In 1882, Jules Ferry, Minister of Public Instruction, decided to create a mission to explore the Regency of Tunisia. The expedition was headed by the botanist Ernest Cosson, who had already undertaken eight study trips in Algeria between 1852 and 1880. It included the botanist Napoléon Doumet-Adanson and other naturalists. The botanists undertook their work in 1883 and 1884. In 1884, a geological section under Georges Rolland was added to the Tunisian Scientific Exploration Mission. Rolland was assisted by Philippe Thomas from 1885 and by Georges Le Mesle in 1887. Thomas was 42 when he started his exploration of Tunisia. He was assigned to the expedition at the recommendation of the paleontologist Jean Albert Gaudry, and because Jules Ferry knew his ability and perfect knowledge of the Arabic language. Rolland covered the centre of the country, while Thomas worked further south and Le Mesle worked mainly in the north, apart from an expedition to the extreme south. The team gave good descriptions of the Jurassic of the Zaghouan region and the Eocene of the Maktar and Kairouan regions. In 1885–86, Thomas explored the southern area of Tunisia between the meridian of Kairouan and the Saharan chotts, covering a vast area. He also explored the western area, which his colleagues had neglected, particularly the Chaîne du Thaljah or Chaîne du Tseldja mountains that stretch westward from Gafsa into Algeria. He identified the geology of the Gafsa chain with that of the M'fatah massif in Algeria and on 18 April 1885 found phosphates at Jebel Tselja. He found a brown or greenish gray limestone that on average contains 60% tricalcium phosphate near Métlaoui, where the Cretaceous limestone axis of the mountains joins the Eocene foundations. Thomas crossed the Chaîne du Tseldja through the dramatic gorges of the Oued and found identical phosphate deposits on the southern slope of the range extending for at least . Thomas sent rock samples to the École des Mines de Paris for analysis, and on receiving the results on 18 October 1885, informed Dr. Cosson, head of the mission. The discovery had great economic and agricultural importance. With Cosson's authorization, he informed the French Academy of Sciences on 7 December 1885. He completed his observations in the 1886 exploration campaign in southern and central Tunisia up to Kalaat es Senam.