726  PBE-CAMBKIAN    GEOLOGY   OF    NORTH   AMEBICA. 
Beyer,33  in  1897,  maps  and  describes  the  part  of  the  Sioux  quartzite 
formation  exposed  northeast  of  Sioux  Falls  in  sees.  10,  11,  14,  15,  22, 
and  23,  T.  102  N.,  R.  48  W.,  South  Dakota.  The  quartzite  dips  from 
3°  to  7°  SW.  An  accurate  estimate  of  the  thickness  may  not  be  given, 
but  1,500  feet  is  a  liberal  one. 
Slate  is  exposed  in  the  area  in  isolated  outcrops,  but  never  in  con- 
tact with  the  quartzite.  In  composition  it  corresponds  very  closely 
to  the  quartz  slate  of  Irving  and  Van  Hise.a  Intruding  the  slate  are 
diabase  dikes,  which  have  followed  the  bedding. 
The  relations  of  the  slates  and  quartzites  can  not  here  be  ascer- 
tained. However,  from  the  relations  of  the  two  outside  of  the  area 
it  is  believed  that  the  slates  are  the  upward  continuation  of  the  quart- 
zite, and  that  they  have  been  removed  in  large  part. 
The  age  of  the  Sioux  quartzite  is  believed  to  be  pre-Cretaceous.  Its 
reference  to  the  Huronian  may  be  supported  by  the  following  facts : 
The  lithological  characters  of  the  quartzite  are  identical  with  those 
of  the  Baraboo  quartzite  in  Wisconsin,  which  has  been  referred  by 
Irving  and  Van  Hise  to  the  Huronian.  The  diabase  intruding  the 
slate,  supposed  to  be  the  upward  continuation  of  the  quartzite,  is 
strikingly  similar  to  intrusives  which  are  peculiar  to  the  Huronian  in 
the  Lake  Superior  region. 
Wilder,34  in  1899,  describes  and  maps  the  Sioux  quartzites  and 
quartz  porphyries  of  Lyon  County,  Iowa.  No  points  concerning 
stratigraphy  or  age  have  been  added  to  those  already  given  by  other 
writers. 
Todd,35  in  1904,  reports  the  presence  of  gabbro  within  one-half 
mile  of  the  Sioux  quartzite  of  South  Dakota.  He  believes  it  to  be 
intrusive  into  the  quartzite,  although  no  contacts  are  found. 
Darton,36  in  1905,  in  connection  with  the  study  of  the  artesian  wells 
of  eastern  South  Dakota,  maps  the  configuration  of  the  bed  rock. 
The  Sioux  quartzite  ridge  of  the  southeastern  part  of  the  State  is 
found  to  extend  westward  beneath  the  Paleozoic  rocks  nearly  to  Mis- 
souri River.  On  the  north  side  a  few  deep  wells  are  bottomed  in 
granite.  A  few  scattering  records  of  quartzite,  some  of  them  doubt- 
ful, also  appear  both  to  the  north  and  to  the  south  of  this  ridge. 
For  literature  on  correlation  of  this  district  with  the  rocks  of  Lake 
Superior  see  summaries  of  articles  by  Irving,  Van  Hise,  and  others 
in  Chapter  III,  Lake  Superior  region,  pages  108-252. 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
In  a  small  area  covering  the  contiguous  corners  of  Minnesota, 
Iowa,  and  South  Dakota  is  the  Sioux  quartzite,  consisting  mainly  of 
reddish  vitreous  quartzite  but  having  associated  with  it  quartz  slate 
a  Van  Hise,  C.  R.,  and  Irving,  R.  D.,  The  Penokee  iron-bearing  series  :  Tenth  Ann.  Rept. 
U.  S.  Geol.  Survey,  1890,  pp.  370  et  seq. 
