ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY.  725 
defined  fossils  were  discovered,  upon  the  outer  surfaces  of  the  rocks 
are  rounded  outlines  of  what  appear  to  be  organic  remains,  but  the 
peculiar  character  of  the  quartzite  points  toward  the  Azoic  series. 
The  formation  is  tentatively  referred  to  the  Super-Carboniferous, 
Triassic,  or  downward  extension  of  the  Cretaceous;  but  Hall's  opin- 
ion that  this  rock  is  Huronian  is  entitled  to  great  weight. 
White,  in  1870,  describes  the  quartzite  of  Iowa,  Minnesota,  and 
Dakota.  See  summary  under  Lake  Superior  region,  Chapter  III, 
section  3,  Minnesota,  page  197. 
Winchell  (N.  H.),28  in  1885,  finds  in  the  red  quartzite  at  Pipe- 
stone two  fossils,  Lingula  calumet  and  Paradoxides  harberi,  which  are 
taken  as  indicative  that  this  formation,  as  well  as  the  Sioux  quartzite 
of  Iowa  and  Dakota,  the  Baraboo  quartzites  of  Wisconsin,  the  quartz- 
ites  of  southwestern  Minnesota,  and  the  associated  red  gneisses,  fel- 
sites,  and  felsite  porphyries,  are  all  Primordial. 
Upham,29  in  1885,  gives  descriptions  of  the  quartzites  of  Minne- 
haha County,  Dak.  These  are  not  infrequently  ripple  marked  and 
conglomeratic;  they  are  like  the  quartzites  of  Pipestone  County  in 
Minnesota,  and,  like  them,  are  placed  in  the  Potsdam. 
Beyer,30  in  1895,  describes  spotted  slates  associated  with  the  Sioux 
quartzite  series  in  the  northeast  corner  of  Minnehaha  County,  S.  Dak. 
The  quartzite  here  grades  up  into  reddish  slate,  which  in  lithological 
character  corresponds  to  the  quartz  slate  in  the  Penokee  series  of 
Michigan  and  Wisconsin,  described  by  Irving  and  Van  Hise. 
Keyes,31  in  1895,  gives  the  result  of  an  examination  of  the  Sioux 
quartzite.  At  several  points  in  the  bedding  planes  of  the  quartzite 
were  seen  impressions  which  so  much  resembled  those  of  lamelli- 
branchs  of  the  Gardium  and  Cytherea  types,  that,  notwithstanding 
strong  preconceived  notions  of  the  ancient  age  of  the  Sioux  rocks, 
faith  in  their  old  age  was  very  much  shaken.  It  is  concluded  that  the 
Sioux  formation  should  be  considered  as  pre-Cambrian  until  indis- 
putable evidence  to  the  contrary  is  produced,  but  that  there  now 
exist  certain  doubts  concerning  the  accuracy  of  this  view. 
Norton,32  in  1897,  in  a  description  of  the  artesian  wells  of  Iowa, 
discusses  the  attitude'  of  the  Algonkian  floor.  In  the  northwestern 
part  of  the  State  the  Algonkian  outcrops  as  the  Sioux  quartzite. 
From  here  it  sinks  rapidly  to  the  south  and  east,  and  is  discovered 
near  the  area  of  its  outcrop  only  by  the  steep  wells  at  Sioux  City, 
Hull,  and  Le  Mars.  In  the  east-central  part  of  Iowa  is  a  slight  eleva- 
tion of  the  Algonkian  floor,  disclosed  by  the  artesian  well  at  Cedar 
Rapids.  In  Wisconsin  the  Algonkian  outcrops  as  the  Baraboo  quart- 
zite, a  rock  similar  to  the  Sioux  quartzite.  From  this  outcrop  the 
Algonkian  sinks  gently  to  the  southwest,  as  it  is  reached  by  the  drill 
at  Lansing,  Iowa.  At  no  other  place  in  Iowa  has  the  drill  gone  deep 
enough  to  reach  the  crystalline  rocks. 
