Notwithstanding these lessons, some of the Turkoman tribes still went on marauding expeditions, robbing, killing, and enslaving their neighbors. So, in 1878, another strong force of the Cossacks was sent against the pillaging tribes, who were made to release all slaves and abolish slavery. Little by little all Turkistan became Russian territory. Bokhara and Khiva alone keep their old forms of government, but they are practically Russian states and pay Russia annually a stipulated tribute. It is thought that once upon a time Siberia had a much larger population than it has now and the peoples who lived there dwelt farther north. The first colonists lived in the stone age and were contemporaneous with the mammoth, whose remains are found scattered all over the northern part of Siberia and the adjacent islands. In the interior these remains are found imbedded in thick strata of pure blue ice, which is covered by the river gravels of streams that do not now exist. So thick are these layers of ice that they may be likened to the rocks found in lower latitudes. Several of these animals have been found imbedded in the ice in an almost perfect state of preservation, and quantities of their tusks are obtained annually along the northern rivers where the spring freshets have worn away the banks of the streams. Whenever the ivory-tusk hunter sees the end of a tusk sticking out of the river bank, he is soon able to remove it from its resting place with pick and shovel. Great quantities of this fossil ivory are also obtained from the islands to the north of the mainland. As in arctic America, the ground of northern Siberia is frozen solid to the depth of many feet, and even during the hottest summer it thaws down only a few inches. The climate is continental in character, being marked by fierce winds and great extremes both in temperature and moisture. In midsummer the temperature may reach one hundred and ten degrees, while in midwinter it has been known to reach ninety degrees below zero. Roughly speaking, Siberia may be divided into three longitudinal belts: first, the tundra, which borders the Arctic Ocean and extends several hundred miles south of it; second, the forest belt, several hundred miles wide, which extends across the continent; third, the southern part, consisting of desert steppes, swamps, grassy plains, and a few broken forests. The tundra is a vast lowland plain which in winter is a desolate, frozen waste, and in summer a vast swamp of lichens and arctic moss. Here nature is embalmed in eternal frost, and life is a terror-inspiring struggle with cold and hunger.