11] WOODLAND ASSOCIATIONS 43 
soil, (2) the depth of the soil, and (8) the altitude of the 
woodland site. Other factors are of importance with regard 
to the distribution of the various types of ground vegetation 
(see pp. 53 and 71). 
1. The chemical nature of the soil. The soils of the 
slopes of the limestone hills, to which the ash woods are 
limited, are rich in calcium carbonate: on newly exposed 
soils in such situations, the soil, in fact, may consist almost 
wholly of this substance; but commonly the percentage of lime 
varies from about five to about thirty per cent. The percentage 
of lime must be very high in the rock-strata in which the tap 
roots of the ash are fixed. On the chalk of the south-east of 
England, the woods are usually dominated by the beech (Fagus 
sylvatica), though the ash association does occur to some extent 
on the chalk. 
The soils of the sandstones and shales have a very low 
lime-content. In several analyses, the percentage of lime 
(calculated as calcium oxide) was so low as 0°02 per cent.; and 
in no case was it higher than 0°05 per cent. On these soils, 
ash woods do not occur, whereas oak and birch associations are 
numerous and typical. It is not suggested here that the presence 
of lime is the direct cause of the occurrence of ash woods on the 
limestone slopes, or that the absence of it is the direct cause of 
the occurrence of oak and birch woods on the siliceous slopes, as 
much more experimental work must be done by plant physio- 
logists before this classical problem can be approached from a 
point of view which is likely to afford a satisfactory outlook on 
the problems involved: all that is stated is that, so far as this 
district is concerned, the ash woods invariably occur on the 
calcareous strata and never elsewhere, and that the oak and 
birch woods invariably occur on the siliceous rocks and never 
on the calcareous ones. It seems, however, reasonable to suggest 
that the presence or absence of a high lime-content of the soil 
is concerned either directly or indirectly with the present distri- 
bution of the principal types of woodland of this district. 
_ 2. The depth of the soil. The fluvio-glacial sands in 
the west of the district are deep, and hence offer no physical 
obstacle to the growth of the long tap roots of Quercus Robur. 
Woods of Quercus Robur occur on these soils. Further, so far as 
this district is concerned, such woods are restricted to these soils. 
