|“||whenever I have required his aid … I have found him possessed of the highest intelligence and the most unbounded zeal||”|
He discovered radiating fans shaped impressions in the town of Bray in 1840. He showed this to the English palaeontologist Edward Forbes, who named it Oldhamia after him. Forbes declared them to be bryozoans, however later workers ascribed it to other plants and animals. For a while these were considered the oldest fossils in the world.
He became Curator to the Geological Society of Dublin, and in 1845 succeeded John Phillips, nephew of William Smith, in the Chair of Geology at Trinity College, Dublin. He was elected a Fellow of the Royal Society in June 1848.
He married Louisa Matilda Dixon of Liverpool in 1850. He resigned in November that year and took a position as the first Superintendent of the Geological Survey of India. He was to be the first of the Irish geologists to migrate to the Subcontinent. He was followed by his brother Charles, William King, son of the Professor of Geology at Queen's College, Galway; Valentine Ball and more than 12 other Irish geologists.
In India he oversaw a mapping program that focussed on coal bearing strata. The team of geologists made major discoveries. Henry Benedict Medlicott coined the term "Gondwana Series" in 1872. Oldham's elder son Richard Dixon Oldham distinguished three types of pressure produced by earthquakes: now known as P (compressional), S (shear), and L (Love)-waves, based on his observations made after the Great Assam Earthquake of 1897. Richard showed in 1906 the arrival patterns of waves and suggested that the core of the earth was liquid. His younger son Henry became a reader in geography at Kings College, Cambridge.
Oldham resigned from his position in India in 1876 on the grounds of poor health and retired to Rugby in England. In recognition of his lifetime's "long & important services in the science of geology", including Palaeontographica Indica, he was awarded the Royal Society's Royal Medal. He died on 17 July 1878.
- Lee, Sidney, ed. (1895). "Oldham, Thomas". Dictionary of National Biography 42. London: Smith, Elder & Co. pp. 111–112.
- Chisholm, Hugh, ed. (1911). "Oldham, Thomas". Encyclopædia Britannica (11th ed.). Cambridge University Press
- "Lists of Royal Society Fellows 1660-2007". London: The Royal Society. Retrieved 14 July 2010.[dead link]
- "Royal archive winners Prior to 1900 - Royal Medals". London: The Royal Society. Retrieved 14 July 2010.
- Patrick N. Wyse Jackson, 2005 Thomas Oldham. Earth Sciences 2000 Issue 12 
- Darwin correspondence database
- "Author Query for 'Oldham'". International Plant Names Index.