LAKE    SUPERIOR   REGION.  187 
The  Penokee  series  is  compared  with  the  Marquette  Huronian  and 
there  is  found  to  be  a  general  likeness  in  the  rock  succession  in  the  two 
regions.  Very  numerous  detailed  sections  and  outcrops  at  particular 
localities  are  fully  described  and  mapped. 
Wright,113  in  1880,  describes  the  Huronian  series  west  of  Penokee 
Gap.  The  succession  here  found  is  limestone,  chloro-siliceous  schists, 
quartzites,  magnetitic  schists,  Keweenawan;  the  magnetitic  schists 
being  occasionally  interstratified  with  greenstone.  At  Penokee  Gap 
is  found  a  dolomitic  limestone  overlain  with  quartzite  and  chloro- 
siliceous  schists,  which  rests  unconformably  upon  the  Laurentian 
rocks.  The  Huronian  rocks  here  have  a  dip  of  66°  N.,  while  the  Lau- 
rentian rocks  have  a  southern  inclination  of  65°  to  80°.  West  of, 
Numakagon  Lake  the  magnetic  attractions  are  found  to  cease  and 
the  Copper-bearing  series  and  granite  belonging  to  the  Laurentian 
are  found  in  direct  contact.  This  appearance  is  regarded  as  being 
due  to  the  covering  up  of  the  Lower  Huronian  by  the  Copper-bearing 
rocks. 
Sweet,114  in  1880,  describes  the  geology  of  the  western  Lake  Su- 
perior district.  The  geological  formations  here  found  comprise  the 
Laurentian,  Huronian,  and  Keweenawan  systems.  The  Keweenawan 
rocks  are  found  in  a  great  syncline.  In  northern  Wisconsin,  below 
the  Keweenawan,  no  southward-dipping  rocks  are  found  that  are  re- 
ferred to  the  Huronian,  but  in  Minnesota,  along  St.  Louis  River,  are 
strata  which  occupy  a  position  inferior  to  the  Keweenawan  series,  are 
lithologically  like  the  slates  of  Ashland  County,  and  are  cut  by 
numerous  dikes  in  lithological  character  precisely  like  the  rock  at  the 
base  of  the  Copper-bearing  series.  Hence  they  are  regarded  as 
Huronian.  The  Copper-bearing  strata  consist  of  a  detrital  upper 
portion  of  sandstones,  conglomerates,  and  breccias,  having  a  maxi- 
mum thickness  of  9,000  feet,  and  of  eruptive  strata,  consisting  of 
melaphyre,  diabase,  porphyry,  gabbro,  etc.,  having  an  apparent  maxi- 
mum thickness  of  more  than  3G,000  feet.  The  quartzites,  siliceous 
schists,  and  chloritic  slates  along  St.  Louis  River,  referred  to  the 
Huronian,  are  many  thousand  feet  thick.  Upon  the  St.  Louis  slates 
at  one  place  the  Lake  Superior  red  sandstone  and  conglomerate  re- 
pose unconformably.  The  Keweenawan  eruptive  rocks  are  bedded. 
They  have  a  very  persistent  and  rather  uniform  clip  and  strike  in  any 
given  locality.  The  layers  are  seldom  less  than  a  foot  or  two  in 
thickness  and  are  more  often  many  feet  thick,  so  as  to  give  an  ex- 
posure an  unstratified  appearance.  On  one  side  of  each  layer  is  a 
precipitous  and  somewhat  jagged  ridge,  owing  to  the  exposure  of  the 
edge  of  the  layers,  while  on  the  other  side  the  soil  descends  with  the 
inclination  of  the  bedding. 
As  to  the  age  of  the  Copper-bearing  series,  it  can  only  be  said  that 
they  are  older  than  the  Lake  Superior  red  sandstones,  for  when  the 
