186  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
ments  are  in  unconformable  contact  with  the  Lower  Silurian  sand- 
stone. 
The  Keweenawan  system  is  a  distinctly  stratified  one,  in  large  meas- 
ure made  up  of  eruptive  rocks  in  the  form  of  flows.  These  constitute 
the  lower  10,000  feet  of  the  system,  and  above  these  are  found  the 
detrital  rocks,  increasing  in  frequency,  until  they  wholly  exclude  the 
igneous  rocks  in  the  upper  15,000  feet.  The  eruptives  of  the  system 
are  chiefly  diabase,  melaphyre,  and  gabbro.  The  succession  on  Mon- 
treal River  is  (1)  chiefly  diabase  and  diabase  amygdaloid,  with 
little  satisfactory  appearance  of  bedding  and  having  a  width  of  about 
33,000  feet;  (2)  alternations  of  (1)  with  red  sandstone  and  shale,  1,200 
feet;  (3)  bowlder  conglomerate,  1,200  feet;  (4)  alternations  of  shale 
and  quartzless  sandstone,  350  feet;  (5)  red  sandstone  and  shale,  12,000 
feet.  If  the  series  is  regarded  as  a  continuous  one  it  is  at  least  50,000 
feet  thick.  There  are  two  prominent  belts  of  the  Keweenaw  rocks  in 
northern  Wisconsin  lying  parallel  to  each  other  and  having  between 
them  a  synclinal  depression  which  is  occupied  by  Chequamegon  Bay. 
The  Keweenawan  system  is  evidently  newer  than  the  Penokee  system. 
That  the  two  systems  are  actually  nonconformable  in  these  regions 
is  not  evident,  for  in  sections  the  dip  in  passing  from  one  to  the 
other  is  generally  nearly  the  same.  That  there  is  a  real  unconformity 
is  indicated  by  the  facts  (1)  that  in  passing  westward  from  Penokee 
Gap  the  uppermost  beds  of  the  Huronian  are  gradually  cut  off  by 
the  gabbro  that  forms  the  base  of  the  Keweenaw  series;  (2)  that 
there  is  not  an  absolute  conformity  in  dip  between  the  Huronian  and 
the  Keweenawan  rocks;  (3)  that  west  of  Lake  Numakagon  the  dia- 
bases and  other  eruptive  rocks  of  the  Keweenaw  series  appear  com- 
pletely 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  sys- 
tem. At  St.  Louis  River  it  overlies  unconformably  the  Huronian 
schists.  In  Douglas  County  are  several  junctions  of  the  sandstone 
with  the  Keweenawan  rocks.  Here  the  horizontal  sandstones  in  ap- 
proaching 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  re- 
garded as  due  in  part  to  the  naturally  confused  mode  of  deposition 
on  the  cliffy  shore  in  which  the  sandstone  was  originally  deposited; 
but  a  slight  movement  of  the  deep-seated  crystalline  rocks  against 
the  more  superficial  sandstones  would  also  account  for  many  of  the 
phenomena.  That  the  sandstone  formation  rests  unconformably 
upon  the  Keweenawan  system  is  further  shown  by  the  fact  that  in 
the  Dalles  of  the  St.  Croix  the  horizontal  sandstone  and  shales,  with 
characteristic  Primordial  fossils,  lie  upon  the  irregular  and  eroded 
surface  of  a  Keweenawan  melaphyre. 
