Geophysical investigation of clay and limonite deposits related to the lower Paleozoic of central Pennsylvania shows the existence of broad belts of self-potential minima above the outcrops of the Cambrian Gatesburg formation. These S. P. minima are related to the peculiar sandy residual soil covering the Gatesburg. A working hypothesis in terms of shale and liquid junction potentials is advanced to account for the phenomena. This appears to be confirmed by the "mirror image" relationship of S. P. profiles with resistivity profiles. The method may be useful in prospecting for clay deposits and in locating certain geological boundaries.