LAKE    SUPERIOR   REGION.  191 
ings.  The  development  of  life  in  the  Primordial  is  so  abundant  as 
to  lead  to  the  conclusion  that  for  its  evolution  to  this  degree  of  per- 
fection a  vast  prior  period  of  time  was  required,  which  probably 
would  carry  the  life  well  down  into  the  Laurentian  series.  It  is  fur- 
ther suggested  that  the  abundance  of  alkaline  rocks  in  the  Laurentian 
may  be  due  to  the  effects  of  life  in  the  Laurentian  ocean. 
Between  the  Laurentian  and  Huronian  periods  the  Laurentian 
beds  were  closely  folded;  the  sediments  were  changed  by  meta- 
morphism  to  a  thoroughly  crystalline  condition,  and  the  series  was 
profoundly  eroded.  About  the  thus  formed  Laurentian  isles  was 
deposited  the  Huronian.  This  comprises  the  Penokee  series,  the 
Menominee  series,  the  Baraboo  quartzites,  the  quartz  porphyries 
of  central  Wisconsin,  the  quartzites  and  catlinite  of  Barron  and 
Chippewa  counties,  and  the  iron-bearing  series  of  Black  River  Falls. 
These  series  consist  for  the  most  part  of  limestones,  slates,  some- 
times heavily  carbonaceous,  quartzites,  hematitic  and  magnetitic 
schists,  mica  schists,  and  diorites.  The  presence  of  limestones,  car- 
bonaceous shales,  and  iron  ore  is  taken  as  the  evidence  of  life.  After 
the  deposition  of  the  Huronian  it  was  upheaved,  metamorphosed, 
and  eroded  before  the  beginning  of  Keweenawan  time,  although  the 
unconformity  between  the  two  series  in  Wisconsin  is  but  slight  and 
the  above  changes  were  only  partially  accomplished  when  the 
KeweenaAvan  eruptions  began.  The  metamorphism  was  less  in  degree 
than  that  which  has  affected  the  Laurentian  strata,  but  is  more  intense 
than  that  which  the  Keweenawan  series  has  suffered.  It  was  not 
in  general  sufficient  to  obliterate  the  original  grains  and  pebbles, 
nor  to  destroy  ripple  and  rill  marks.  In  the  Huronian  strata  are 
igneous  beds  and  dikes  of  gabbros,  diabases,  and  diorites,  the  age  of 
which  is  not  certain.  They  may  be,  so  far  as  yet  known,  in  part  con- 
temporaneous and  in  part  subsequent,  or  wholly  the  one  or  the  other. 
The  rocks  of  the  Keweenawan  period  consist  of  interstratified  igne- 
ous and  sedimentary  beds;  the  former  mainly  diabases,  with  some 
gabbros,  melaphyres,  and  porphyries ;  the  latter  conglomerates,  sand- 
stones, and  shales  derived  mainly  from  the  igneous  rocks.  The 
maximum  thickness  is  about  45,000  feet,  of  which  the  upper  15,000 
feet  is  sedimentary.  The  bottom  of  the  Lake  Superior  basin  was 
gradually  subsiding  during  the  time  of  the  formation  of  these  beds. 
While  tilted,  they  are  not  contorted  or  metamorphosed.  There  is 
in  this  period  no  direct  evidence  of  the  existence  of  life.  Over  the 
great  conglomerate  of  the  Penokee  and  Porcupine  mountain  regions 
is  a  black  shale  that  simulates  the  shales  of  later  ages  formed  in 
association  with  life.  After  the  close  of  the  Keweenawan  period, 
before  the  Potsdam  sedimentation  began,  there  was  a  period  of  ero- 
sion.    How  great  this  interval  was  has  not  yet  been  determined; 
