designated  the  Aticokan 
series.  The  granites  and  gneisses  of  the  basal  complex  are  cut  by  vari- 
ous dikes,  which  are  of  three  kinds,  those  which  supplied  pebbles  to 
the  conglomerate  at  the  base  of  the  Steep  Eock  series;  those  which 
traverse  both  the  basement  complex  and  the  Steep  Rock  series  but 
have  been  subjected  to  the  folding 5  and,  third,  a  single  massive  dike 
which  is  subsequent  to  the  latest  period  of  folding. 
The  formations  of  the  Steep  Rock  series,  in  ascending  order,  are  con- 
glomerate, lower  limestone^  ferruginous  horizon,  in terbedded  crystal- 
line traps,  calcareous  green  schists,  upper  conglomerate,  greenstones 
and  greenstone- schists,  agglomerate,  and  dark  gray  clay-slate.  It  is 
then  a  series  of  sediments  and  interbedded  eruptives. 
Along  the  whole  course  of  the  lake  this  series  dips  at  very  steep  an- 
gles, ranging  from  60°  to  80°  away  from  the  basement  rocks,  upon 
which  they  hang  as  a  time-Avorn  fringe  having  no  extension  inland.  The 
basal  part  of  the  Steep  Rock  series  is  a  bed  having  a  maximum  thick- 
ness of  nearly  a  hundred  feet,  presenting  the  various  phases  of  a  con- 
glomerate, coarse  and  fine,  and  quartzite  and  quartz-schists  with  feld- 
spar. The  lowest  member  contains  rounded  and  waterworn  pebbles  of 
quartz  and  greenstone,  the  largest  being  a  foot  in  diameter.  Near  the 
junction  of  the  Steep  Rock  series  and  basal  complex  both  are  some- 
times very  similar  in  composition,  so  that  it  is  impossible  to  draw  the 
lines  between  them  by  this  criterion.  There  is  an  apparent  transition 
from  one  rock  into  the  other.  The  transition  zone  has  a  highly  schist- 
ose structure  in  the  regional  direction,  which  crosses  the  course  of  con- 
tact and  the  bedding  nearly  at  right  angles  and  is  traced  from  the  tran- 
sition zone  into  the  undoubted  granite  into  which  it  gradually  dies  out. 
This  transition  is  explained  as  due  to  probable  disintegration  of  the 
basement  complex  before  the  Steep  Rock  series  was  deposited;  combined 
with  subsequent  powerful  dynamic  movements  which  have  affected  both 
series. 
%The  Steep  Rock  series  is  folded  into  an  eastern  synclinal,  a  middle 
anticlinal,  and  a  western  synclinal,  the  latter  being  faulted.  The  axes 
of  these  folds  have  a  high  pitch  to  the  southward,  varying  from  00°  to 
nearly  90°.  Throughout  the  whole  area  is  a  regional  cleavage  which 
has  a  nearly  uniform  direction  transverse  to  all  the  members  of  the 
Steep  Rock  series  and  also  the  contact  between  this  series  and  the 
basement  complex.  This  has  largely  obliterated  the  original  la  mi  na- 
tion of  the  sediments  and  is  now  the  dominant  structure  It  is  there- 
fore the  last  force  which  has  left  its  marks  upon  the  rocks  of  (he  lake. 
Before  this  last  force  acted  upon  the  rocks,  the  Steep  Rock  series  had 
been  folded  into  a  southwestward  dipping  monoclinal  which,  under  the 
action  of  the  cleavage-producing  force  in  a  northeast  and  southwest 
