itanhise.]  LAKE    SUPERIOR    REGION.  113 
terns  are  actually  nonconformable  in  these  regions  is  not  evident,  for 
in  sections  the  dip  in  passing  from  one  to  the  other  is  generally  nearly 
bhe  same.  That  there  is  a  real  unconformity  is  indicated  by  the  facts 
(1)  that  in  passing  westward  irom  Penokee  gap  the  uppermost  beds  of 
bhe  Huronian  are  gradually  cut  off  by  the  gabbro  that  forms  the  base 
[>f  the  Keweenaw  series;  (2)  that  there  is  not  an  absolute  conformity  in 
dip  between  the  Huronian  and  Keweenawan  rocks ;  (3)  west  of  lake 
Numakagon  the  diabases  and  other  eruptive  rocks  of  the  Keweenaw 
series  appear  completely  to  cover  the  Huronian. 
The  lake  Superior  sandstone  is  always  in  a  horizontal  position  and  is 
more  highly  siliceous  than  the  sandstones  of  the  Keweenawan  system. 
A.t  the  St.  Louis  river  it  overlies  unconformably  the  Huronian  schists, 
[n  Douglas  county  are  several  junctions  of  the  sandstone  with  the  Ke- 
weenawan rocks.  Here  the  horizontal  sandstones  in  approaching  the 
eruptive  rocks  of  the  Keweenawan  system  are  found  to  be  brecciated 
and  tilted,  the  original  lines  of  deposition  being  sometimes  entirely 
obliterated.  These  peculiar  appearances  are  regarded  as  due  in  part  to 
the  naturally  confused  mode  of  deposition  on  the  cliffy  shore  in  which 
bhe  sandstone  was  originally  deposited;  but  a  slight  movement  of  the 
deep-seated  crystalline  rocks  against  the  more  superficial  sandstones 
would  also  account  for  much  of  the  phenomena.  That  the  sandstone 
formation  rests  unconformably  upon  th  Keweenawan  system  is  further 
mown  by  the  fact  that  in  the  Dalles  of  the  St.  Croix  the  horizontal 
sandstone  and  shales,  with  characteristic  Primordial  fossils,  lie  upon 
bhe  irregular  and  eroded  surface  of  a  Keweenawan  melaphyre. 
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,137  in  1880,  describes  the  Huronian  series  west  of  Penokee 
^ap.  The  succession  here  found  is  limestone,  chloro-silicious  schists, 
^uartzites,  magnetic  schists,  Keweenawan;  the  magnetic  schists  being 
occasionally  interstratitied  with  greenstone.  At  Penokee  gap  is  found 
idolomitic  limestone  overlain  with  quartzite  and  chloro-siliceous  schists, 
^hich  rest  unconformably  upon  the  Laurentian  rocks.  The  Huronian 
ocks  here  have  a  dip  of  66°  to  the  north,  while  the  Laurentian  rocks 
lave  a  southern  inclination  from  65°  to  80°.  West  of  Numakagon  lake 
he  magnetic  attractions  are  found  to  cease  and  the  Copper- bear  mi- 
series and  granite  belonging  to  the  Laurentian  are  found  in  direct  con- 
act.  This  appearance  is  regarded  as  being  due  to  the  covering  up  of 
he  Lower  Huronian  by  the  Copper-bearing  rocks. 
Sweet,138  in  1880,  describes  the  geology  of  the  western  lake  Superior 
li strict.  The  geological  formations  here  found  comprise  the  Laurentian, 
luronian,  and  Keweenawan  systems.  The  Keweenawan  rocks  are  found 
u  a  great  synclinal.  In  northern  Wisconsin,  below  the  Keweenawan, 
lo  southward  dipping  rocks  are  found  referred  to  the  Huronian,  but  in 
Bull.  86 8 
