ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY.  733 
area  GO  miles  long  and  20  miles  broad.  These  rocks  are  folded  into 
close  rolls  having  north-south  axial  planes,  and  are  cut  by  a  regional 
cleavage  which  is  usually  nearly  vertical.  The  ordinary  metamor- 
phism  is  believed  to  be  largely  due  to  the  folding.  The  sedimen- 
tary rocks  are  cut  by  various  masses  of  intrusive  granite- and  basic 
rocks  of  Algonkian  age,  and  also  by  later  eruptives.  The  largest 
granite  mass  is  8  or  10  miles  long  and  nearly  as  broad.  A  number 
of  other  masses  are  of  considerable  size.  Adjacent  to  these  batho- 
liths  occur  numerous  large  dikes  of  granite.  In  passing  away  from 
them  dikes  become  less  abundant  and  smaller  in  size,  until  they 
finally  disappear.  Close  to  the  large  granite  masses  the  dikes  have 
typical  granitic  structures;  farther  away  they  grade  into  peg- 
matitic  rocks,  and  these,  by  a  lessening  of  the  feldspar,  grade  into 
ordinary  quartz  veins,  showing  at  places  a  banded  structure.  There 
can  be  little  doubt  that  the  dikes  are  dependent  upon  the  batholiths 
of  granite.  There  is  equally  little  doubt  that  the  quartz  veins  are 
water  impregnations.  Between  the  two  there  appear  to  be  grada- 
tions. About  the  great  batholiths  the  sedimentary  rocks  are  thor- 
oughly crystalline,  being  transformed  to  mica  schists  and  mica 
gneisses,  with  secondary  structures  parallel  to  the  intrusive  masses, 
the  original  structures  being  destroyed.  In  places  the  mica  schists 
and  mica  gneisses  are  thoroughly  pegmatized.  The  zonal  belt  of 
schist  passes  by  gradation  into  the  ordinary  cleaved  slate  of  the 
district. 
Resting  unconformably  upon  the  deeply  denuded  edges  of  the 
Algonkian*  schists  and  granite  is  the  nearly  horizontal  middle  Cam- 
brian sandstone. 
In  degree  of  folding,  character,  and  mineral  composition  the  sedi- 
mentary rocks  resemble  a  part  of  the  upper  Huronian  of  the  Lake 
Superior  region.  The  recent  extension  by  drilling  of  the  iron-bear- 
ing upper  Huronian  rocks  of  the  Cuyuna  district  of  Minnesota  west- 
ward into  North  Dakota  suggests  a  possible  connection  with  the 
schists  of  the  Black  Hills,  which  are  lithologically  and  structually 
similar  to  the  rocks  of  the  Cuyuna  district. 
The  Black  Hills  afford  one  of  the  best  instances  known  of  the 
metamorphic  effect  of  great  intrusive  masses  of  granite. 
SECTION  4.     MISSOURI. 
SUMMARY   OF  LITERATURE. 
King  (H.),52  in  1851,  states  that  the  Primitive  formation  is  met 
with  near  a  point  about  70  miles  south  of  St.  Louis  and  30  miles  west 
of  the  Mississippi.  It  consists  chiefly  of  granite,  syenite,  and  por- 
phyry, and  rises  in  conelike  elevations  or  detached  ridges  to  the  height 
