The scientific career of Patrick Blackett is intimately connected with the cloud chamber invented by C.T.R. Wilson, who received a Nobel Prize in Physics 1927. The cloud chamber shows the tracks of rays of ionising radiation, e.g., cosmic rays. By developing the cloud chamber technique and thereby making fundamental discoveries in particle physics, Blackett received his Nobel Prize in 1948. It is a curious fact that Wilson invented his apparatus to study the formation of raindrops, a subject belonging to geophysics, and that Blackett eventually left the field of particle physics and also turned his interests to geophysics. This can be heard from the present lecture, which was delivered at Blacketts’ only participation in the Lindau meetings. The main part of the lecture tells the story of rock magnetism and how it can help detect both reversals of the Earth magnetic field and continental drift. But it is also of some interest to understand how Blackett came to this new field of study. It appears that his interest actually started with a rather bold hypothesis, with relevance for the theories of gravitation and electromagnetism. After formulating his theory of gravitation around 1917, Albert Einstein spent the major part of his scientific life unsuccessfully trying to find a unification between gravity and electromagnetism. It is even said that on his deathbed, in 1955, he asked for paper and pencil to make a final attempt. Blackett, in 1947, published a paper in which he pointed out that some observations of the magnetic fields of astronomical bodies could be explained if a rotating body gave rise to a magnetic field (Einstein had also thought about this idea). As a good experimental physicist, Blackett then set about developing methods to measure small magnetic fields in the laboratory. But, again as a good experimental physicist, when he could not measure any magnetic fields around rotating objects, he gave up the theory. Instead he started looking for another field of study where his magnetic measuring technique could be useful and he found it in rock magnetism. There he could make a real contribution to the verification of the theories of reversals of the Earth magnetic field and the continental drift by measuring the tiny magnetic traces left in rocks. Both these theories are today believed to hold true, but Blackett’s own theory of gravitational dynamics belongs today more to the realm of science fiction than to science.