South African geologist and palaeobotanist. Alexander du Toit was one of South Africa's most influential and versatile scientists. Highly esteemed by his contemporaries – the geologist R.A. Daly called him "the world's greatest field geologist" – he added important knowledge to a variety of subjects (palaeobotany, petrology, hydrogeology, geomorphology, and economic geology), but is most famous as a champion of the theory of continental drift.

Du Toit was a descendent of Huguenot settlers who arrived in the Cape in 1687, and was born on the family estate near Cape Town. After graduating from South Africa College, he qualified in mining engineering at the Royal Technical College in Glasgow and studied geology at the Royal College of Science in London. He was appointed as a lecturer at both the Royal Technical College and the University of Glasgow but returned to South Africa in 1903 to work as an assistant geologist for the Geological Commission of the Cape of Good Hope.

For the next 17 years he mapped the geology of more than 100,000 square miles and drew many of the boundaries shown in the first maps published by the Geological Survey of South Africa. Travelling alone, on foot or by bicycle, with a plane table strapped to his back, he worked almost continuously in the field, mapping the Stormberg series in the east (between Herschel in the north and Cala in the south), the more arid northern country (Bechuanaland, Griqualand West and the districts of Prieska, Kenhardt, Britstown, Carnarvon and Victoria West), and Basutoland and the south and southeastern coasts. He also made the first systematic collections of fossil plants in South Africa, mainly in the Molteno Beds. His carefully detailed reports and maps have been the starting point for all later geological investigations of the region, and they supplied him with the geological evidence to support his theoretical work in the 1920s and 1930s.

His first books and papers were published during this period: a revised and expanded edition of the A.W. Rogers Introduction to the Geology of Cape Colony, a textbook entitled Physical Geography for South African Schools, and several papers that established him as an authority on groundwater in South Africa. This led to his being seconded, at the outbreak of the First World War, as a hydrogeologist to the Defence Force, in charge of providing water to the troops fighting in the South West African campaign. After the war, he was made chief geologist in the Irrigation Department, a senior position that freed him from routine fieldwork and gave him more time to write. He produced a series of important papers and books on the geology of South Africa in quick succession, including a paper from 1921 that suggested for the first time that the southern continents were the fractured remains of a supercontinent.

In 1923 the Carnegie Institution gave him a grant to compare the geology of South America and South Africa. Published in 1927, (the same year du Toit resigned from government service to work as a consulting geologist for De Beers Consolidated Mines) the report was based on the results of his fieldwork in Argentina, Uruguay, Paraguay and Bolivia and showed that in South America the same species of plant and animal fossils occur in the same complex sequence as they do in South Africa, India and Australia.

Interested in any scrap of information that might support the hypothesis of continental drift, he travelled widely and corresponded with professionals from all parts of the world – geologists, miners, geophysicists, botanists, zoologists. Finally, in 1937, he published Our Wandering Continents, which he dedicated to the German meteorologist and originator of the theory of continental drift, Alfred Wegener. In it he hypothesised that about 205 million years ago Pangea, the single supercontinent postulated by Wegener, had divided into two land masses: a northern supercontinent, Laurasia, that settled near the Equator and a southern supercontinent, Gondwanaland, that drifted toward the south pole, where it later fractured into South America, Africa, India, Australia, and Antarctica. Although he provided a wealth of geological evidence for past land connections, he was unable to offer a mechanism for the movement of continents, and so it was not until the discovery of palaeomagnetism and developments in oceanography in the 1960s, when the hypothesis of continental drift was absorbed into the larger theory of plate tectonics, that the quality of his vision was fully acknowledged.

Du Toit served as a president of the South African Association for the Advancement of Science and of the South African Archaeological Society; he was twice president of the Geological Society of South Africa, a member of the Geological Society of America, and the first South African-born geologist to be made a fellow of the Royal Society of London. A year after his death the Geological Society of South Africa instituted the biennial Du Toit Memorial Lectures to commemorate aspects of his work. A crater on Mars has been named in his honour.