vil] MOORLAND ASSOCIATIONS 175 
to the drier habitats of these species as compared with the 
habitat of the cotton-grasses (Hriophorum spp.) and their 
ecological allies. In the transitional moors of heather and 
cotton-grass, the shallow roots of Calluna vulgaris, Vaccinium 
Myrtillus, and Empetrum nigrum allow of their growth side 
by side with Hriophorum vaginatum whose functional roots 
are more deeply embedded in the peat; for, in summer and 
autumn, the upper layers of peat are frequently dry whilst the 
lower layers remain extremely wet. 
(7) Changesin the habitat. It has already been stated 
that the peat on the highest moors is in a state of denudation 
and now dominated by Vaccinium Myrtillus, Empetrum nigrum, 
and Rubus Chamaemorus. An examination of the plant-remains 
composing the peat of such associations proves that the vegeta- 
tion was previously dominated, and almost exclusively dominated, 
by Eriophorum. The degeneration of an Eriophorum moor 
results in the peat becoming drier; and this results in the 
dying out of the more hydrophilous species, such as Lrio- 
phorum vaginatum and £E. angustifoluum and the successful 
invasion of other plants, such as the bilberry, which are 
structurally better adapted to the drier conditions. 
(8) Rainfall. It would appear that the local differences in 
the mean annual rainfall of the different parts of the moorland 
area have little or no relation to the local distribution of the 
different moorland associations in the Peak District. Generally 
speaking, the mean annual rainfall of the moorland area varies 
from about 40 inches (102 cm.) per annum to 55 inches (140 cm.) 
or rather more. It has been suggested (Smith and Rankin, 
1903: 155) that the areas where the heather (Calluna vulgaris) 
is dominant have, on the whole, a lower mean annual rainfall than 
the areas over which the cotton-grass (Hriophorum vaginatum) 
is dominant; but, judging from the statistics supplied by Dr 
H. R. Mill (see page 25), the suggestion does not appear to bea 
fruitful one so far as this district is concerned. It is well known 
that the highest local rainfall of a district is not exactly at the 
summit of a hill, but some distance to the leeward of that hill. 
For example, in the present district, the highest indicated 
rainfall (see British Rainfall) is not on the Peak itself, but 
in Fairbrook Clough, which is a few miles to the leeward, that 
is, to the north-east of the Peak; and this tract of highest 
