EARTH RESISTIVITIES 55 
third, Jefferson City dolomite was brought against Kimmswick lime- 
stone. 
The geologic section of the region and a brief description of the 
fault are given to aid in understanding the discussion which follows. 
Alluvium Feet 
Pleistocene glacial debris 
Pennsylvanian 
Cherokee formation o-50 
Mississippian 
St. Louis limestone 75 
Keokuk-Warsaw-Salem group 140 
Burlington limestone 150 
Chouteau limestone 55 
Grassy Creek shale 140 
Devonian 
Callaway limestone 12-15 
Ordovician 
Maquoketa shale 150 
Kimmswick limestone 140 
Plattin limestone 125 
Joachim dolomite 65 
St. Peter sandstone 125 
Jefferson City dolomite 125 
The Cap-au-Gres fault, a normal fault having a stratigraphic 
throw of about 450 feet in the locality worked, cuts across Mississippi 
River at old Cap-au-Gres, Illinois, and extends into Illinois and 
Missouri, trending about N. 85° W. The rocks dip away very gently 
north of the fault plane but are dragged up in excess of 45° on the 
south, the downthrown side, flattening out within half a mile. This 
structure brings up the greater part of the geologic section within 
this small area. 
SANDSTONE FAULTED AGAINST DOLOMITE 
Graph No. 14 carries the resistivity profiles of the measurements 
at 5 different depths made at the set-ups along a line across the fault 
where the St. Peter sandstone was faulted against the Joachim dolo- 
mite. The traverse was also continued across the greater part of the 
upturned geologic section. 
The stations at which the determinations were made, were located 
at fairly regular intervals along a small valley in which the soil thick- 
ness was probably as uniform as could be found. The line of electrodes 
was always north-south, across the strike. Most of the stations were 
100 feet apart, under which conditions the 1oo-foot maximum spacing 
of the potential electrodes formed a continuous length of potential 
measurement. The current circuits were overlapping, of course, in 
those cases. At any rate, the position of the stations is plotted to 
scale along the abscissa. Resistivity is shown on the ordinate. The 
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