Oldham, Thomas (1816–78), geologist, was born 4 May 1816 in Dublin, eldest son of Thomas Oldham of the Grand Canal Co. of Ireland and Margaret Oldham (née Boyd). He was educated privately and then entered TCD, where he graduated BA (1836) and MA (1846). Soon after 1836 he entered the University of Edinburgh, where he attended the geological lectures of Robert Jameson, and also undertook some engineering projects in the city. Two years later he was employed by the ordnance survey under Joseph Ellison Portlock (qv) on the survey of Londonderry. His early publications in Ireland included papers on the temperature gradients in mines, on some carboniferous trilobites, and on the rocks at Bray Head, south of Dublin. There in 1840 he discovered unusual radiating and fan-shaped markings in the Cambrian slates. He officially reported these in 1844 and they were named Oldhamia in his honour by the palaeontologist Edward Forbes (1815–54) in 1848. For many years these trace fossils were thought to be the earliest examples of life on earth.
In 1843 Oldham was appointed curator and assistant secretary of the Geological Society of Dublin. He spent much of his time enhancing the holdings of the society's museum, which was housed in the custom house in Dublin, through his own collecting. In 1844 he returned to TCD as assistant to the professor of civil engineering, and in 1845 was appointed professor of geology, and also in that year local director of the geological survey of Ireland. He was elected a fellow of the Geological Society of London (1843) and FRS (1848), and served as president of the Geological Society of Dublin 1848–50. In 1846 he embarked with his officers on an ambitious programme of the geological mapping of Ireland, first producing a map of Co. Wicklow. Eventually in 1890, long after Oldham had departed Ireland's shores and indeed this life, the programme was completed.
In 1850 he was appointed the first superintendent (later designated director) of the geological survey of India. In India he employed a staff of about twelve geologists, including his younger brother Charles Aemilius Oldham (1831–69), who between them in the first decade mapped the geology of much of Bengal and central India – an area twice that of Britain. His Indian publications concentrated on determining the coal resources of the subcontinent; at least sixteen papers on the subject were published by him in the survey's Memoirs, and he also produced monographs on the important fossil plant faunas of the Rajmahal Hills. On the grounds of poor health Oldham resigned his Indian position in 1876 and retired to Rugby, England. He received a gold medal for his work from the emperor of Austria in 1873, and the Royal Medal from the Royal Society in 1875. He died in Rugby 17 July 1878. He married (17 October 1850), at St Peter's, Dublin, Louisa Matilda Dixon of Liverpool; they had five sons and one daughter.
Their third son, Richard Dixon Oldham (1858–1936), geologist and seismologist, was born 31 July 1858 in Rugby, and was educated there and at the Royal School of Mines in London. He joined the geological survey of India in 1879, following in the footsteps of his father and his uncle Charles Aemilius. In India Richard Oldham carried out extensive fieldwork in the Himalayas, in the desert of Rajputana, where he prospected for coal, in the Andaman Islands, and in Burma. He wrote over seventy papers on Indian geology, including the second edition of the Manual of the geology of India (1893), and served as director of the survey from 1896 to 1897. On 12 July 1897 a catastrophic earthquake shook Assam and many lives were lost. Following this, Oldham headed an investigation that led him to important seismological conclusions, which remain his greatest contribution to geology. While the existence of seismic waves was already known, Oldham in 1900 distinguished three types produced by earthquakes: now known as P (compressional), S (shear), and L (Love) waves. The former two pass through the Earth while the latter travel around it. He retired in 1903 suffering from the tropical disease sprue, and returned to live in Britain and France, between which he divided his time. In retirement he studied the changes that had affected the Rhône delta since Roman times. In 1906 he showed, from arrival patterns of waves from various earthquakes, that the earth had a core (its existence had been suspected), which he thought was liquid. It was later shown to be composed of iron and nickel, having a solid centre surrounded by a liquid portion. He received the Lyell medal of the Geological Society of London in 1911, and served as president of the society 1920–22. He was FRS (1911) and honorary fellow of the Imperial College of Science, London (1931). Oldham never married, and he died 15 July 1936 at the Gwalia Hotel, Llandrindod Wells, Wales.