In 1925, following his retirement as a consulting mining engineer, Uncle William joined Selection Trust. Selection Trust was a company set up by the American financier Alfred Chester Beatty in 1913 to inaugurate mines round the world. Chester Beatty did not travel himself, but he chose suitable people to establish the mines in each area, and Uncle William was chosen to set up copper mines in Northern Rhodesia.
He began at what has become his best known project, at the Roan Antelope site at Luanshya, where an outcrop was already known, but by a program of deep drilling he demonstrated that there were substantial deep deposits. Work then moved 60 miles to the south to Mufilira which is still a very prolific mine. Selection Trust continued to expand, mining diamonds on the Gold Coast and in Sierra Leone, and even forming a mining company to work lead zinc deposits in Yugoslavia. He was a director of all these companies and in addition he was vice chairman of the parent company Selection Trust Ltd. He was clearly that very valuable asset, someone who not could not only understand the geology but also understood management. These all proved extremely profitable and he remained with Selection Trust until he retired at the end of 1945, at the age of 76.
Two subdirectories provide further information. about this. The first is an obituary of Uncle William in Horizon which was a magazine of the Selection Trust which contains some interesting information. I then add a note from Wikipedia on Selection Trust.