AREA    NORTH   AND   NORTHEAST   OF   LAKE    HURON.  "    421 
The  Laurentian  rocks  northwest  of  the  Huronian  area,  outside  of 
the  area  of  the  sheet,  are  considered  to  belong  to  the  older  division  of 
the  system. 
The  Huronian  rocks  comprise  quartzites,  sericite,  chlorite,  horn- 
blende, and  arkose  schists,  clay  slates,  graywackes,  and  dolomites. 
They  have  a  general  synclinal  structure.  The  quartzites  of  the  ridges 
northwest  of  Killarney  form  the  southern  side  of  the  basin  and  those 
of  the  Cloche  Mountains  the  northern  side.  Along  the  southern  side 
of  the  major  syncline  are  several  subordinate  folds.  Associated 
greenstones  are  less  conspicuous  than  in  the  Huronian  rocks  of  the 
Sudbury  area  to  the  north.  Those  present  are  more  largely  developed 
in  the  tract  on  the  south  side  of  Lake  Panache  than  elsewhere. 
In  the  space  between  the  Cloche  Mountains  and  the  range  which 
runs  eastward  from  McGregor  Point  to  Sturgeon  Lake,  including  Bay 
of  Islands,  McGregor  Bay,  and  the  land  thence  eastward  to  the  junc- 
tion of  the  two  chains,  the  rocks  belong  to  a  local  division  of  the 
Huronian,  which  may,  for  present  convenience,  be  called  the  arkose 
series  with  its  associated  rocks.  Structurally  this  area  would  appear 
to  occupy  the  central  part  of  the  synclinal  area  between  the  above- 
mentioned  conspicuous  quartzite  ranges.  Although  various  forms  of 
arkose  or  graywacke  are  the  prevailing  rocks  within  this  space,  there 
are  in  different  parts  of  it  considerable  quantities  of  gray  quartzites 
and  fine  quartz  conglomerates,  mixed  agglomerates  and  breccias, 
sericitic  and  micaceous  schists,  impure  dolomites,  and  eruptive  green- 
stones. 
As  to  the  origin  of  these  rocks,  the  thick  unstratified  and  brecciated 
graywacke  or  arkose  may  represent  consolidated  masses  of  volcanic 
ashes  or  mud  with  stones,  which  were  thrown  upon  the  land  or  into 
shallow  water,  while  the  stratified  varieties  may  have  consisted  of 
similar  ejectamenta  thrown  into  deeper  water,  where  they  became 
arranged  in  layers  as  we  find  them.  Some  of  these  rocks,  whether 
stratified  or  otherwise,  may  represent  volcanic  products  which  were 
originally  thrown  into  the  sea  in  a  molten  or  heated  condition  and 
became  broken  up  and  almost  completely  disintegrated. 
A  study  of  the  different  phases  of  the  graywackes  and  their  asso- 
ciated rocks  in  this  region  would  appear  to  prove  that  the  former 
constituted  the  crude  material  from  which  both  the  quartzites  and  the 
clay  slates  were  derived  by  the  modifying  and  separating  action  of 
water.  Again,  by  the  action  of  time,  pressure,  heat,  and  other  meta- 
morphosing agents  upon  different  varieties  of  graywacke,  some  of  our 
granites,  syenites,  gneisses,  and  possibly  other  crystalline  rocks  were 
probably  formed. 
Solid  and  slaty  argyllites  are  found  along  Long  Lake,  an  expan- 
sion of  Whitefish  River,  and  slate  conglomerates  occur  on  both  sides 
of  Bear  Lake  and  between  Cat  and  Leech  lakes.     However,  these 
