rding to principles to be explained
in the next chapter, tend to increase the severity of the winters. We
may conclude, therefore, that, before the land reached so far to the
north, the temperature of the Siberian winter and summer was more nearly
equalized; and a greater degree of winter's cold may, even more than a
general diminution of the mean annual temperature, have finally
contributed to the extermination of the mammoth and its contemporaries.

On referring to the map (p. 79), the reader will see how all the great
rivers of Siberia flow at present from south to north, from temperate to
arctic regions, and they are all liable, like the Mackenzie, in North
America, to remarkable floods, in consequence of flowing in this
direction. For they are filled with running water in their upper or
southern course when completely frozen over for several hundred miles
near their mouths, where they remain blocked up by ice for six months in
every year. The descending waters, therefore, finding no open channel,
rush over the ice, often changing their direction, and sweeping along
forests and prodigious quantities of soil and gravel mixed with ice. Now
the rivers of Siberia are among the largest in the world, the Yenesei
having a course of 2500, the Lena of 2000 miles; so that we may easily
conceive that the bodies of animals which fall into their waters may be
transported to vast distances towards the Arctic Sea, and, before
arriving there, may be stranded upon and often frozen into thick ice.
Afterwards, when the ice breaks up, they may be floated still farther
towards the ocean, until at length they become buried in fluviatile and
submarine deposits near the mouths of rivers.

Humboldt remarks that near the mouths of the Lena a considerable
thickness of frozen soil may be found at all seasons at the depth of a
few feet; so that if a carcass be once imbedded in mud and ice in such a
region and in such a climate, its putrefaction may be arrested for
indefinite ages.[150] According to Prof. Von Baer of St. Petersburg, the
ground is now frozen permanently to the depth of 400 feet, at the town
of Yakutzt, on the western bank of the Lena, in lat. 62 degrees N., 600
miles distant from the polar sea. Mr. Hedenstrom tells us that,
throughout a wide area in Siberia, the boundary cliffs of the lakes and
rivers consist of alternate layers of earthy materials and ice, in
horizontal stratification;[151] and Mr. Middendorf informed us, in 184