124 VEGETATION OF THE PEAK DISTRICT [CH. 
surface soils on the flatter plateaux are not washed away; and 
hence they receive no replenishment of new soil from the sub- 
jacent rock: consequently, as leaching continues, the percentage. 
of lime in the less steep localities becomes more and more 
reduced as time passes. If this reasoning be correct, it follows 
that calcareous heaths should be more characteristic of flatter 
and exposed situations than of steep hill-slopes; and this is 
actually the case. 
It is most interesting to note that the humus-loving plants 
of the calcareous heath, such as the heather (Calluna vulgaris), 
are shallow-rooted plants, and that the lime-loving species, such 
as the burnet (Poterium Sanguisorba), are deep-rooted plants. 
Thus the roots of the lime-loving species are able to reach the 
lower layers of the soil where the lime-content remains high ; 
and the roots of the humus-loving species perform their work 
in the upper layers where the lime-content is low and the 
humus-content high. The calcareous heath is therefore a 
complementary plant community (cf. Woodhead, 1906: 345), 
where species of antagonistic requirements live side by side 
in virtue of their roots occupying different levels in the soil. 
The leaching of lime from calcareous soils has, of course, 
long been known; and it is to be expected in districts like the 
Pennines, the Mendips, and the west of Ireland, where the 
rainfall is high. The importance of the process in ecological 
plant geography is that by this means a soil may in time 
become so changed in character as to support a totally different 
group of plant associations from those which first occupied it. 
By this process of leaching, it is conceivable that a particular 
tract of calcareous pasture may ultimately disappear from a 
given spot and be replaced by siliceous grassland or even by 
heath or moor; and similarly it is possible that an ash wood may 
in time be superseded by an oak wood. Such a process is, in 
its general effects, comparable with the changes which occur 
in the conversion of a “ Niedermoor” (“ Flachmoor,” in part) 
or fen characterised by an alkaline peat into a “Hochmoor,” 
or true moorland, characterized by an acidic peat. 
A single plant formation is, within a district of uniform 
climate, marked by a generally uniform type of soil. When, 
by any means, the soil becomes radically changed, then a new 
plant formation has also been called into being on the site of 
