114  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
Minnesota,  along  the  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  char- 
acter precisely  like  the  rock  at  the  base  ot  the  Copper-bearing  series. 
They  are  hence  regarded  as  Huronian.  The  Copper-bearing  strata  con- 
sist of  a  detrital  upper  portion  of  sandstones,  conglomerates,  and  breccias, 
having  a  maximum  thickness  of  9,000  feet,  and  of  eruptive  strata,  con- 
sisting of  melaphyre,  diabase,  porphyry,  gabbro,  etc.,  having  an  appar- 
ent maximum  thickness  of  over  36,000  feet.  The  quartzites,  siliceous 
schists  and  chloritic  slates  along  the  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  repose 
un  con  form  ably.  The  Keweenawan  eruptiverocks  are  bedded.  They  have 
a  very  persistent  and  quite  uniform  dip  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  exposure  an  unstratified  appear- 
ance. 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  be  only  said  that 
they  are  older  than  the  lake  Superior  red  sandstone ;  for  when  the  latter 
is  conglomeratic  the  pebbles  are  almost  invariably  derived  from  the 
Keweenaw  series.  Also  the  perfectly  horizontal  sandstones  approach 
in  that  condition  within  15  or  20  feet  of  the  dipping  crystalline  rocks; 
and  from  this  it  is  assumed  that  they  unconformably  overlie  them.  At 
Black  river  falls  on  the  lower  Black  river,  at  the  gorge  of  Copper  creek, 
and  along  the  west  bank  of  Middle  river,  the  horizontal  sandstones  are' 
found,  in  approaching  the  eruptives  of  the  Copper-bearing  series,  to  be- 
come uptilted,  breccia  ted,  and  in  some  cases  conglomeratic,  and  some- 
times wholly  lose  their  structure.  The  eruptives  in  all  these  cases  dip 
away  from  the  uptilted  sedimentary  rocks. 
Chamberlin  and  Strong,159  in  1880,  describe  the  geology  of  the 
upper  St.  Croix  district.  The  Keweenaw  series  is  composed  of  twc 
classes  of  rocks,  massive  crystalline  beds  which  owe  their  origin  to  th^< 
succession  of  outflows  of  molten  rocks,  and  conglomerates,  sandstones 
and  shales  derived  from  the  wear  of  these  igneous  rocks  and  from  the- 
older  formations.  They  are  in  part  mterstratined  with  the  igneous 
rocks  and  in  part  overlie  them.  The  eruptives  are  mainly  diabase  and 
diabase  amygdaloid,  although  melaphyre  is  found.  The  Keweenawar. 
beds  were  deposited  in  essentially  a  horizontal  condition,  were  bent  intc 
their  present  trough-like  form,  and  ercded,  and  upon  their  upturned' 
edges  was  deposited  the  Potsdam  sandstone.  This  is  shown  by  the  faci 
that  at  St.  Croix  falls  the  horizontally  stratified  sandstone  is  found  with 
in  a  few  feet  of  an  exposure  of  highly  inclined  Keweenawan  melaphyre 
containing  numerous  fragments  derived  from  it.  This  sandstone  has 
characteristic  Potsdam  fossils.    At  one  place  near  the  falls  the  Potsdam  I 
