LAKE    SUPERIOR   REGION.  283 
probable  that  the  conglomerates  represent  an  important  interval  of 
erosion,  perhaps  equivalent  to  the  one  shown  by  Van  Hise  and  others 
between  the  Upper  and  Lower  Huronian  in  the  States  to  the  south. 
The  Coutchiching  rocks  are  all  formed  of  clay  sand,  more  or  less 
metamorphosed ;  in  general  they  are  biotite  schists  or  gneisses,  the 
quartz  showing  a  clastic  origin.  The  Coutchiching  passes  up  by 
transition  into  the  Keewatin,  and  there  is  no  reason  why  the  two 
together  should  not  be  classed  as  Huronian. 
Following  Lawson's  estimate,  the  Keewatin  and  Coutchiching  series 
together  sum  up  50,000  feet  in  thickness. 
The  term  Laurentian  is  employed,  as  Lawson  and  other  Canadian 
geologists  are  accustomed  to  employ  it — in  a  petrographical  and  struc- 
tural sense — for  crystalline  gneisses  and  granites  underlying  the 
Huronian,  although  it  is  evident  that  these  rocks  have  consolidated  at 
a  time  later  than  the  Huronian. 
As  described  by  Lawson,  the  Laurentian  (Lower  Archean)  "  occurs 
in  large  isolated  central  areas,  more  or  less  completely  surrounded  by 
the  schists  of  the  upper  Archean,  the  encircling  belts  anastomosing 
and  forming  a  continuous  mesh  work."  It  consists  chiefly  of  a  coarse 
reddish,  often  porphyritic  rock,  usually  granite  in  the  central  part  of 
the  area,  but  showing  a  foliation,  generally  parallel  to  the  periphery, 
where  it  comes  in  contact  with  the  Huronian. 
Throughout  the  region  the  Laurentian  is  in  eruptive  contact  with 
the  Huronian,  and  nowhere  is  a  basal  conglomerate  of  the  Huronian 
found.  Near  the  contact  with  the  Huronian  strips  and  fragments  of 
the  Huronian  are  embedded  in  the  gneiss ;  also  dikes  of  granite,  peg- 
matite, or  felsite  generally  run  from  the  gneiss  into  the  Huronian. 
The  larger  areas  of  gneiss  and  granite  are  evidently  batholiths.  Some 
of  the  smaller  granite  bosses  may  be  stumps  of  old  volcanoes.  The 
Huronian  schists  usually  dip  rather  steeply  away  from  the  gneiss,  at 
an  angle  seldom  less  than  45°.  Finer  grained  granites  cut  both  the 
Keewatin  and  the  Laurentian.  However,  it  is  not  easy  to  say 
whether  a  given  granite  is  Laurentian  or  a  later  granite. 
It  is  believed  that  as  a  result  of  the  piling  up  of  a  thickness  of  8  or 
10  miles  of  sediments  and  eruptive  materials,  represented  by  the  Kee- 
watin and  Coutchiching  rocks,  the  slowly  rising  isogeotherms  soft- 
ened or  fused  the  foundation,  which  rose  into  domes,  the  inner  parts 
solidifying  as  granite,  and  the  outer,  more  viscid  portions  having 
their  constituents  dragged  into  rough  parallelism  with  the  adjoining 
solid  rocks  and  forming  gneiss. 
As  the  Huronian  rocks  south  of  Lake  Superior  and  in  New  Bruns- 
wick are  described  by  Van  Hise  and  Dawson  as  presenting  basal  con- 
glomerates resting  unconformably  on  the  Laurentian,  it  is  suggested 
that  in  these  cases  the  thickness  of  the  sediments  was  not  great  enough 
to  depress  the  Laurentian  floor  to  the  level  of  fusion  or  plasticity ;  or 
