A decree in 1892 divided the "terres vacantes" into a domainal system, which privatized extraction rights over rubber for the state in certain private domains, allowing Leopold to grant vast concessions to private companies. In other areas, private companies could continue to trade but were highly restricted and taxed. The domainal system enforced an in-kind tax on the Free State's Congolese subjects. As essential intermediaries, local rulers forced their men, women and children to collect rubber, ivory and foodstuffs. Depending on the power of local rulers, the Free State paid below rising market prices. In October 1892, Leopold granted concessions to a number of companies. Each company was given a large amount of land in the Congo Free State on which to collect rubber and ivory for sale in Europe. These companies were allowed to detain Africans who did not work hard enough, to police their vast areas as they saw fit and to take all the products of the forest for themselves. In return for their concessions, these companies paid an annual dividend to the Free State. At the height of the rubber boom, from 1901 until 1906, these dividends also filled the royal coffers. The Free Trade Zone in the Congo was open to entrepreneurs of any European nation, who were allowed to buy 10- and 15-year monopoly leases on anything of value: ivory from a district or the rubber concession, for example. The other zone—almost two-thirds of the Congo—became the "Domaine Privé", the exclusive private property of the state. In 1893, Leopold excised the most readily accessible portion of the Free Trade Zone and declared it to be the "Domaine de la Couronne", literally, "fief of the crown". Rubber revenue went directly to Leopold who paid the Free State for the high costs of exploitation. The same rules applied as in the "Domaine Privé". In 1896 global demand for rubber soared. From that year onwards, the Congolese rubber sector started to generate vast sums of money at an immense cost for the local population. Early in Leopold's rule, the second problem—the British South Africa Company's expansionism into the southern Congo Basin—was addressed. The distant Yeke Kingdom, in Katanga on the upper Lualaba River, had signed no treaties, was known to be rich in copper and thought to have much gold from its slave-trading activities. Its powerful "mwami" (big chief), Msiri, had already rejected a treaty brought by Alfred Sharpe on behalf of Cecil Rhodes.