ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY. 
the  Siamo  slate,  and  the  Freedom  dolomite  with  heavily  ferruginous 
horizon  at  its  base  to  the  iron-bearing  Xegaunee  formation,  which  is 
known  to  have  resulted  from  the  alteration  of  iron  carbonate.  The 
sedimentary  series  will  here  be  called  Huronian  and  the  basement 
complex  will  be  called  Archean. 
WATERLOO,  PORTLAND,  AND  LAKE  MILLS. 
SUMMARY   OF  LITERATURE. 
Percival,  in  1856,  discusses  the  geology  of  this  district.  See  sum- 
mary under  Baraboo,  page  717. 
Irving,14  in  1873,  maintains  the  pre-Potsdam  age  of  the  Portland 
quartzite  on  the  same  ground  as  the  pre-Potsdam  age  of  the  Baraboo 
ranges.  There  is  a  close  similarity  between  the  Baraboo  and  Port- 
land quartzites  and  the  rocks  in  northern  Wisconsin  and  Michigan 
which  are  now  regarded  as  Huronian. 
Chamberlin,  in  1877,  describes  the  geology  of  this  district.  See 
summary  under  Fox  Kiver  Valley,  page  722. 
Buell,15  in  1892,  describes  and  maps  the  Waterloo  quartzite  areas. 
These  are  a  series  of  detached  outcrops  resting  unconformably  under 
the  Lower  Silurian  of  southern  Wisconsin.  Within  the  quartzite 
are  occasional  layers  of  conglomerate.  The  different  outcrops  are 
apparently  parts  of  a  synclinal  fold.  As  a  result  of  the  shearing 
much  of  the  quartzite  has  been  crushed,  and  sericite  has  developed. 
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. 
Warner,16  in  190-1,  maps  and  describes  the  quartzite  of  the  Water- 
loo, Portland,  and  Lake  Mills  areas,  collectively  referred  to  as  the 
Waterloo  quartzite.  The  quartzite  outcrops  have  distribution  and 
structure  such  as  to  suggest  that  they  represent  parts  of  the  edge  of 
a  great  eastward-pitching  syncline  of  quartzite.  The  quartzite  is 
almost  identical  lithologically  with  the  Baraboo  quartzite,  and  its 
synclinal  axis  is  along  the  line  of  the  axis  of  the  Baraboo  syncline. 
There  is  little  reason  to  doubt  that  the  Baraboo  and  Waterloo  quartz- 
ites are  of  the  same  age.  If  this  be  the  case,  one  would  expect  to  find 
slate  and  ferruginous  dolomite  formations  within  the  Waterloo 
quartzite  syncline,  as  in  the  Baraboo  syncline.  On  this  basis  War- 
ner has  mapped  the  theoretical  distribution  of  such  formations  be- 
neath the  Paleozoic  rocks.  Like  the  Baraboo  quartzite,  the  Waterloo 
quartzite  is  referred  to  the  Huronian,  and  its  similarity  with  the  Mid- 
dle Huronian  series  is  emphasized.  Well  drilling  outside  of  the 
Waterloo  syncline  shows  the  presence  of  a  granite  basement, 
55721— Bull.  360—09 46 
