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THROUGH SIBERIA
down to a certain depth. Now the soil, whether it
consists of beds of rock or of loose strata, and whether
it is frozen or thawed, is always somewhat porous, and
it contains moisture and aqueous vapour. But, as the
temperature increases with the depth, the tension of
vapour must be higher at the deeper levels than nearer
the surface. If, then, a thin layer of ice—which often
seems to be formed in the earth in such conditions—
crystallizes out at a certain depth in the perpetually
frozen soil, this layer will continually increase through
the condensation of aqueous vapour from the subjacent
layers of earth, where the tension of vapour is higher.
In this way the layer of ice may constantly grow in the
course of years to almost any thickness. With the
immense force of crystallization of water the ice will
easily raise the layers of earth lying above it. It is also
conceivable that in summer, when the layers of earth
thaw above the ice, both aqueous vapour, and liquid
water which penetrates to the upper side of the layer
of ice, will be condensed and freeze upon the latter,
with the result that it will increase on the upper side
also, unless the temperature is high enough for the thaw
to reach the ice itself.
That layers of ice, sometimes of great thickness,
may be formed in the earth in this manner wherever
the conditions are favourable, appears to me perfectly
natural ; and thus we have, in my opinion, a simple
explanation of the frequent occurrence of this
phenomenon in Siberia and elsewhere. The existence
of such great differences in the temperature of the soil
within short distances as we find here at Taldan and in
other p^ces, must be explained by differences in the
heat-conducting powers of the soil. The general
experience here is that the topmost layer of earth is of
special importance. On the flat tracts, where the soil
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