242  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Peridotite  is  found  in  only  one  exposure  in  this  valley,  3  miles 
southeast  of  Morton.  The  relations  to  the  other  rocks  of  the  area 
could  not  be  determined.  Cutting  the  gneisses  and  gabbro  schists 
throughout  the  area  are  numerous  dikes  of  diabase.  They  vary  in 
width  from  a  fraction  of  an  inch  to  175  feet.  Their  age  is  probably 
Keweenawan. 
Southeast  of  Redstone  and  near  New  Ulm  are  exposures  of  quart- 
zite  associated  with  coarse  quartzite  conglomerate.  Near  Redstone 
the  strike  of  the  quartzites  is  N.  60°  to  70°  W.,  and  their  dip  varies 
from  5°  to  27°  N.  .  In  New  Ulm  the  strike  is  N.  15°  E.,  and  the  dip 
varies  from  10°  to  15°  SE.  The  quartzite  is  believed  to  be  the  same 
as  the  quartzite  found  in  a  deep  well  at  Minneopa  Falls,  near  Man- 
kato,  Minn.,  which  is  covered  by  a  quartzite  conglomerate  of  Middle 
Cambrian  age.  The  quartzite  of  Redstone  and  New  Ulm  is  above 
the  Archean  granite  and  gneiss.  It  is  believed  to  be  of  Huronian 
age,  but  whether  Upper  or  Lower  is  unknown. 
Overlying  the  crystalline  rocks  are  Cretaceous  shales  and  sand- 
stones, which  appear  in  rare  exposures  in  the  valley  and  glacial  drift. 
Winchell  (Alexander),204  in  1900,  prefaces  a  detailed  petro- 
graphical  description  of  certain  phases  of  the  gabbroicl  rocks  of 
Minnesota  with  a  brief  account  of  the  general  succession  in  structure 
of  formations  in  northeastern  Minnesota.  This  is  essentially  the  same 
as  given  by  N.  H.  Winchell  in  Vols.  IV  and  V  of  the  Minnesota 
Survey.  The  correlation  of  this  succession  with  the  succession  deter- 
mined by  the  United  States  Geological  Survey  is  discussed. 
Van  Hise  and  Clements,205  in  1900,  together  make  observations 
on  the  rocks  of  the  Minnesota  coast  from  Duluth  to  Canada.  The 
Beaver  Bay  diabase  and  the  red  rock  of  Irving's  Beaver  Bay  group 
are  found  not  to  be  surface  lava  flows,  but  intrusives  which  cut  the 
lavas  of  the  Keweenawan.  The  anorthosite  of  the  district  is  almost 
always  clearly  associated  with  the  diabase.  It  occurs  in  it  in  in- 
numerable fragments,  varying  from  those  of  small  size  to  large 
masses,  although  this  relation  to  the  diabase  can  not  be  shown  for 
some  large  masses  of  the  anorthosite.  The  red  rock  intrudes  both 
the  lavas  and  the  Beaver  Bay  diabase.  Where  the  diabase  is  cut 
through  by  numerous  dikes  of  the  red  rock,  of  varying  size,  it  seems 
to  be  altered  throughout.  It  there  contains  red  feldspar  and  assumes 
the  aspect  of  Irving's  orthoclase  gabbr6;  in  fact,  its  chemical  com- 
position seems  to  have  been  effected  by  the  minute  penetration  of  the 
magma  and  its  emanations. 
Winchell  (N.  H.),206  in  1901,  publishes  a  geological  atlas  of 
Minnesota  with  synoptical  description  of  plates.  This  volume  con- 
tains maps  and  general  conclusions  found  in  Vols.  IV  and  V  of  the 
Minnesota  Survey.  One  additional  map  is  published,  a  general  geo- 
logical map  of  the  State. 
