LAKE    SUPERIOR   REGION.  165 
sandstone  has  been  changed  to  a  vitreous,  largely  recrystallized  quartz- 
ite,  which  now  shows  only  here  and  there  vague  evidence  of  its  clastic 
character. 
The  Sturgeon  formation  varies  in  thickness  from  probably  more 
than  1,000  feet  in  the  Sturgeon  River  tongue  to  less  than  100  feet  at 
places  in  the  Felch  Mountain  range,  and  is  altogether  absent  in  the 
northeastern  part  of  the  district. 
In  the  southeastern  part  of  the  district  the  Sturgeon  quartzite  is 
overlain  by  the  Randville  dolomite.  In  the  central  part  of  the  dis- 
trict the  quartzite  between  the  Archean  and  the  Randville  is  so  thin 
that  it  can  not  be  represented  on  the  maps  as  a  separate  formation. 
In  the  northeastern  part  of  the  district  a  quartzite,  resting  on  the 
Archean,  but  occupying  a  higher  position  stratigraphically  than  the 
Randville  dolomite,  is  overlain  by  an  iron-bearing  formation.  It  ap- 
pears, therefore,  that  the  Sturgeon  sea  gradually  overrode  the  dis- 
trict, and  that  at  the  time  the  Sturgeon  quartzite  was  deposited  in  the 
southeastern  part  of  the  district  the  Archean  was  not  yet  submerged 
in  the  central  and  northeastern  parts.  However,  since  the  quartzite 
resting  on  the  Archean  in  the  latter  area  can  not  be  separated  litho- 
logically  from  the  Sturgeon  quartzite,  both  are  given  the  same  forma- 
tion color,  but  the  later  quartzite  is  given  a  separate  letter  symbol. 
The  quartzite  color  therefore  represents  the  transgression  deposits  of 
the  same  general  lithological  character,  rather  than  a  formation  all 
parts  of  which  have  exactly  the  same  age.  While  nowhere  in  the 
district  is  there  any  marked  discordance  between  the  schistosity  of 
the  Archean  and  the  Sturgeon  quartzite,  the  conglomerates  at  the 
base  of  the  latter  formation  in  the  Sturgeon  River  tongue  are  be- 
lieved to  indicate  a  great  unconformity  between  the  Archean  and  the 
Lower  Huronian  series.  The  change  from  the  Sturgeon  deposits  to 
those  of  the  Randville  was  a  transition. 
The  Randville  dolomite  is  a  nonclastic  sediment,  and  is  believed 
to  mark  a  period  of  subsidence  and  transgression  of  the  sea  to  the 
northeast,  resulting  in  deeper  water  for  much  of  the  district.  Since 
the  Randville  dolomite  has  its  full  thickness,  on  the  Fence  River  just 
east  of  the  western  Archean  oval,  and  does  not  appear  at  all  about  the 
Archean  oval  a  short  distance  to  the  northeast,  it  is  probable  that  the 
shore  line,  during  Randville  time,  was  between  these  two  areas  and 
that  the  land  arose  somewhat  abruptly  toward  the  northeast.  As  the 
Randville  formation  has  a  thickness  of  1,500  feet,  it  probably  rep- 
resents a  considerable  part  of  Lower  Huronian  time. 
Following  the  deposition  of  the  Randville  dolomite,  deposits  of 
very  different  character  occur  in  different  parts  of  the  district.  These 
deposits  are:  (1)  The  Mansfield  formation,  (2)  the  Hemlock  volcanic 
formation,  and  (3)  the  Groveland  formation. 
