Continental drift was not taken seriously by scientists; Wegener’s enthusiasm led him to cite many lines of evidence that did not stand up to scrutiny, damaging the impact of his ideas. Above all, no physical mechanism could be envisaged. Harold JeffreysHarold Jeffreys (1891–1989) was a mathematician who applied classical mechanics to astronomy and planetary science, in doing so shaping the development of geophysics in the UK. He successfully modelled the shape and strength of the Earth, used the developing field of seismology to delve into the Earth’s interior, found layers within the mantle and determined that the outer part of the Earth’s core was fluid. His work was crucial in locating earthquake origins, which later played a key role in mapping plate motions. He did not accept the idea of continental drift, and remained sceptical about plate tectonics. notably dismissed the whole idea; Arthur HolmesArthur Holmes (1890–1965) Ernest Rutherford pioneered the use of radioactivity to determine the age of rocks; Arthur Holmes refined the method and developed it into a geological timescale, pushing back the age of the Earth to more than four thousand million years by the 1940s. He was especially interested in mantle convection and expanded on his ideas in his textbook Principles of Physical Geology. This wide-ranging textbook, first published in 1944 with a second edition in 1965, influenced generations of Earth scientists, including McKenzie. He read it as an undergraduate and enthusiastically told his mother, a landscape architect, about it. She was reading the book, but decided it was too heavy to take to bed, so she cut it up into manageable sections. “I was outraged,” says McKenzie. “I made her buy me a replacement copy.”, in contrast, was a keen proponent of both continental drift and mantle convectionMethod by which the heat from the inner core is transported to the Earth’s surface. . His book, Principles of Physical Geology, influenced Dan in his first year geology course. “It was a wonderful book. I’m sorry I never met him”.