LAKE    SUPERIOR   REGION.  125 
Keweenaw  Point,  is  the  horizontally  stratified  Silurian  sandstone, 
forming  a  generally  level  country.  The  conformable  cupriferous  and 
Huronian  schists  dip  to  the  northward  at  angles  of  50°  to  70°,  but  in 
approaching  Gogebic  Lake  from  the  west  the  pre-Silurian  erosion 
has  made  a  deep  indentation  across  the  cupriferous  series  and  the 
Huronian,  as  well  as  into  the  Laurentian,  so  that  a  short  distance  west 
of  the  lake  these  rocks  end  in  steep  and  high  declivities,  at  the  base 
of  which  lies  the  level  country  of  the  Silurian  sandstone.  On  On- 
tonagon River  the  Silurian  sandstone  is  nearly  horizontal,  while 
about  150  steps  from  the  base  of  the  cliff  are  outcrops  of  Laurentian 
schists  having  a  dip  of  45°  to  60°  SE.  The  nearest  outcrop  of  the 
cupriferous  series  is  about  4  miles  distant,  and  it  strikes  nearly  east 
and  west  and  dips  50°  N.  The  lithology  of  the  copper-bearing  rocks 
of  the  Portage  Lake  district  is  fully  given.  The  rocks  are  mela- 
phyres  and  amygdaloids,  interstratified  with  conglomerates.  The 
paragenesis  of  the  minerals  associated  with  the  copper  is  worked 
out.  Several  detailed  cross  sections  at  Portage  Lake  are  given,  and 
one  cross  section  at  the  Central  mine  describing  the  thickness  and 
character  of  the  alternating  rocks  in  great  detail. 
Marvine,35  in  1873,  gives  in  the  greatest  detail  the  structure  and 
lithology  of  the  alternating  trappean  and  detrital  beds  of  the  copper- 
bearing  rocks  on  the  Eagle  River  section.  The  correlation  of  the 
Houghton  and  Keweenaw  rocks  is  fully  discussed.  The  Albany, 
Boston,  and  Allouez  conglomerates  are  regarded  as  the  same  bed. 
Stratigraphically  eleven  out  of  fifteen  conglomerates  have  equivalents 
in  both  the  Houghton  and  the  Keweenaw  regions.  The  conglom- 
erate beds  of  Keweenaw  Point  are  not  mere  local  deposits,  but  are 
unusually  persistent,  and  while  a  bed  may  thin  out  and  lose  its  char- 
acter as  a  conglomerate,  it  may  still  exist  as  a  mere  seam.  In  one 
instance  a  band  extends  for  at  least  50  miles,  varying  in  thickness 
from  a  few  to  more  than  75  feet.  It  is  therefore  concluded  that  the 
changes  which  formed  the  melaphyres  ceased  to  act  over  extended 
areas  during  the  time  of  the  formation  of  the  sandstones  and  con- 
glomerates. The  abundance  of  acidic  rocks  among  the  conglomerate 
was  noted,  and  opposite  Calumet  the  former  presence  of  predominant 
quartz  porphyry  was  inferred. 
Rominger,36  in  1873,  places  the  Lake  Superior  sandstone  as 
Potsdam,  finding  it  directly  overlapped  by  the  calciferous  formation. 
At  Presque  Isle  and  Granite  Point  the  horizontal  sandstones  are 
found  resting  upon  the  crystalline  rocks,  there  being  at  the  former 
place  a  conglomerate  which  rests  unconformably  upon  the  dolomite 
of  Presque  Isle.  The  sandstones  on  the  eastern  shore  of  Keweenaw 
Point  retain  their  horizontal  position  and  lithological  character  to 
such  a  degree  that  the  different  strata  can  be  parallelized  without 
difficulty  with  those  of  the  more  eastern  localities.     Near  the  center 
