164  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
The  Archean  is  believed  to  be  wholly  an  igneous  group,  and  there- 
fore no  estimate  of  its  thickness  can  be  given.  It  covers  a  broad  area 
in  the  eastern  part  of  the  district,  and  from  this  several  arms  project 
west.  West  of  the  main  area  there  are  two  large  oval  areas  of 
Archean. 
The  Lower  Huronian  series,  from  the  base  upward,  comprises  the 
Sturgeon  quartzite,  from  100  to  more  than  1,000  feet  thick;  the  Rand- 
ville  dolomite,  from  500  to  1,500  feet  thick;  the  Mansfield  slate,  from 
100  to  1,900  feet  thick;  the  Hemlock  volcanic  formation,  from  1,000 
to  10,000  or  more  feet  thick;  and  the  Groveland  formation,  about  500 
feet  thick.  A  minimum  thickness  for  the  series  is  about  2,200  feet, 
and  a  possible  maximum  thickness  is  more  than  1(>,000  feet.  How- 
ever, in  the  latter  ease  a  large  part  of  the  series  is  composed  of  vol- 
canic material.  It  is  not  likely  that  the  sediments  at  any  one  place 
are  as  much  as  5,000  feet  thick. 
The  Upper  Huronian  is  a  great  slate  and  schist  series,  which  it  is 
not  possible  to  separate  on  the  maps  into  individual  formations.  It  is 
impossible  to  give  even  an  approximate  estimate  of  the  thickness  of 
this  series. 
Various  igneous  rocks  intrude  in  an  intricate  manner  both  the 
Upper  Huronian  and  the  Lower  Huronian  series. 
In  the  following  paragraphs  the  descriptions  of  the  formations 
are  summarized  somewhat  more  in  detail. 
Archean. — The  Archean  consists  mainly  of  massive  and  schistose 
granites  and  of  gneisses.  Nowhere  in  the  Archean  have  any  rocks  of 
sedimentary  origin  been  discovered.  The  Archean  has  been  cut  by 
various  igneous  rocks,  both  basic  and  acidic,  at  different  epochs. 
These  occur  in  the  form  both  of  bosses  and  of  dikes,  the  latter  some- 
times cutting,  but  more  ordinarily  showing  a  parallelism  to,  the  folia- 
tion of  the  schistose  granites.  The  granites  must  have  formed  far 
below  the  surface,  and  therefore  must  have  been  deeply  denuded  be- 
fore the  transgression  of  the  Lower  Huronian  sea.  The  Archean 
granites  and  gneisses  and  the  earlier  intrusives  alike  have  been  pro- 
foundly metamorphosed,  and  at  various  places  have  been  completely 
recrystallized. 
Lower  Huronian. — The  Sturgeon  quartzite,  the  first  deposit  of  the 
advancing  sea,  when  formed  consisted  mainly  of  sandstone,  but  in 
places  at  the  base  it  consisted  of  coarse  conglomerate.  The  conglom- 
erate is  best  seen  in  the  Sturgeon  River  tongue.  Elsewhere  evidence 
of  conglomeratic  character  at  the  base  of  the  formation  is  seen,  but 
the  metamorphism  has  been  so  great  as  nearly  to  destroy  the  pebbles. 
However,  in  the  Sturgeon  River  tongue  is  a  great  schistose  conglom- 
erate which,  while  profoundly  metamorphosed,  still  gives  evidence  of 
the  derivation  of  its  material  from  the  older  Archean  rocks.     The 
