508  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225. 
slate  of  the  interior.  As  exposed  in  Hardin  County,  this  formation  is 
at  leas!  !<»<>  feet  thick.  There  are  no  opportunities  to  determine  either 
the  total  thickness  of  the  formation  or  the  nature  of  the  beds  below  it. 
Igneous  rocks. — In  both  Pope  and  Hardin  counties  there  are  several 
dikes  of  dark-colored,  noncrystalline,  igneous  material.  For  the  most 
part  they  are  mica-peridotite,  similar  to  that  described  by  J.  S.  Diller,a 
from  the  adjacent  portion  of  Kentucky.  In  the  high  hill  known  as 
Downeys  Bluff,  at  Rosiclare,  there  is  a  dike  and  a  small  intrusive 
sheet  of  rock  consisting  mainly  of  biotite,  augite,  and  magnetite,  and 
probably  somewhat  different  from  the  mica-peridotite.  Pieces  of 
ordinary  diabase,  said  to  have  been  encountered  in  sinking  a  well  in 
Rosiclare,  possibly  represent  another  dike.  Certain  other  much- 
altered  igneous  rocks  occur  in  the  district,  but  these  have  not  yet 
been  studied  in  sufficient  detail  to  determine  their  nature.  In  all, 
seven  different  dikes  have  so  far  been  located — -two  in  Pope  and  five  in 
Hardin  counties.  Their  intrusion  was  not  marked  by  any  notable 
metamorphism  of  the  surrounding  rocks. 
Geological  structure. — In  a  broad  way  the  district  represents  a  trun- 
cated dome,  circled  to  the  north  and  east  by  an  escarpment  formed  by 
the  basal  formation  of  the  coal  measures  of  the  Pennsylvanian  series, 
and  cut  off  on  the  west  by  the  Mesozoic  and  Tertiary  deposits  of  the 
Mississippi  embayment.  The  central  portion  of  the  dome  consists  in 
the  main  of  the  rocks  of  the  Mississippian  series.  Involved  with  them 
are  certain  fault  blocks  of  Pennsylvanian,  and  the  small  area,  previously 
noted,  of  Devonian  shales.  The  whole  dome,  up  to  the  bounding 
escarpment,  has  been  planed  down  to  a  moderately  smooth  plain  now 
forming  the  upland,  and  passing  at  its  western  edge  under  a  deposit  of 
gravels,  probably  of  Tertiary  age. 
The  area  is  one  within  which  there  are  a  very  large  number  of  faults. 
These  are,  so  far  as  has  been  observed,  normal,  and  follow  a  great 
variety  of  directions.  The  fault  planes  divide  the  area  into  a  series  of 
very  irregular  but  always  sharp-sided  polygons.  The  amount  of 
faulting  varies  from  a  few  inches  to  600  feet  and  perhaps  more.  Indi- 
vidual fault  planes  may  be  traced  a  mile  or  more,  and  lines  or  zones 
along  which  faulting  has  occurred  may  be  traced  several  times  as  far. 
There  is  little,  if  any,  folding  present.  The  small  dome  in  which  the 
Devonian  shale  outcrops  is  itself  bounded  by  normal  fault  planes,  and 
the  bending  seems  to  be  incidental  to  the  more  important  phenomena 
of  fracturing.  The  region  is  seemingly  one  in  which  vertical  uplift  has 
been  more  important  than  horizontal  compression.  The  dikes  occupy 
crevices  along  which,  so  far  as  observation  goes,  there  has  been  no 
faulting.  There  are  in  Illinois  no  conclusive  data  as  to  the  relations  of 
the  intrusion  to  the  faulting,  though  the  sill  found  in  Downey's  Bluff 
is  apparently  cut  off  to  the  north  by  a  fault.     If  this  interpretation  be 
«  Am.  Jour.  Sci.,  3d  series,  vol.  44, 1892,  p.  288. 
