94  P.RE-CAMHKIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
explained  as  due  to  a  fault,  the  horizontal  sandstone  being-  regarded  as 
of  the  same  age  as  the  conformable  overlying  sandstone  of  the  cuprif- 
erous series.  One  objection  to  this  explanation  is  the  enormous  amount 
of  dislocation  required,  amounting  to  several  miles.  Again,  near  Hough- 
ton there  are  two  patches  of  sandstone  lying  on  the  upturned  melaphyre 
beds.  In  the  horizontal  sandstone  near  the  so-called  fault  are  abundant 
pebbles  of  melaphyre  and  conglomerate  of  the  cupriferous  series.  But 
the  most  decided  facts  found  by  Maj.  Brooks  and  the  author  are  in  the 
country  between  the  Bad  river  in  Wisconsin  and  the  middle  branch  of 
the  Ontonagon,  east  of  lake  Gogebic.  Here  the  quartzites  and  schists 
of  the  Huronian  formation  are  bordered  on  the  south  by  the  Laurentian 
gneisses,  and  are  overlain  conformably  by  the  bedded  inelaphyres  and 
inter  stratified  sandstones  of  the  cupriferous  series.  Between  these 
ridges  forming  the  south  mineral  range  and  the  main  range  of  Kewee- 
naw point  is  the  horizontally  stratified  Silurian  sandstone,  forming 
a  generally  level  country.  The  conformable  cupriferous  and  Huron- 
ian schists  dip  to  the  northward  at  angles  from  50°  to  70°,  but  in  ap- 
proaching 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  the  Ontonagon 
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  (>f)°  southeast.  The  nearest  outcrop  of  the  cupriferous 
series  is  about  4  miles  distant,  and  it  strikes  nearly  east  and  west  and 
dips  50°  to  the  north.  The  lithology  of  the  copper-bearing  rocks  of  the 
Portage  lake  district  is  fully  given.  The  rocks  are  melaphyres  and 
amygdaloids,  interstratihed  with  conglomerates.  -The  paragenesis  of 
the  minerals  associated  with  the  copper  is  worked  out.  Several 
detailed  cross  sections  are  given  at  Portage  lake,  and  one  cross  section 
at  the  Central  mine  describing  the  thickness  and  character  of  the 
alternating  rocks  in  great  detail. 
Marvtne,103  in  1873,  gives  in  the  greatest  detail  the  structure  and 
lithology  of  the  alternating  trappean  and  detfital  beds  of  the  copper- 
bearing  rocks  on  the  Eagle  river  section.  The  correlation  of  the  Hough- 
ton and  Keweenaw  rocks  is  fully  discussed.  The  Albany,  Boston,  and 
Allouez  conglomerates  are  regarded  as  the  samebed.  Stratigraphically 
eleven  out  of  fifteen  conglomerates  have  equivalents  in  both  the  Hough- 
ton and  Keweenaw  regions.  The  conglomerate  beds  of  Keweenaw 
point  are  not  mere  local  deposits,  but  are  unusually  persistent,  and 
while  a  bed  may  thin  out  and  lose  its  character  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  over  75  feet.  It 
is  therefore  concluded  that  the  changes  which  formed  the  melaphyres 
