192  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
quite  possibly  tit*1  lower  Cambrian  formations  of  Great  Britain  and 
Bohemia  bridge  the  entire  interval. 
Irving,122  in  1888,  gives  a  systematic  account  of  the  lithology  of 
Wisconsin.  Among  the  eruptive  rocks  are  placed  diabase,  melaphyre, 
gabbro,  norite,  diorite,  peridotite,  syenite,  porphyry,  and  granite. 
Among  the  schistose  rocks  are  gneiss,  mica  schist,  hydromica  schist, 
actinolite  schist,  tremolite  schist,  hornblende  schist,  augite  schist, 
chlorite  schist,  talc  schist,  magnetite  schist,  hematite  schist,  quartz 
schist,  quartzite  in  part,  chert  schist,  and  jasper  schist.  Among  the 
half  fragmental  rocks  are  quartzite,  clay  slate,  and  novaculite.  The 
explanation  of  the  origin  of  gneiss  or  lamellar  granite  by  metamor- 
phism,  the  structure  being  regarded  as  residual  sedimentation,  is 
not  satisfactory.  Many  rocks  which  have  been  called  metamorphic 
are  placed  as  eruptive,  and  it  seems  not  improbable  that  the  same 
origin  is  to  be  attributed  to  some  rocks  with  a  strongly  developed 
schistose  structure.  The  hornblende  schists  are  regarded  as  altered 
forms  of  augite  schists. 
Wooster,123  in  1884,  describes,  on  St.  Croix  River  near  Osceola 
Mills,  the  Potsdam  sandstone  carrying  fossils,  which  grades  down 
into  a  conglomerate  containing  pebbles  from  the  KeweenawTan,  Lau- 
rentian,  and  Huronian  series.  The  sandstone  and  conglomerate  rest 
unconformably  upon  the  underlying  Keweenawan  rocks! 
Irving  and  Van  Hise,  in  1890  and  1892,  give  a  detailed  description 
of  the  Penokee-Gogebic  series  of  Michigan  and  Wisconsin.  See  sum- 
mary in  Chapter  III,  section  1,  Michigan,  pages  140-141. 
Van  Hise,  in  1890,  describes  a  base-level  in  north-central  Wiscon- 
sin and  Keweenaw  Point.  See  summary  in  Chapter  III,  section  1, 
Michigan,  page  154. 
Berkey,124  in  1897  and  1898,  describes  and  maps  the  geology  of 
the  St.  Croix  Dalles  of  Wisconsin  and  Minnesota.  Keweenawan 
eruptives  are  exposed  at  numerous  localities,  and  particularly  along 
the  river,  where,  by  their  erosion,  they  have  formed  the  dalles  of  St. 
Croix  River.  The  dip  is  about  15°  S.,  which  would  give  a  thickness 
to  the  rocks  in  sight  of  4,000  feet.  At  several  localities  the  Basal 
sandstone  unconformably  overlies  the  Keweenawan  eruptives  with 
visible  contacts,  the  Basal  sandstone  including  the  sandstone  and 
shale  series  between  the  Keweenawan  and  the  St.  Lawrence  shales. 
Grant,125  in  1900,  describes  and  maps  the  Upper  and  Lower 
Keweenawan  copper-bearing  rocks  of  Douglas  County,  Wis.  The 
Lower  Keweenawan  appears  in  a  broad  belt  running  northeast-south- 
west across  the  county,  widening  toward  the  southwest,  and  in  a 
small  belt  cutting  through  the  southeast  corner  of  the  county.  It 
consists  mainly  of  basic  lava  flows,  associated  with  which,  in  the 
area  in  the  southeast  corner  of  the  county,  are  a  few  beds  of  con- 
glomerate composed   of  debris  of  the  closely   adjacent   underlying 
