 It was during his time at the observatory that he made the crucial observation that led to his discovery of geomagnetic reversal.In 1905, he found that rocks in an ancient lava flow at Pontfarin in the commune of Cézens (part of the Cantal "département") were magnetised in a direction almost opposite to that of the present-day magnetic field.From this, he deduced that the magnetic North Pole of the time was close to the current geographical South Pole, which could only have happened if the magnetic field of the Earth had been reversed at some point in the past.