anhise.]  LAKE    SUPERIOR    REGION.  117 
yarded  as  a  sedimentary  accumulation,  on  the  grounds  (1)  of  foliation 
jud  stratification;  (2)  of  the  alternating  bands  of  varying  chemical  consti- 
ution ;  (3)  of  the  verging  of  one  kind  of  rock  into  another  laterally ;  and 
4)  of  the  presence  of  kinds  of  rocks  not  known  to  be  produced  by 
gneous  agencies.  The  thickness  of  these  sediments  is  enormous.  An 
stiinate  of  30,000  feet  is  probably  not  too  great. 
The  calcareous  and  carbonaceous  beds  of  the  Laurentian  of  Canada, 
,s  well  as  the  Archean  limestones  and  iron  ore  beds  of  New  York,  are 
onsidered  to  be  Huronian  rather  than  Laurentian,  and  if  this  is  so, 
here  is  present  in  the  Laurentian  no  positive  evidence  of  life,  although 
investigations  in  the  future  may  reveal  evidences  of  organic  beings.  The 
Levelopment  of  life  in  the  Primordial  is  so  abundant  as  to  lead  to  the 
onclusion  that  for  its  evolution  to  this  degree  of  perfection  a  vast 
>rior  period  of  time  was  required  which  probably  would  carry  the  life 
rell  down  into  the  Laurentian  series.  It  is  further  suggested  that  the 
abundance  of  alkaline  rocks  in  the  Laurentian  may  be  due  to  the  effects 
»f  life  in  the  Laurentian  ocean. 
Between  the  Laurentian  and  Huronian  periods  the  Laurentian  beds 
rere  closely  folded ;  the  sediments  were  changed  by  metamorphism  to  a 
horoughly  crystalline  condition,  and  the  series  was  profoundly  eroded. 
Ibout  the  thus  formed  Laurentian  isles  was  deposited  the  Huronian. 
?his  comprises  the  Penokee  series,  the  Menominee  series,  the  Baraboo 
[iiartzites,  the  quartz -porphyries  of  central  Wisconsin,  the  quartzites 
nd  catlinite  of  Barron  and  Chippewa  counties,  and  the  iron-bearing 
eries  of  Black  river  falls.  These  series  consist  for  the  most  part  of 
imestones,  slates,  sometimes  heavily  carbonaceous,  quartzites,  heina- 
itic  and  magnetitic  schists,  mica-schists  and  diorites.  The  presence  of 
imestones,  carbonaceous  shales,  and  iron  ore  are  taken  as  the  evidence 
»f  life.  After  the  deposition  of  the  Huronian  it  was  upheaved,  meta- 
aorphosed  and  eroded  before  the  beginning  of  Keewenawan  time, 
although  the  unconformity  between  the  two  series  in  Wisconsin  is 
>ut  slight  and  the  above  changes  were  only  partially  accomplished 
trhen  the  Keweenawan  eruptions  began.  The  metamorphism  was  less 
n  degree  than  that  which  has  affected  the  Laurentian  strata,  but  is 
Qore  intense  than  that  which  the  Keweenawan  series  has  suffered.  It 
\ras  not  in  general  sufficient  to  obliterate  the  original  grains  and  peb- 
)les,  nor  to  destroy  ripple  and  rill  marks.  In  the  Huronian  strata  are 
gneous  beds  and  dikes  of  gabbros,  diabases,  and  diorites,  the  age  of 
rhich  is  not  certain.  They  may  be,  so  far  as  yet  known,  in  part  cou- 
emporaneous  and  in  part  subsequent,  or  wholly  the  one  or  the  other. 
The  rocks  of  the  Keweenawan  period  consist  of  interstratified  igne- 
us  and  sedimentary  beds;  the  former  mainly  diabases,  with  some 
abbros,  melaphyres,  and  porphyries;  the  latter  conglomerates,  sand- 
tones,  and  shales,  derived  mainly  from  the  igneous  rocks.  The  maxi- 
aum  thickness  is  about  45,000  feet,  of  which  the  upper  15,000  feet  is 
edimentary.    The  bottom  of  the  lake  Superior  basin  was  gradually 
