Emil W. Cohen, Danish-Jewish geologist and explorer, acquired Prussian citizenship while studying chemistry and physics at the universities of Heidelberg and Berlin, Germany, from 1863. He was an assistant at the Mineralogical Institute at the University of Berlin from 1867 to 1869 and carried out geological surveys along the Rhine. During 1870-1871 he served in the Franco-Prussian War as a civilian attached to the medical corps. After the war a firm of gem-dealers in Hamburg, Lippert & Co., asked him to visit South Africa to report on the diamond fields of Griqualand West. He arrived at the Cape in mid-1872 and stayed for 15 months. After studying the diamond fields he visited the Transvaal and travelled on to Mozambique, making geological observations and collecting rocks, minerals, meteorites, and some fossils.
His visit formed the basis of numerous publications. While still in South Africa he wrote six papers describing the diamond fields, as well as the gold-fields at Lydenburg and Marabastad. These papers all appeared in the Neues Jahrbuch für Mineralogie in 1872-1873. He was the first to describe the geology of Dutoitspan, in 1872. He observed the hollows filled with decomposed tuff-like rock containing diamonds and fragments of other rocks and minerals, and saw them surrounded by shales and greenstone which ended abruptly against the tuff in vertical walls. The contents of the pipes he took to be the result of tuff eruptions which carried up fragments of the crystalline, diamondiferous rocks below, through the shales and greenstone. Thus he was the first to offer a volcanic explanation for the matix rock of the diamond pipes. Later that year, having travelled and exchanged ideas with George W. Stow*, he wrote a general account of the geology of Griqualand West. Moving on to the Transvaal he visited the salt pan (Tswaing) north of Pretoria, but only years later described its geology in "Ueber eine nördlich von Pretoria in Granit gelegene Salzpfanne" (Mineralogische und Petrographische Mittheilungen, 1896). As he had found no volcanic rocks in it, he thought it must have resulted from gas explosions. In June and July 1873 he travelled on foot from Lydenburg to the gold-fields on the Waterval River and back, and then on to Delagoa Bay. His description of these travels, "Erläuterende bemerkungen zu der routenkarte einer reise von Lydenburg nach den goldfeldern und von Lydenburg nach der Delagoa Bai im östlichen Süd-Afrika", was published in the Jahresbericht der geographischen Gesellschaft in Hamburg in 1875, and also as a monograph of 116 pages. It included an overview of the geology of the eastern Transvaal and Lebombo Mountains.
Cohen's observations relating to the disputed stratigraphy of the south-western Cape did not add much to existing knowledge, perhaps because the topic was of secondary importance to him. Thus in his paper, "Geognostisch-petrographische Skizzen aus Süd-Afrika, Pt. II" (Neues Jahrbuch für Mineralogie, 1887), he made a most curious stratigraphical suggestion which arose from his confusion of the Table Mountain quartzites with the Witteberg quartzites in the Ceres area.
Though some of his samples were given to various experts to report on, Cohen was himself one of the founders of modern petrography. His work initiated petrographical studies of South African rocks, and he was the first to use a polarising microscope to study South African samples. He also had many of his rock samples chemically analysed. He and his students ascertained the mineralogical composition of many South African rocks, so elucidating their origins. Most of the results were published in the Neues Jahrbuch der Mineralogie... over a period of more than 30 years. For example, in 1874 he described the petrography of the granite and slate at Seapoint and Platteklip (Table Mountain), and of the Bokkeveld beds. He later also described the petrography of the Dwyka tillite and the diabase intrusions in the Karoo. Though he did not visit Lesotho, samples obtained from James Orpen enabled him to describe the pipe-amygdales of the Stormberg lavas in 1875.
In 1878 Cohen became assistant professor of petrography at the University of Strasbourg, France, and a member of the geological commission of Alsace-Lorraine. His Zusammenstellung petrographischer Untersuchungsmethoden... was published by the university's Petrographische Institut in 1881, followed by several later expanded editions. After undertaking a journey of geographical exploration in South America he was appointed professor of mineralogy and geology at Greifswald, in north-western Germany, in 1885. He retained this post to his death in 1905. During the last ten years of his career he focussed on the study of iron meteorites, producing some 15 publications on their composition, microscopy, and mineralogy. His book, Meteoritenkunde (Stuttgart, 3 vols, 1894-1905) was a pioneering study of the structure of meteorites and was partly based on samples he collected in South Africa. He furthermore described his investigations of South African meteorites from Griqualand East, Great Namaqualand, and the Transkei in the Annals of the South African Museum (1900, Vol. 2(2), pp. 9-19, 21-29; 1906, Vol. 5(1), pp. 1-16). Another major work by him, Sammlung von mikrophotogaphien zur Veranschaulichung der mikroskopischen Structur von Mineralen und Gesteinen (1881-1885, 3rd ed. 1900) contributed much to knowledge of the microscopic structure and mineralogical composition of rocks. The mineral cohenite (an iron carbide which he found in terrestrial nickel-iron from northern Greenland in 1897) was named in his honour.
Cohen became a corresponding member of the South African Philosophical Society in August 1878, and was still its only corresponding member in 1900.