56 W. D. KELLER 
geologic cross section along the traverse is drawn at the bottom of the 
graph in its proper position with respect to the ten stations. 
For convenience in analyzing Graph No. 14 let us proceed along 
the traverse from north to south, left to right along the page. 
The first stations, 58 and 60, are on the highly resistive St. Peter 
sandstone. There is a fairly uniform increase in resistivity with depth; 
the figures on the whole are high; the curves are roughly parallel; in 
other words, they are typical sandstone curves. The increase in re- 
sistivity of 60 over 58 is believed to be due to increased silicification 
near the fault plane, evidenced by the fact that the sandstone in its 
outcrops nearest the fault shows a little crushing and some cracks and 
veinlets filled with quartz and calcite. 
A sharp drop in all profiles occurs at 91. The writer does not believe 
this to be due to low resistive Plattin limestone or Joachim dolomite, 
which would be taken in below at greater depth, but rather to possible 
clay gouge and similar material perhaps well saturated by movement 
of water along the plane. 
Upon advancing to g2 the current is dominantly in Plattin lime- 
stone and the resistivity increases. It rises to a maximum over the 
center of the limestone series at 94. 
The low profile over the Maquoketa and Grassy Creek shales 
needs no comment after one has seen their normal curves. Station 69 
shows higher resistivity because the south current electrode has been 
extending over the Chouteau and Burlington limestone. 
Further advance to 68 over limestone is indicated by a rise in the 
profile. This set-up is one of the most interesting made. Since the in- 
strument was set up at approximately the center of the limestone 
formation, one would expect, without further analysis of the condi- 
tions, a normal increase in resistivity with depth. However, exactly 
the opposite is what happens. Each increasing depth shows a cor- 
responding decrease in resistivity. But after consideration of the 
geology of the traverse the explanation is easily made. At a 20-foot 
depth the outer electrodes are separated 60 feet, still on limestone; but 
as the potential electrodes are separated wider the current electrode 
spacing is tripled, and they pass off the limestone at each side, going 
onto the Grassy Creek shale at the north and the intercalated shale 
and limestone of the Keokuk-Warsaw group at the south, whereby 
the resistivity is lowered. With increased spacing more shale is taken 
into the circuit, outweighing the limestone, and consequently the 
curve declined all of the way to the end. 
The conclusion that a covered fault can be located by an earth 
resistivity method is self-evident. There is nothing original or new 
578 
