A paleomagnetic investigation of oceanic crust recovered from ODP Hole 801C was carried out to determine the origin of the Jurassic Quiet Zone and to study the geologic history of ODP Site 801. A total of 370 samples from 400 m of Middle to Upper Jurassic basement were studied from this hole. The basalts of Hole 801C record a total of six polarity intervals downcore which are evidence for a period of rapid reversals of the geomagnetic field during the Middle and Upper Jurassic. The vector cancellation resulting from these vertically superimposed polarity intervals is responsible for the magnetic quiescence of the Jurassic Quiet Zone. The magnetic signature of the visibly altered and unaltered basalts from this hole suggest three episodes of secondary alteration. The first of these episodes occurred at or near the time of extrusion of the tholeiitic basalts, whereas the other two probably occurred respectively at the Oxfordian/Callovian boundary and during the mid-Cretaceous.