42 VEGETATION OF THE PEAK DISTRICT [CH. 
equal strictness, to calcareous soils. The associations of Quercus 
sessiliflora, and of Betula pubescens, as contrasted with the 
association of Quercus Robur, are also edaphic associations; for 
the former occur solely on the shallow siliceous soils of the 
sandstones and shales whilst the latter is limited to the deep 
fluvio-glacial sands. 
On the other hand, the association of Quercus sessiliflora, as 
contrasted with that of Betula pubescens, is related to those 
climatic factors connected with altitude; for both associations 
occur on shallow siliceous soils, but the former occurs at 
altitudes below 1000 feet (805 m.) and the latter at altitudes 
between 1000 feet and 1250 feet (381 m.). 
The mean annual rainfall of the sites of the various woodland 
associations varies locally from about 35 inches (89 cm.) to about 
50 inches (127 cm.); but, so far as one is able to judge, there 
is, in this district, no definite relationship between the different 
woodland associations and local differences in rainfall. On the 
whole, it may perhaps be said that the woods of Quercus Robur 
have the lowest rainfall and those of Betula pubescens the highest 
rainfall; whilst the rainfall of the woods of Quercus sessiliflora 
and of Fraxinus excelsior is intermediate and approximately 
equal in amount. 
As regards the water-content of the soils of the various 
woods, that of the woods of Quercus Robur is the lowest, that 
of the woods of Q. sessiliflora and Fraxinus excelsior is rather 
higher, varying from moderately dry to very wet, and that of © 
the woods of Betula pubescens is, on the whole, the highest. 
The surface soils of the birch association and of the two oak 
associations show a marked tendency to form acidic humus, a 
tendency which is scarcely discernible in the soils of the ash 
woods. The nature of the surface soil is doubtless important 
in relation to the germination of the seeds of the dominant 
species, and therefore in relation to the rejuvenation of the 
woods; but few or no experiments appear to have been con- 
ducted with the view of testing such a hypothesis. In any 
case, the surface soil cannot have much to do with the biology 
of mature trees with deep tap roots, such as the ash. 
Factors, then, which appear to be of importance in accounting 
for the differential distribution of the various woodland associa- 
tions of the Peak District are (1) the chemical nature of the 
