G. Schmitz-Dumont published a paper, "Licht und Electricit?t sind wesentlich verschiedene Formen einer Energie" (Light and electricity are essentially different forms of energy) in the Electrotechnische Zeitschrift (1891). By March 1896 he had arrived in the South African Republic (Transvaal) and was employed in its civil service as mining inspector at Johannesburg. During that year he also served as acting government mining engineer and was responsible for writing the latter's annual report, which was published with the annual report of the head of the Department of Mines for 1896. Included as an appendix to his report was his "Toelichting by de geologiesche kaart van Pretoria en van de geologische doorsnede, Z-N, van Vereeniging naar Pretoria" (Explanation of the geological map of Pretoria and of the geological section, South to North, from Vereeniging to Pretoria). Attached was a coloured geological map of Pretoria and environment on a scale of 1:148 752 and a geological section from the Vaal River to Pretoria, both based on geological surveys by Scmitz-Dumont. According to Hall (1922) he also wrote "Schets der formatie ten noorden der Magaliesbergen en in de Waterbergen" (Sketch of the formation north of the Magaliesberg and in the Waterberg, 4p), which was included in the annual report of the head of the Department of Mines for 1897.
During 1896 Schmitz-Dumont expressed his views on the origin of the Witwatersrand gold-bearing conglomerates, though it is not clear whether these views were published. However, criticisms of his "river-bed theory" by J. Kuntz*, and Schmitz-Dumont's reply to these criticisms, were printed in the Transactions of the Geological Society of South Africa (1896, Vol. 2, pp. 128-130; 141-142). He disagreed with the generally prevailing marine beach origin of the conglomerates and proposed that the main reef represented the gravels deposited by an east-flowing river passing from a narrow valley onto a broad plain. To account for the width of the conglomerates he assumed that more than one river bed was formed on the plain. The overlying quartzites he thought were deposited on a delta or in an off-shore environment, after the continent had subsided.
Schmitz-Dumont ended his position as mining inspector to become manager of the New Goch Gold Mines. However, by December 1897 he had been appointed state mining engineer and applied for naturalization as a citizen of the South African Republic. In September 1896 he was appointed as a member of the curatorium (or council) of the Staatsmuseum (later the Transvaal Museum), Pretoria, in the place of Josef Klimke*. By 1899 he was a member of the Chemical and Metallurgical Society of South Africa. He died in the battle of Spioenkop, near Ladysmith, during the Anglo-Boer War (1899-1902).