188  PRE-CAMBRIAN   GEOLOGY    OF    NORTH   AMERICA. 
latter  is  conglomeratic  the  pebbles  are  almost  invariably  derived  from 
the  Keweenaw  series.  Also  the  perfectly  horizontal  sandstones  ap- 
proach in  that  condition  within  15  or  20  feet  of  the  dipping  crystal- 
line rocks,  and  from  this  it  is  assumed  that  they  unconformable  overlie 
them.  At  Black  Kiver  Falls  on  lower  Black  River,  at  the  gorge  of 
Copper  Creek,  and  along  the  west  bank  of  Middle  River  the  hori- 
zontal sandstones  are  found,  in  approaching  the  eruptives  of  the 
Copper-bearing  series,  to  become  uptilted,  brecciated,  and  in  some 
cases  conglomeratic,  and  sometimes  wholly  lose  their  structure.  The 
eruptives  in  all  these  cases  dip  away  from  the  uptilted  sedimentary 
rocks. 
Chamberlin  and  Strong,115  in  1880,  describe  the  geology  of  the 
upper  St.  Croix  district.  The  Keweenaw  series  is  composed  of  two 
classes  of  rocks — massive  crystalline  beds  which  owe  their  origin  to 
the  succession  of  outflows  of  molten  rocks,  and  conglomerates,  sand- 
stones, and  shales  derived  from  the  wear  of  these  igneous  rocks  and 
from  the  older  formations.  They  are  in  part  interstratified  with  the 
igneous  rocks  and  in  part  overlie  them.  The  eruptives  are  mainly 
diabase  and  diabase  amygdaloid,  although  melaphyre  is  found.  The 
Keweenawan  beds  were  deposited  in  essentially  a  horizontal  condi- 
tion, were  bent  into  their  present  troughlike  form  and  eroded,  and 
upon  their  upturned  edges  was  deposited  the  Potsdam  sandstone. 
This  is  shown  by  the  fact  that  at  St.  Croix  Falls  the  horizontally 
stratified  sandstone  is  found  within  a  few  feet  of  an  exposure  of 
highly  inclined  Keweenawan  melaphyre  containing  numerous  frag- 
ments derived  from  it.  This  sandstone  has  characteristic  Potsdam 
fossils.  At  one  place  near  the  falls,  in  a  small  gorge,  the  Potsdam  is 
found  directly  superimposed  upon  the  melaphyre.  The  melaphyre  is 
cut  by  vertical  planes  of  division  which  are  fairly  smooth  and  uni- 
form; but  there  is  another  persistent  set  which  is  much  less  smooth, 
though  persistent  and  constant  in  direction.  These  planes  are  usually 
in  detail  slightly  uneven  and  undulatory  and  are  separated  by  sev- 
eral feet.  They  are  believed  to  represent  the  dip  of  the  igneous  beds. 
It  is  upon  the  persistence  of  these  inclined  beds,  taken  in  connection 
with  their  parallel  lithological  habit,  that  the  determinations  of  dip 
are  based.  Outside  of  the  district  the  northward-dipping  diabase 
is  found  on  one  side  of  Numakagon  River,  while  on  the  other  is  seen 
the  Laurent ian  granite. 
Brooks,110  in  1880,  gives  the  geology  of  the  Menominee  iron  region. 
The  Lower  Silurian  sandstone  is  found  capping  the  older  rocks  near 
Lake  Eliza.  The  Laurentian  granite,  gneiss,  and  crystalline  schist 
series  is  not  subdivided.  No  limestones,  dolomitic  marbles,  conglom- 
erates, calcareous  or  arenaceous  chloritic  schists  are  considered  as 
belonging  to  the  Laurentian  system.  It  is  not  certain  whether  this 
series  occurs  in  Wisconsin  within  the  area  surveyed. 
