120  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
suggests  the  idea  that  it  is  Azoic ;  but  the  gradual  transition  from  the 
unaltered  sandstone  of  the  Sault  to  the  altered  sandstone  of  Neebish 
Rapids  and  the  jasper  conglomerates  of  the  western  shore  of  Campe- 
ment  d'Ours,  favors  the  equivalency  of  the  sandstone  and  the 
quartzite,  as  does  also  the  fact  that  the  fossiliferous  Chazy  limestone 
is  found  directly  upon  the  quartzite  at  Sulphur  Island. 
Williams  and  Blandy,28  in  1862,  describe  the  trap  ranges  of  Por- 
tage Lake  as  being  about  3  miles  wide  and  consisting  of  amygdaloidal 
trap,  occasionally  intercalated  with  sandstones  and  conglomerates. 
The  dips  vary  from  60°  to  75°,  becoming  more  nearly  horizontal 
toward  the  northwest,  until  finally  the  sandstone  which  succeeds  it 
becomes  absolutely  horizontal. 
Kimball,29  in  1865,  divides  the  rocks  of  the  Marquette  region  into 
two  formations,  Laurentian  and  Huronian,  the  former  including  the 
granite  ridges,  while  the  latter  nearly  agrees  with  Foster  and  Whit- 
ney's limits  for  the  Azoic.  The  crystalline  rocks  south  of  Keweenaw 
Point  are  pre-Paleozoic,  while  the  greenstones  of  that  point  are  inter- 
calated conformably  with  the  Paleozoic.  The  specular  iron  ore  and 
beds  of  specular  conglomerate  are  heavy-bedded  strata  and  schists  in 
which  none  of  the  phenomena  of  aqueous  deposits  are  wanting.  They 
exhibit  not  only  stratification,  but  anticlinal  and  synclinal  folds. 
From  a  stratigraphic  point  of  view  the  Huronian  greenstones,  schists, 
and  iron  ores  of  Marquette  exhibit  characters  which  render  quite  un- 
tenable the  theory  of  the  exotic  character  of  any  portion  of  them. 
Agassiz,30  in  1867,  finds  at  two  ravines  near  Torch  Lake — one  the 
Douglass  Houghton — that  the  sandstone  rests  unconformably  upon 
the  trap.  The  trap  dips  42°  N.,  while  the  sandstone,  100  feet  distant, 
lies  nearly  horizontal,  with  no  trace  of  an  anticlinal  axis  between. 
The  sandstone  contains  waterworn  fragments  of  the  trap.  The 
sandstone  north  of  the  range  is  conformable  with  the  trap,  but  the 
sandstone  south  is  plainly  of  a  different  age. 
Credner,31  in  1869,  describes  the  Laurentian  and  Huronian  sys- 
tems in  the  Upper  Peninsula  of  Michigan.  The  Laurentian  system 
is  the  gneiss  granite  formation,  which  includes  many  varieties  of 
massive  rocks,  as  well  as  hornblende,  chlorite,  and  other  schists,  and 
also  thin  layers  of  dolomitic  limestones.  The  Huronian  system  is  the 
iron-bearing  formation.  The  general  succession,  beginning  at  the 
base,  is :  Quartzite,  in  its  upper  parts  often  iron  stained,  2,500  feet ; 
crystalline  dolomitic  limestone,  containing  argillite,  chlorite  schist, 
and  layers  of  quartz,  seldom  conglomeratic,  2,500  to  3,500  feet;  more 
or  less  siliceous  hematite,  600  to  1,000  feet;  ferruginous  chlorite  schist, 
1,200  feet;  dark  clay  slate,  with  beds  of  hard  quartzite,  8,500  feet; 
chlorite  schist,  with  beds  of  diorite,  1,300  feet;  talc  schist,  with  va- 
rious impurities,  100  feet;  aphanitic  to  granular  diorite,  2,300  feet; 
talc  schist,  with  various  impurities,  1,500  feet. 
