AS CONTRASTED WITH EPPING FOREST. 341
residua of deposits formerly occupying a higher level than their
present situation, and let down by the valley-ward flow of the
clay on which they rest, a progress too molecular to be termed
landslip, and too deep-seated to come under the denomination of
soil-creep. Landslips consist in the descent of one or more
continuous masses, gliding down more or less regular internal
planes of disconnection, as distinct from outward falls of cliff or
escarpment. Soil-creep, on the other hand, is chiefly, though
not wholly, due to alternations of frost and thaw, whilst the
descent of sheets of gravel and sand, without disturbance of the
bedding-planes, by lateral flow of the subjacent clay under their
weight, is the result of simple saturation, which frost would
rather hinder than help. It is, however, doubtful if in this
country the influence of frost ever penetrates deep enough to
affect the process in question.
Detailed study of the environment of the beech groves elicits
the general rule that patches of gravel, of but few feet in thick-
ness, and resting on so gentle a slope of clay as to be almost
continuously water-logged, constitute the favourite habitat of the
beech. Bagshot or Westleton pebble-beds, or Glacial (or more
recent ?) gravels, seem to be indifferently selected, the hydrological
conditions being the main factors in the great question, "To be,
or not to be," for Fagus sylvatica.
The chemical elements present in the soil, or in the percolating
waters, may have a minor influence on the ultimate development
of the trees, a neutral or alkaline condition being indicated as
favourable, acidity as hostile. The Chalk supports many noble
beech-forests, and the ashes of the timber contain a large pre-
ponderance of lime over other bases, whilst sulphates and
chlorides occur only in very limited proportions. In Hainhault
there is a much less quantity of gravel, and the fine sand of the
Bagshot Beds, as mentioned in a previous note (p. 246), is more
or less charged with sulphuric acid from decomposing pyrites.
Probably these considerations are not the sole causes of the
difference between Epping and Hainhault in respect of beech-
trees, but in the complex relations which affect the existence of
every form of life some factors are of paramount, others of minor
or no importance in each case, and I venture to think that those
I have indicated are amongst the foremost for the subject of his
note.