136 ON SOME PLATEAU DEPOSITS AT FELSTEAD AND STEBBING.
most persons. These agencies cause a complete disintegration of
the soil, and there is a sensible movement of the mass due to the
redistribution of the molecules. If there are lumps of chalk in the
soil (Boulder Clay) the change is more striking. In the case of
complete penetration by frost, sufficient moisture being present,
the lumps of chalk suffer disintegration, and when the thaw comes
they are quickly broken up and diffused through the soil (see "Notes
on the Prolonged Frost," Essex Naturalist, vol. v., page 66).
The effect of this diffusion is to make the soil afterwards more
pervious to water; and the effect of rain-water containing carbonic
or other acids then percolating through the soil is to attack the
calcareous matter and gradually take it away in solution. Boulder
Clay is ordinarily almost or quite impervious to water; but if, by any
means, it can be completely penetrated by frost, a door is at once
opened for the admission of atmospheric agencies, and this door
will not again be closed until most or all of the calcareous matter
is removed.
In order to apply this reasoning to the cases under considera-
tion, we must suppose longer or colder winters than we have at
present. My experience is that our winters make no appreciable
difference to the weathering of the Boulder Clay when it is kept
moderately dry.5 The conditions required for change seem to be such
as those now obtaining in some parts of Siberia or Canada, but not
where the soil is permanently frozen, a thaw being essential to the
work of decalcification. Cases are quoted where the loams extend
to a depth of ten feet. In order to affect the Boulder Clay to such an
extent, it seems that the winter temperature must have been much
lower than at present, and the country correspondingly sterile;
moreover, we know that the drainage was less perfect than at
present, and the swamps may have assisted the frost materially in
its work.
In inquiring as to when such arctic conditions prevailed,' we
must take into account the work done by erosion since. This is
considerable, for some loams now occupy such a position that,
supposing those conditions of frost to return again, the loams would,
on account of the loss of their lateral supports, slide down the slope
on the return of the thaw. In fact, a frost and thaw of that
character would be prolific of landslips in England. The extent to
5 A perpendicular section of Boulder Clay made here about twenty years ago, with slightly
overhanging ledge, has not suffered appreciably by weathering.