274  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
illations,  which  together  constitute  the  Steep  Rock  series.  Lying 
a cross  the  edges  of  the  Steep  Rock  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  Aticokan  River  and  are 
designated  the  Aticokan  series.  The  granites  and  gneisses  of  the 
basal  complex  are  cut  by  various  dikes,  which  are  of  three  kinds — 
those  which  supplied  pebbles  to  the  conglomerate  at  the  base  of  the 
Steep  Rock  series;  those  which  traverse  both  the  Basement  Complex 
and  the  Steep  Rock  series  but  have  been  subjected  to  the  folding;  and, 
third,  a  single  massive  dike  which  is  subsequent  to  the  latest  period 
of  folding. 
The  formations  of  the  Steep  Rock  series  are,  in  ascending  order, 
conglomerate,  lower  limestone,  ferruginous  horizon,  interbedded 
crystalline  traps,  calcareous  green  schists,  upper  conglomerate,  green- 
stones 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 
angles,  ranging  from  60°  to  80°,  away  from  the  basement  rocks,  upon 
which  they  hang  as  a  time-worn  fringe  having  no  extension  inland. 
The  basal  part  of  the  Steep  Rock  series  is  a  bed  having  a  maximum 
thickness  of  nearly  a  hundred  feet,  presenting  the  various  phases  of  a 
conglomerate,  coarse  and  fine,  and  quartzite  and  quartz  schists  with 
feldspar.  The  lowest  member  contains  rounded  and  waterwTorn  peb- 
bles of  quartz  and  greenstone,  the  largest  being  a  foot  in  diameter. 
Near  the  junction  of  the  Steep  Rock  series  and  basal  complex  both 
are  sometimes  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  schistose  structure  in  the  regional  direction,  which  crosses  the 
course  of  contact  and  the  bedding  nearly  at  right  angles  and  is  traced 
from  the  transition  zone  into  tne  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  poAverful  dynamic  move- 
ments which  have  affected  both  series. 
The  Steep  Rock  series  is  folded  into  an  eastern  syncline,  a  middle 
anticline,  and  a  western  syncline,  the  latter  being  faulted.  The  axes 
of  these  folds  have  a  high  pitch  to  the  south,  varying  from  60°  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  to  the  contact  between  this  series  and  the 
Basement  Complex.  This  has  largely  obliterated  the  original  lamina- 
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  the  lake. 
Before  this  last  force  acted  upon  the  rocks,  the  Steep  Rock  series  had 
